Laser Surface Modification of HVOF Coatings for Improvement of Corrosion and Wear Performance

Transcription

1 Laser Surface Modification of HVOF Coatings for Improvement of Corrosion and Wear Performance A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering and Physical Science 2013 Mohsen Mohamed Rakhes School of Materials

2 TABLE OF CONTENTS LIST OF FIGURES... 6 LIST OF TABLES ABSTRACT DECLARATION COPYRIGHT STATEMENT ACKNOWLEDGEMENTS NOMENCLATURE LIST OF PUBLICATIONS RELATED TO THIS THESIS CHAPTER 1 INTRODUCTION RESEARCH MOTIVATION AND RATIONALE AIMS AND OBJECTIVES OF THE RESEARCH THESIS OUTLINE CHAPTER 2 LITERATURE REVIEW INTRODUCTION......, CORROSION OF METALS Fundamentals of Electrochemical Corrosion The electrode potential Corrosion thermodynamics Kinetics of corrosion under polarisation Types of corrosion Polarisation curve Electrochemical impedance spectroscopy (EIS) WEAR Classifications and Types of Wear Wear tests Pin-on-Disc wear test THERMAL SPRAY COATINGS

3 TABLE OF CONTENTS Principles of Thermal Spray Coatings Types of Thermal Spray Coatings High Velocity Oxy-Fuel Thermal Spraying Coating materials Microstructure of HVOF-Sprayed coatings Corrosion Performance of HVOF-Sprayed WC-Co-base coatings Wear Performance of HVOF-Sprayed WC-Co-base coatings LASER SURFACE ENGINEERING Lasers Laser beam Interaction with Materials Laser Surface Treatment Laser induced rapid solidification Effect of laser Surface treatment on microstructure and electrochemical properties of HVOF Coatings Effect of laser Surface treatment on hardness and wear resistance of HVOF Coatings CHAPTER 3 EXPERIMENTAL PROCEDURES INTRODUCTION MATERIALS Specimen preparation for laser treatment LASER PROCESSING Laser System Laser processing setup MATERIAL CHARACTERISATION Sample preparation Scanning Electron Microscope (SEM/EDS) X-ray diffraction (XRD) Surface profile Porosity measurements Electron probe micro-analyser (EPMA) CORROSION TESTS Immersion test Sample preparation for polarisation and EIS tests

4 TABLE OF CONTENTS Polarisation Electrochemical Impedance Spectroscopy (EIS) Test WEAR TEST Pin-On-Disc tester Wear test conditions MICROHARDNESS MEASUREMENT CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS INTRODUCTION DEFINITION OF THE TERMS FORMATION OF POROSITY WITHIN LASER MELTED SURFACE FORMATION OF CRACKS WITHIN LASER MELTED SURFACE OPERATING WINDOWS FOR LASER TREATMENT Summary CHAPTER 5 MATERIALS CHARACTERISATION INTRODUCTION POWDERS Morphological observation XRD analysis AS-SPRAYED COATING Surface views of HVOF coatings Cross sectional view of HVOF coatings XRD analysis of HVOF coatings Porosity measurements of HVOF coatings LASER SURFACE TREATED HVOF COATINGS Surface views of laser melted HVOF coatings Cross section of laser treated HVOF coatings XRD of laser treated HVOF coatings Porosity measurements of laser treated HVOF coatings Summary CHAPTER 6 CORROSION TESTS INTRODUCTION

5 TABLE OF CONTENTS 6.2 IMMERSION TEST Tribaloy 800 (T800) coating T800+21WC T800+43WC T800+68WC Inductivity coupled plasma- optical emission spectrometer (ICP-OES) test POLARISATION TEST ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY TEST (EIS) Summary CHAPTER 7 RESULTS ON HARDNESS AND WEAR RESISTANCE MEASUREMENTS INTRODUCTION MICROHARDNESS WEAR TEST RESULTS T T800+21WC T800+43WC T800+68WC Summary CHAPTER 8 CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK INTRODUCTION Conclusions Suggestions for future work APPENDIX A APPENDIX B REFERENCES Word count: 54,075 5

6 LIST OF FIGURES Figure 2.1 Current flow in corrosion cell [17] Figure 2.2 Metal in an aqueous solution containing metal ions; an electrical double layer has formed [19] Figure 2.3 Determination of electrode potential with the aid of a reference electrode [19] Figure 2.4 Anodic and cathodic polarisation curves [19] Figure 2.5 Crevice corrosion [27] Figure 2.6 Pitting corrosion [28] Figure 2.7 (a) Transgranular cracks in an austenitic steel produced in a chloride environment (100x); (b) Intergranular cracks in a ferritic stainless steel produced in a high temperature caustic environment (50x) [30] Figure 2.8 Schematic diagram of the polarisation behaviour [16] Figure 2.9 AC wave forms for an applied potential and a resulting current Figure 2.10 Complex plane showing impedance vector Z [36] Figure 2.11 Nyquist plot with impedance vector Figure 2.12 Bode and phase angle plots [32] Figure 2.13 Randle equivalent circuit (a) and Nyquist plot (b) Figure 2.14 The Equivalent circuit for a coated substrate [37] Figure 2.15 Abrasion wear [51] Figure 2.16 Adhesion wear [51] Figure 2.17 Fatigue wear [53] Figure 2.18 Diagram of Pin-on-Disk configuration wear test [48] Figure 2.19 Basic principle of thermal spraying [55] Figure 2.20 Classification of thermal spray processes [56] Figure 2.21 Schematic diagram of the HVOF process [57] Figure 2.22 Cross-section of T800 HVOF coating on stainless steel substrate Figure 2.23 Schematic diagram of adhesion of a particle to a substrate asperity. (1) asperities, (2) particle in flight, (3) particle mechanically locked to the substrate, (4) substrate [55] Figure 2.24 Thermal spray coating microstructure showing common features [56]

7 LIST OF FIGURES Figure 2.25 Cross-section of T800-68WC HVOF coating on stainless steel substrate Figure 2.26 Potentiodynamic polarisation curves of different coatings in the aerated 5 wt. H2SO4 solution [5] Figure 2.27 Schematic representation of the reaction processes taking place on the WC-Co surface as a consequence of the galvanic coupling between Co and WC [77] Figure 2.28 Electrochemical polarisation curves of HVOF-sprayed coatings in 0.1 M HCl solution [70] Figure 2.29 Potentiodynamic polarisation of Co and WC in aerated 1 N H 2 SO 4 [85] Figure 2.30 Potential-pH equilibrium diagram for the system cobalt-water, at 25 o C, potential with respect to standard hydrogen electrode [85] Figure 2.31 Potential-pH equilibrium diagram for the system tungsten-water, at 25 o C, potential with respect to standard hydrogen electrode [85] Figure 2.32 Electrochemical behaviour of Co-W alloys in aerated 1 N H 2 SO 4 [93] Figure 2.33 Total (pin and disc) specific wear rates for different HVOF coating composition under dry oscillating wear [105] Figure 2.34 Two-level energy system (a), Spontaneous emission (b), absorption (c), Stimulated emission (d) Figure 2.35 Laser components Figure 2.36 Amplification process Figure 2.37 Semiconductor laser-junction region [111] Figure 2.38 Semiconductor laser [111] Figure 2.39 absorption as a function of wavelength for carbon steel and aluminium [114] Figure 2.40 Time resolved intensity distribution within a 1.6 s interval (a) HPDL system (b) Nd-YAG laser [116] Figure 2.41 Laser surface processing [119] Figure 2.42 laser surface treatment techniques; (a) laser transformation hardening, (b) laser surface melting, (c) laser alloying, (d) laser cladding Figure 2.43 Effect of temperature gradient G and growth rate R on the morphology and size of solidification microstructure [122] Figure 2.44 SEM morphologies of the laser clad Tribaloy T-800 on stainless steel (AISI 304), shows: (1) substrate; (2) planar crystallization region; (3) cellular growth zone; (4) fine dendritic microstructure; (5) overlap zone between tracks. power density 200 W/mm 2, scanning speed 240 mm/min and powder feed rate g/min [123]

8 LIST OF FIGURES Figure 2.45 SEM morphologies of the laser clad Tribaloy T-800 on stainless steel (AISI 304) power density 200 W/mm 2 scanning speed 240 mm/min and powder feed rate g/min: (a) Cellular structure; (b) dendritic microstructure [123] Figure 2.46 SEM morphologies of laser clad of the Stellite 6. The dark dendritic phase is the cobalt-based matrix and the white phase is formed by the M 7 C 3 eutectic carbides: (a) slow processing conditions (V b =1.67 mm s -1 ); (b) fast processing conditions (V b =167 mm s -1 ) [126] Figure 2.47 Potentiodynamic polarisation curves of HVOF WC24Cr 3 C 2-6Ni coatings in 3.5% NaCl solution [129] Figure 2.48 Polarisation curves of laser melting and HVOF sprayed Inconel 625 coatings [10] Figure 2.49 optical micrograph of (a) HVOF sprayed and (b) laser remelted high chromium nickel-chromium coating after one week immersion test in 3.5 wt.% NaCl [11] Figure 2.50 Microhardness of cross-sections of HVOF WC-CoCr coatings before and after laser treatment [128] Figure 2.51 Friction coefficient and wear rate of the coatings [129] Figure 3.1 Schematic diagram of cutting process Figure 3.2 Laser processing setup Figure 3.3 Laser treatment experiment Figure 3.4 Cross-sectioned of SEM samples mounted using conducting resins Figure 3.5 Schematic of scanning electron microscope [133] Figure 3.6 Schematic of electron beam interaction with material [135] Figure 3.7 Waves reflected from successive planes of a crystal Figure 3.8 MicroXAM surface mapping microscope [137] Figure 3.9 Schematic diagram showing sample prepared for the immersion test Figure 3.10 Sample prepared for polarisation or EIS test Figure 3.11 (a) ACM Gill AC instrument, (b) complete setup for the experiment, (c) A schematic of experimental setup used in polarisation test Figure 3.12 Setup of open circuit potential Figure 3.13 Setup of polarisation experiment Figure 3.14: Setup of AC impedance experiment Figure 3.15 Diagram of a pin on disc configuration wear test Figure 3.16 Parameters of wear track (a) track diameter and track width, (b) track depth Figure 4.1 Schematic shown partially laser melting of HVOF coating

9 LIST OF FIGURES Figure 4.2 Overlapping tracks of laser treated T800+21WC HVOF coating showing surface of partially melting of HVOF coating Figure 4.3 SEM cross-section image of HVOF T800 (a) partially laser melting T800 HVOF coating (b) high magnification of melted T800 HVOF layer Figure 4.4 SEM cross-section image of HVOF T800+21WC (a) partial laser melting of T800+21WC HVOF coating (b) high magnification of melted T800+21WC HVOF layer Figure 4.5 Schematic shown fully laser melting of HVOF coating (a) melting of HVOF coating only (no dilution) (b) melting of substrate (dilution) Figure 4.6 Overlapping tracks of the laser-treated the T800+43WC HVOF coating, showing surface of large melting of HVOF+43WC coating Figure 4.7 SEM cross-section image of HVOF coating T800+43WC fully laser melted (a) without dilution (b) with dilution Figure 4.8 SEM cross-section image of HVOF coating T800+68WC fully laser melted (a) without dilution (b) with dilution Figure 4.9 Schematic illustration of partially and full melting of WC Figure 4.10 SEM cross-section image of HVOF T800+21WC (a) HVOF coating (b) high magnification of the coating Figure 4.11 SEM cross-section image of HVOF T800+21WC (a) partially laser melted WC near the surface (b) partially melted WC particles and fully melted T Figure 4.12 SEM cross-section image of HVOF T800+43WC (a) partially laser melted WC (b) partially melted WC particles and fully melted T Figure 4.13 SEM cross-section image of HVOF T800+68WC (a) partially laser melted WC (b) partially melted WC particles and fully melted T Figure 4.14 porosity in laser treated area (a) porosity near the surface and a vertical crack in T800+43WC HVOF laser-treated (800 W, 40 mm/sec) (b) porosity in T800+68WC HVOF laser treated (800 W, 20 mm/sec) Figure 4.15 low porosity in the laser treated area (a) T800+43WC HVOF, laser-treated (800 W, 5 mm/s) (b) T800+68WC HVOF, laser treated (800 W, 3 mm/s) Figure 4.16 porosity in the laser treated area (a) T800 HVOF, laser treated area (600 W, 20 mm/s) (b) T800+21WC HVOF, laser treated (800 W, 40 mm/s) Figure 4.17 low porosity in the laser treated area (a) T800+43WC HVOF, laser treated (550 W, 3 mm/s) (b) T800+68WC HVOF, laser treated (500 W, 3 mm/s)

10 LIST OF FIGURES Figure 4.18 SEM microstructure of the Laves phase in thet800 HVOF coating after laser treatment Figure 4.19 vertical crack in T800 HVOF laser treated (a) small crack at (600W, 5 mm/s) (b) big crack with high dilution at (800W, 10 mm/s) Figure 4.20 a small vertical crack in T800 HVOF laser treated with a partially- melted coating (500W, 3 mm/s) Figure 4.21 a vertical crack in a T800+21WC HVOF laser-treated surface (a) a small crack at (800W, 5 mm/s) (b) a large crack and porosity at laser parameters (600W, 40 mm/s) Figure 4.22 a vertical crack and porosity in a T800+43WC HVOF lasertreated surface, with a partially-melted coating (800W, 80 mm/s) Figure 4.23 a vertical crack and porosity in a T800+68WC HVOF lasertreated surface with a partially-melted coating (800W, 20 mm/s) Figure 4.24 The variation of the ratio of melting depth/coating thickness with scanning speed for different laser powers for a T800 HVOF coating Figure 4.25 The variation of the ratio of melting depth/coating thickness with scanning speed for different laser powers for a T800+21WC HVOF coating Figure 4.26 The variation of the ratio of melting depth/coating thickness with scanning speed for different laser powers for a T800+43WC HVOF coating Figure 4.27 The variation of the ratio of melting depth/coating thickness with scanning speed for different laser powers for a T800+68WC HVOF coating Figure 5.1 SEM morphology of T800 powder Figure 5.2 SEM micrographs of the T WC powder Figure 5.3 SEM micrographs of (a) T WC (b) T WC powders Figure 5.4 Mapping images: (a) T800 powder (b) T800+21WC powder (c) T800+43WC powder (d) T800+68WC powder Figure 5.5 XRD pattern of the T800 stock powder Figure 5.6 XRD pattern of the T800+21WC stock powder Figure 5.7 XRD pattern of the T800+43WC stock powder Figure 5.8 XRD pattern of the T800+68WC stock powder Figure 5.9 SEM micrographs of surface view of T800 HVOF coatings with (a) low magnification (b) high magnification Figure 5.10 SEM micrographs of surface view of T800+21WC HVOF coatings with (a) low magnification and (b) high magnification

11 LIST OF FIGURES Figure 5.11 SEM micrographs of surface view of T800+43WC HVOF coatings with (a) low magnification and (b) high magnification Figure 5.12 SEM micrographs of surface view of T800+68WC HVOF coatings with (a) low magnification and (b) high magnification Figure 5.13 MicroXam (a) 2D and (b) 3D profiles of four different areas on T800 HVOF surface Figure 5.14 MicroXam (a) 2D and (b) 3D profiles of four different areas on T800+21WC HVOF surface Figure 5.15 MicroXam (a) 2D and (b) 3D profiles of four different areas on T800+43WC HVOF surface Figure 5.16 MicroXam (a) 2D and (b) 3D profiles of four different areas on T800+68WC HVOF surface Figure 5.17 Comparison of surface roughness of various HVOF coatings Figure 5.18 Comparison of surface area ratios of various HVOF coatings Figure 5.19 SEM micrograph of cross section of T800 HVOF coating Figure 5.20 SEM micrographs of T800 HVOF coating at the interface between the coating and substrate (a) and typical microstructure (b) Figure 5.21 SEM image of cross-section of T800 HVOF Figure 5.22 SEM micrograph of T800 HVOF microstructure Figure 5.23 EDX results of various phases in T800 HVOF coating (1) Laves phase (2) Eutectic Laves phase + Co solid solution (3) Co solid solution Figure 5.24 SEM micrograph of cross section of T800+21WC HVOF coating Figure 5.25 SEM micrographs of T800+21WC HVOF coating at the interface between coating and substrate (a) and with high magnification (b) Figure 5.26 SEM micrograph of cross section of T800+21WC HVOF coating with high magnification Figure 5.27 SEM micrograph of cross section of T800+43WC HVOF coating with high magnification Figure 5.28 SEM micrograph of cross section of T800+43WC HVOF coating Figure 5.29 SEM micrographs of T800+43WC HVOF coatings at interface between coating and substrate (a) and with high magnification (b) Figure 5.30 SEM micrograph of cross section for T800+68WC HVOF coating Figure 5.31 SEM micrographs of T800+68WC HVOF coating at interface between coating and substrate (a) and with high magnification (b) Figure 5.32 EDX results of various phases in T800+WC HVOF coating (1) White phase, (2) dark matrix and (3) grey phase

12 LIST OF FIGURES Figure 5.33 (a) SEM micrographs of T800+68WC HVOF coating and (b) EDX line scan analysis of the line marked in (a) Figure 5.34 EDX analyses at splat boundary (1) and near splat boundary (2) for T800 HVOF coating Figure 5.35 Line scan for oxygen trough splat boundary for T800 HVOF coating Figure 5.36 EPMA mapping of T800 HVOF coating Figure 5.37 EPMA mapping of T800+43WC HVOF coating Figure 5.38 XRD pattern of T800 HVOF coating Figure 5.39 XRD pattern of T800+21WC HVOF coating Figure 5.40 XRD pattern of T800+43WC HVOF coating Figure 5.41 XRD pattern of T800+68W HVOF coating Figure 5.42 Porosity of T800 HVOF coating Figure 5.43 Porosity of T800+21WC HVOF coating Figure 5.44 Porosity of T800+43WC HVOF coating Figure 5.45 Porosity of T800+68WC HVOF coating Figure 5.46 SEM micrographs of laser treated surface view of T800 HVOF coating at low magnification (a) and high magnification (b) Figure 5.47 SEM micrographs of laser treated surface of T800+21WC HVOF coating at low magnification (a) and high magnification (b) Figure 5.48 SEM micrographs of laser treated surface of T800+43WC HVOF coatings at low magnification (a) and high magnification (b) Figure 5.49 SEM images of surface of laser treated T800+68WC HVOF coating at low magnification (a) and high magnification (b) Figure 5.50 MicroXam (a) 2D and (b) 3D profiles of four different areas on laser treated T800 HVOF coating Figure 5.51 MicroXam (a) 2D and (b) 3D profiles of four different areas on laser treated T800+21WC HVOF coating Figure 5.52 MicroXam (a) 2D and (b) 3D profiles of four different areas on laser treated T800+43WC HVOF coating Figure 5.53 MicroXam (a) 2D and (b) 3D profiles of four different areas on laser treated T800+68WC HVOF coating Figure 5.54 Surface roughness of various laser treated HVOF coatings Figure 5.55 Average of surface area ratio of laser treated HVOF coatings Figure 5.56 SEM micrograph of cross section of laser treated T800 HVOF coating Figure 5.57 SEM micrograph of laser melted T800 HVOF coating

13 LIST OF FIGURES Figure 5.58 EDX analysis of various phases in laser melted T800 HVOF coating (1) Laves phase (2) Eutectic (Laves phase + Co solid solution) (3) Co solid solution Figure 5.59 SEM micrograph of cross section of laser treated T800+21WC of HVOF coating Figure 5.60 SEM micrographs of laser treated T800+21WC HVOF coating showing (a) interface between the coating and substrate (b) microstructure in laser melted area Figure 5.61 SEM micrograph of cross section for laser treated T800+21WC HVOF coating Figure 5.62 EDX results of various phases in T800+21WC HVOF coating after laser treatment (1) Co-Cr binder phase (2) Co solid solution phase (3) partially melted WC Figure 5.63 SEM micrograph of cross section of laser treated T800+43WC HVOF coating showing partially melted WC Figure 5.64 SEM micrograph of laser treated T800+43WC HVOF coating showing (a) interface between the substrate and the coating (b) microstructure in laser melted area Figure 5.65 SEM micrograph of cross section of laser treated T800+43WC HVOF coating showing partially melted WC Figure 5.66 EDX results of phases (marked as 1, 2, 3 in Figure 5.65) in T800+43WC HVOF coating (1) T800 matrix (2) Co-Cr binder phase (3) partially melted WC Figure 5.67 (a) SEM micrograph of laser treated T800+68WC (b) EDX line scan analysis on T800 and WC particles Figure 5.68 EPMA mapping of laser treated T800 HVOF coating Figure 5.69 EPMA mapping of laser treated T800+43WC HVOF coating with partially melting of WC particles Figure 5.70 EDX results of different areas of T800+43WC HVOF coating after laser treatment Figure 5.71 EDX line scan of laser treated T800+43WC HVOF coating Figure 5.72 SEM micrograph of cross section of laser treated T800+68WC HVOF coating Figure 5.73 SEM micrographs of laser treated T800+68WC HVOF coating showing (a) interface between the coating and substrate and (b) microstructure of melted area Figure 5.74 SEM micrograph of cross section of laser treated T800+68WC Figure 5.75 EDX results in different areas of laser treated T800+68WC HVOF coating Figure 5.76 EDX results in different areas of laser treated T800+68WC HVOF coating

14 LIST OF FIGURES Figure 5.77 EDX results of line scan of laser treated T800+43WC HVOF coating showing Fe distribution along the coating thickness Figure 5.78 XRD pattern of T800 HVOF coating after laser treatment Figure 5.79 XRD pattern of T800+21WC HVOF coating after laser treatment Figure 5.80 XRD pattern of T800+43WC HVOF coating after laser treatment Figure 5.81 XRD pattern of T800+68WC HVOF coating after laser treatment Figure 5.82 Porosity of laser treated T800 HVOF coating Figure 5.83 Porosity of laser treated T800+21WC HVOF coating Figure 5.84 Porosity of laser treated T800+43WC HVOF coating Figure 5.85 Porosity of laser treated T800+68WC HVOF coating Figure 6.1 SEM cross-section images of T800 HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 24 h Figure 6.2 SEM cross-section images of T800 HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 48 h Figure 6.3 SEM cross-section images of T800 HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 72 h Figure 6.4 SEM cross-section images of T800 HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 96 h Figure 6.5 SEM cross-section images of T800+21WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 48 h Figure 6.6 SEM cross-section images of T800+21WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 72 h Figure 6.7 SEM cross-section images of T800+21WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 96 h Figure 6.8 SEM cross-section images of T800+43WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 48 h Figure 6.9 SEM cross-section images of T800+43WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 72 h Figure 6.10 SEM cross-section images of T800+43WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 96 h

15 LIST OF FIGURES Figure 6.11 SEM cross-section images of T800+68WC HVOF coating: a, c as received HVOF coatings, b, d after laser treatment, all are immersed for 48 h Figure 6.12 SEM cross-section images of T800+68WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 72 h Figure 6.13 SEM cross-section images of T800+68WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 96 h Figure 6.14 Mean concentration of the elements released in 3M H 2 SO 4 after 7 days of immersion of T800+43WC HVOF coating before and after laser treatment Figure 6.15 Polarization curves of the HVOF coatings in 0.5 M H 2 SO 4 solution Figure 6.16 Polarization curves of HVOF coatings before and after laser treatment in 0.5 M H 2 SO 4 (a) T800 (b) T800+21WC (c) T800+43WC (d) T800+68WC Figure 6.17 A typical polarization curve of T800+43WC HVOF coating Figure 6.18 Microcrevice corrosion of T800+21WC HVOF coating after immersion in 3 M H 2 SO 4 for 108 h Figure 6.19 Galvanic corrosion of T800+43WC HVOF coating after polarisation test Figure 6.20 Cross section of T800 HVOF coating after polarization test in 0.5 M H 2 SO 4 solution Figure 6.21 Surface corrosion of laser treated T800 coating after polarization test in 0.5 M H 2 SO Figure 6.22 Surface of laser treated T800 coating after polarization test in 0.5 M H 2 SO 4 solution Figure 6.23 Cross section of laser treated T800+21WC coating after polarization test in 0.5 M H 2 SO 4 solution Figure 6.24 Cross section of T800+68WC HVOF coating after polarization test in 0.5 M H 2 SO Figure 6.25 Cross section of T800+43WC laser treated coating after polarization test in 0.5 M H 2 SO Figure 6.26 Nyquist plots of HVOF coatings for different times (a) T800 (b) T800+21WC (c) T800+43WC and (d) T800+68WC Figure 6.27 Nyquist plots of laser treated HVOF coatings for different times (a) T800 (b) T800+21WC (c) T800+43WC and (d) T800+68WC Figure 6.28 Equivalent circuit proposed for the coating system Figure 6.29 Impedance spectra after 3 hours of immersion for various coatings before and after laser treatment: (a) T800, (b) T800+21WC, (c) T800+43WC, and (d) T800+68WC

16 LIST OF FIGURES Figure 6.30 Impedance spectra after 12 hours of immersion for various coatings before and after laser treatment: (a) T800, (b) T800+21WC, (c) T800+43WC, and (d) T800+68WC Figure 6.31 Coating resistance R p before and after laser treatment for various time of immersion (a) T800 (b) T800+21WC (c) T800+43WC (d) T800+68WC Figure 6.32 Charge transfer resistance R ct before and after laser treatment for various time of immersion (a) T800 (b) T800+21WC (c) T800+43WC (d) T800+68WC Figure 6.33 Cross section of T800 HVOF coating after EIS test at 12h immersion time (a) cross section of the coating (b) interface Figure 6.34 Cross section of T800 HVOF coating after EIS test at 12h immersion time (a) surface of the coating (b) corrosion of Co solid solution within splats (c) cracks within splats Figure 6.35 Cross section of laser treated T800 HVOF coating after EIS test at 12h immersion time (a) cross section of the coating (b) zoom of the upper side of the coating Figure 6.36 Cross section of T800+68WC HVOF coating after EIS test at 12h immersion time (a) cross section of the coating (b) interface (c) zoom of upper side of the coating Figure 6.37 Cross section of laser treated T800+68WC HVOF coating after EIS test at 12h immersion time (a) cross section of the coating (b) corrosion area Figure 6.38 Cross section of T800 HVOF coating after EIS test at 48h immersion time (a) cross section of the coating (b) corrosion at the upper side of the coating Figure 6.39 Cross section of laser treated T800 HVOF coating after EIS test at 48h h immersion time (a) cross section of the coating (b) zoom at the upper side of the coating Figure 6.40 Cross section of T800+21WC HVOF coating after EIS test at 48h immersion time (a) upper side of the coating (b) interface Figure 6.41 Cross section of laser treated T800 HVOF coating after EIS test at 48h immersion time (a) cross section of the coating (b) upper side of the coating Figure 6.42 Cross section of T800+68WC HVOF coating after EIS test at 48h immersion time (a) cross section as HVOF coating (b) cross section after laser treated (c) upper side of HVOF coating (d) upper side of laser treated coating Figure 7.1 Hardness profiles of various HVOF coatings before and after laser treatment (a) T800 (b) T800+21WC (c) T800+43WC (d) T800+68WC

17 LIST OF FIGURES Figure 7.2 SEM micrographs of microhardness indentations on different phases of T800+43WC HVOF coating Figure 7.3 SEM micrographs of microhardness indentations on different phases of laser treated T800+43WC coating Figure 7.4 SEM micrographs of microhardness indentations (a) on asreceived HVOF coating and (b) after laser treatment Figure 7.5 Wear rates for various coatings before and after laser treatment Figure 7.6 SEM micrographs of worn surface of T800 HVOF coating Figure 7.7 SEM micrographs of worn surface of laser-treated T800 HVOF coating Figure 7.8 SEM micrographs of worn surface of T WC HVOF coating Figure 7.9 SEM micrographs of worn surface of laser-treated T WC HVOF coating Figure 7.10 SEM micrographs of worn surface of T WC HVOF coating Figure 7.11 SEM micrographs of worn surface of laser-treated T W coating Figure 7.12 SEM micrographs of worn surface of T WC HVOF coating Figure 7.13 SEM micrographs of worn surface of laser-treated T WC coating

18 LIST OF TABLES Table 2.1 Equivalent circuit elements [34] Table 2.2 Comparision of characteristics for various thermal spraying processes [3] Table 2.3 Main lasers and their properties [110] Table 2.4 Wavelengths of a selected range of diode laser materials [112] Table 3.2 Conditions of wear test Table 4.1 A laser operating conditions used for various HVOF coating with observations Table 4.2 Laser operating conditions and coating characteristics of the selected coatings Table 5.1 Surface roughness and surface area ratios of various HVOF coatings Table 5.2 EDX chemical analysis of T800 HVOF coating, wt.% Table 5.3 Coating thickness measured Table 5.4 Measurement of porosity of various HVOF coatings Table 5.5 Optimum parameters obtained in laser processing and coatings features Table 5.6 Surface roughness parameters of laser treated surface Table 5.7 Measurement of porosity of various HVOF coatings after laser treatment Table 6.1 Mean concentration of the elements (units of µg/ml) released into the electrolyte solution after 7 days of immersion of T800+43WC HVOF coating before and after laser treatment Table 6.2 Corrosion current density (Icorr) and corrosion potential (Ecorr) of HVOF coatings before and after laser treatment in 0.5 M H2SO4 solution Table 6.3 Electrochemical parameters obtained from EIS spectra of HVOF coatings after 3 hours of immersion Table 6.4 Electrochemical parameters obtained from EIS spectra of laser coatings after 3 hours of immersion Table 6.5 Electrochemical parameters obtained from EIS spectra of HVOF coatings after 12 hours of immersion Table 6.6 Electrochemical parameters obtained from EIS spectra of laser coatings after 12 hours of immersion Table 7.1 Microhardness values of various HVOF coatings before and after laser treatment

19 ABSTRACT Metal Matrix Composite (MMC) coatings, comprised of a hard ceramic phase embedded in a metallic matrix, are increasingly being applied for many industrial applications to provide cost effective protection against wear and corrosion. Such coatings are commonly produced by thermal spray. Although the most advanced thermal spray techniques, such as high-velocity oxy-fuel (HVOF), produce MMC coatings with total porosity levels lower than 1%, due to the nature of thermal spray MMC coatings, corrosion still takes place. The corrosion processes are dominated by the complex microgalvanic and interfacial mechanisms, as well as by porosity, due to the existence of various defects in HVOF MMC coatings. As a result, HVOF coatings do not ultimately meet the requirements in certain service conditions in operating environments. Therefore, there is a need to find a method of modification of coatings, with significantly reduced microstructural defects and improved cohesive and adhesive strength so that the service life of the coated components can be increased. This work aims to investigate the effects of laser surface treatment on the corrosion and wear performance for Tribaloy 800 (T800), and T800-based WC HVOFsprayed MMC coatings onto 316L stainless steel substrate. Laser surface treatments have been carried out using a 1.5 kw high power diode laser. Laser operating windows for various coatings have been established for the relationships between the laser operating conditions and melt pool dimensions, in the consideration of formation of cracks and porosity within laser-treated surface layers. Microstructural analysis of the powders, and various coatings before and after laser treatments has been conducted by means of optical and SEM (with EDX) microscopy, electron probe micro-analysis (EPMA), white-light interferometery, and X-ray diffraction, to characterise morphology, chemical composition and phase. Corrosion performance of various coating was evaluated using immersion testing in 3 M H 2 SO 4 at ph ~ 1.27 at room temperature for different periods of time (including 24, 48, 72, 96 and 168 hours), followed by Inductivity Coupled Plasma-optical emission spectrometer (ICP-OES) technique, potentiodynamic polarisation in 0.5 M H 2 SO 4, and electrochemical impedance 19

20 ABSTRACT spectroscopy studies in 0.5 M H 2 SO 4 solution after 1, 3, 6, 12, 24, and 48 hours. Inaddition, dry sliding wear behaviour measured by pin-on-disk and microhardness test of various coatings before and after laser treatment were evaluated. The results indicated that it was possible to achieve full control of melt depth and the degree of melting, particularly full or partial melting of WC particles by proper selection of the laser processing parameters while preventing dilution. Significant improvement of corrosion and wear resistance has been achieved after laser treatment as a result of the elimination of discrete splat-structure, removal of microcrevices and porosity, as well as the reduction of microgalvanic driving force between the WC and the metal matrix by formation of new phases at the interfaces. The degree of melting of WC particles controls the corrosion properties of the laser-treated HVOF coatings. Moreover, the results also suggested that partial melting of WC had positive effect on wear resistance of the coatings. 20

21 DECLARATION No portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institution of learning MOHSEN RAKHES 21

22 COPYRIGHT STATEMENT [i] [ii] [iii] [iv] The author of this thesis (including any appendices and/or schedules to this thesis) owns any copyright in it (the Copyright ) and he has given The University of Manchester the right to use such Copyright for any administrative, promotional, educational and/or teaching purposes. Copies of this thesis, either in full or in extracts, may be made only in accordance with the regulations of the John Rylands University Library of Manchester. Details of these regulations may be obtained from the Librarian. This page must form part of any such copies made. The ownership of any patents, designs, trade marks and any and all other intellectual property rights except for the Copyright (the Intellectual Property Rights ) and any reproductions of copyright works, for example graphs and tables ( Reproductions ), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property Rights and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property Rights and/or Reproductions. Further information on the conditions under which disclosure, publication and exploitation of this thesis, the Copyright and any Intellectual Property Rights and/or Reproductions described in it may take place is available from the Head of School of Materials. 22

23 THIS WORK IS DEDICATED TO MY PARENTS & WIFE 23

24 ACKNOWLEDGEMENTS I am extremely thankful to Allah for help me to carrying out this research. I would like to express my thanks for my kind supervisor Dr Zhu Liu for her great guidance, help, and encouragement. Very special thanks to Dr Gareth Littlewood and Dr Karen Shapiro for their help, support, and comments on the work. I am thankful to all staff and research assistants working in both centres (CPC and LPRC) for their support. I am thankful to the experimental officers in LPRC, Dr Marc Schmidt and Dr David Whitehead for their help. I want to express my gratitude to Dr. Elena Koroleva, Prof. Peter Skeldon, Ms Judith Shackleton, Mr Teruo Hashimoto, Mr Mark Harris, Paul Jordan, Steve Blatch, Peter Carroll, and Mr Kenneth Gyves for their help and support. I wish to thank my colleagues: Ejaz, Zakria, Sohaib, Yasir, Ismail, Wei, Abdeslam, Bader, Ahmed, Dr Fernando, and Dr Ana for their advice. I am thankful to my father and my mother and all my family members for their efforts and pray to provide me strength to compete this work. I would like to express my indebtedness to my wife for her support during my work. Finally, I would also like to express my gratitude to the people who have contributed in different ways towards the completion of this thesis. 24

25 NOMENCLATURE NOMENCLATURE HVOF GNP EDL OCP EIS AC DC CPE pf nf MMC AFM ICP HPDL SEM EDS EBSD FEG SE BSE XRD EPMA WDS SCE OCP G EMF F E E ref E test High Velocity Oxygen Fuel Gross National Product Electrical Double Layer Open Circuit Potential Electrochemical Impedance Spectroscopy Alternating Current Direct Current Constant Phase Element Pico-Farad Nano-Farad Metal Matrix Composite Atomic force microscope Inductively coupled plasma High Power Diode Laser Scanning Electron Microscope Energy Dispersive Spectrometry Electron Back-Scattered Diffraction Field Emission Gun Secondary Electrons Backscattered Electrons X-ray Diffraction Electron Probe Micro-Analyser Wavelength Dispersive Spectroscopy Saturated Calomel Electrode Open Circuit Potential Gibbs Free-Energy Electromotive Force Faraday s Constant Electromotive Force of the Electrochemical Cell Electrode Potential of Reference Electrode Electrode-Potential-of-the-Test-Electrode 25

26 NOMENCLATURE C dl R ct R s C coat R pore CPE R a E 1 E 0 C p K h G R WDS S dr ICP OES Polarisation Double-Layer Capacitance Charge-Transfer Resistance Electrolyte Resistance Capacitance of coating Pore Resistance Constant Phase Element Roughness Energy level for upper state Energy level for lower state Heat Capacity Thermal Conductivity Thermal Diffusivity Temperature Gradient Growth Rate Wavelength Dispersive X-ray Surface Area Ratio Inductivity Coupled Plasma Optical Emission Spectrometer 26

27 LIST OF PUBLICATIONS RELATED TO THIS THESIS Journal papers Rakhes, M. E. Koroleva, and Z. Liu, Improvement of corrosion performance of HVOF MMC coatings by laser surface treatment, Surface Engineering, (10): p Oral/ Poster papers Rakhes, M. and Z. Liu, Laser surface modification of HVOF MMC coatings for improvement of tribological performance, NASM 3, April 2010, Manchester, UK, (Oral Presentation). Rakhes, M. E. Koroleva, and Z. Liu, Improvement of corrosion performance of HVOF MMC coatings by laser surface treatment, 36 th International Matador conference, July 2010, Manchester, UK, (Oral Presentation). Rakhes, M. and Z. Liu, Laser surface modification of HVOF coatings for improvement of corrosion and wear performance, NACE Corrosion 2010, March 2010, San Antonio, Texas, USA, (Poster Presentation). Local Seminars/ Posters Rakhes, M. and Z. Liu, Laser surface modification of HVOF coatings for improvement of corrosion resistance, PG Conference, School of Materials, The University of Manchester, Manchester, UK, May 2008, (Oral Presentation). Rakhes, M. and Z. Liu, Laser surface modification of HVOF coatings for improvement of corrosion and wear performance, PG Conference, School of Materials, The University of Manchester, Manchester, UK, May 2009, (Poster Presentation). 27

28 LIST OF PUBLICATIONS RELATED TO THIS THESIS Rakhes, M. and Z. Liu, Laser surface modification of HVOF coatings for improvement of corrosion resistance, PG Conference, School of Materials, The University of Manchester, Manchester, UK, May 2010, (Oral Presentation). Awards First Prize in Corrosion for best poster at Postgraduate Student Conference 2009, School of Materials, The University of Manchester. 28

29 Chapter 1 Introduction This chapter presents the motivation and rationale of this research, and defines the overall aim of the work. The structure of the thesis is also provided. 1.1 Research motivation and rationale Thermal spray processes, especially high velocity oxygen fuel (HVOF), are widely used coating techniques in many gas and oil industrial applications to protect materials from various degradation processes such as wear, erosion, high temperature and corrosive atmosphere [1-3]. Metal matrix composite (MMC) coatings, in which a mixture of hard particles and metallic binder materials are normally applied on metal substrates by HVOF spraying, provide advantages to the surface mechanical properties especially wear resistance. A typical example of such coatings is tungsten carbide (WC) particles with metallic binders of Co, Ni or Co Cr which has been applied extensively in heavy-loaded conditions, due to its excellent wear resistance [4-7]. However, when applied in corrosive environments, such coatings must exhibit an acceptable resistance against corrosion. Corrosion resistance of thermal spray coatings depends on many factors. Among them, the corrosion behaviour of metallic binders, especially the content of chromium [5, 8], is a determining factor, while the microstructure of the thermal spray coatings [9] also plays an important role. Due to the defects of thermally sprayed coatings including pores (isolated or interconnected), micro-cracks, gaps and oxide inclusions between splats and the bond strength at the coating/substrate interface by mechanical interlocking mechanism, these coatings might be insufficient to protect the surface in severe service conditions containing aggressive chemicals. In addition, inhomogeneous structures of the HVOF coatings limit their effectiveness to be used as physical barriers to inhibit corrosive species from getting into contact with the substrate to be protected. As a result, improvement against corrosion by 29

30 CHAPTER 1 INTRODUCTION such coatings is limited in most cases. In order to further improve the corrosion resistance of the coating properties, laser surface modification could be considered as a potential technique to eliminate or reduce the defects of the coatings. Laser surface modification via re-melting, or heat treatment, offers many advantages including precisely controlled treatment dimensions, particularly in depth, and minimum heat-affected zones, resulting in no thermal effects on substrate materials. Moreover, the laser processes are non-contact, suitable for components with complicated geometry, and easy to automate. Laser surface melting of HVOF coatings to improve corrosion resistance and/or wear resistance has been investigated by a number of researchers. Tuominen et al. [10, 11] reported the improvement of corrosion properties of Inconel 625 and nickel-chromium HVOF coatings on mild steel substrate using Nd:YAG laser surface melting. Liu et al. [12], at the University of Manchester, studied the effect of High Power Diode Laser (HPDL) surface melting on the corrosion and wear performance for Inconel 625 and Inconel 625 based WC HVOF coatings, showing that the laser treatments improved both corrosion resistance and wear resistance. Some other researchers intended to improve hardness and wear resistance of HVOF coatings using lasers. Chen et al. [13] attempted to improve wear resistance of WC-12Co HVOF coatings using CO 2 laser surface melting, showing that the wear resistance was reduced as a result of decreasing WC particle size by laser treatment. From the previous work, it is clear that laser surface melting of HVOF coatings reduced/eliminated various defects of the coatings and subsequently improved the properties of the coatings, particularly in corrosion resistance. However, one of the main purposes of applying such coatings is for protection against wear. If laser surface treatment is only capable of improving corrosion properties, but deteriorating the wear performance, it would be less beneficial to apply such laser techniques. Therefore it is essential to ensure that laser surface treatment is able to improve corrosion resistance at least without sacrificing wear performance. It would be an extra bonus if laser surface treatment could improve both corrosion and wear resistance. From the literature, it was also realised that different degrees of laser surface melting, especially for metal matrix composite coatings, in terms of fully/partially melting of the ceramic particles and formation of new phases, had significant impacts on the resultant properties. Therefore, it was believed that proper controlling of laser operating 30

31 CHAPTER 1 INTRODUCTION conditions to achieve different degrees of melting of HVOF coatings, would be a major issue to achieve optimised improvement of properties. In this project, a HPDL has been applied for surface modification of Tribaloy (T800) and T800-based WC metal matrix composite HVOF coatings, to gain detailed knowledge of how laser surface modification affects corrosion and wear performance of those coatings. 1.2 Aims and objectives of the research This work aims to improve corrosion resistance of the following HVOF coatings on 316L stainless steel substrate by laser surface melting using HPDL, with possibility of improving wear performance: Tribaloy 800 (T800) T WC (wt.%) T WC (wt.%) T WC (wt.%) The investigations include microstructural analysis, corrosion behaviour, wear resistance and hardness testing of various coatings before and after laser surface treatments. The specific objectives of this project are: 1) To establish laser operating windows for various HVOF coatings to achieve i) defect-free layers, i.e. elimination of porosity, microcracks and splats microstructure. ii) control of the melt pool dimensions, in particularly melt depth; iii) control of the degree of full/partial melting of the WC particles. 2) To investigate the microstructural changes of various HVOF coatings before and after laser treatments, in terms of surface morphology, phase transformation and homogenisation of WC particles; 31

32 CHAPTER 1 INTRODUCTION 3) To study the corrosion behaviour of various HVOF coatings before and after laser surface treatments in different laser operating conditions, and to understand the corrosion mechanisms of various coatings; 5) To investigate the wear performance and hardness of various HVOF coatings before and after laser surface treatments in different laser operating conditions, and to understand the wear mechanisms of various coatings; 6) To establish the best laser operating conditions for various coatings with respect to wear performance and corrosion properties. 1.3 Thesis outline This thesis comprises eight chapters and two appendices, of which this first chapter is an introduction. Chapter 2 covers the fundamental aspects of corrosion and various corrosion techniques such as polarisation and electrochemical impedance spectroscopy (EIS). Also, the principles of laser theory, laser beam interaction with materials and various laser treatment techniques are introduced. In addition, wear performance and hardness are discussed. It also presents a review of the HVOF thermal spray technique. This chapter also reviews the state-of-art literature on the subjects related to corrosion and wear performance of WC-Co HVOF sprayed coatings, along with laser surface treatment of HVOF coatings for improvement of corrosion and wear properties. Chapter 3 gives details of the experimental equipment and procedures used in this thesis. It includes the materials used for HVOF coatings, laser processing setup, and preparation of the samples for laser treatment and for different characterisation techniques adopted in this work before and after laser treatment. Chapter 4 presents the optimisation of laser operating conditions by studying the influence of laser operating conditions on melting of various HVOF coatings, and establishment of laser operation windows to produce the melted layers of the HVOF coatings being crack-free and porosity-free. 32

33 CHAPTER 1 INTRODUCTION Chapter 5 presents the results and discussion on materials characterisation of powders, then HVOF coatings before and after laser treatment, in terms of surface morphology, phase transformation, elemental and WC particle distribution within the laser-treated layers. Chapter 6 presents the results obtained from the different corrosion techniques used to evaluate corrosion performance of the different HVOF coatings before and after laser treatment, and discussion on corrosion mechanisms involved. Chapter 7 presents results obtained from the wear and hardness tests of HVOF coatings before and after laser treatment, and discussion on possible wear mechanisms involved. Chapter 8 is the last chapter of this thesis, which presents general conclusions reached from this work. It also makes recommendations for future work. Appendix A presents Nyquist plots with tables of electrochemical parameters obtained from EIS test for various HVOF coatings before and after laser treatment at 1, 6, 24, 48 hours of immersion in 0.5M H 2 SO 4 electrolyte. Appendix B presents Bode plots obtained from EIS test for various HVOF coatings before and after laser treatment at 1, 3, 6, 12, 24, 48 hours of immersion time. 33

34 Chapter 2 Literature Review 2.1 Introduction This literature review explores various subjects related to this work. The main contributions of the literature include discussion of the fundamental mechanism of corrosion and various corrosion techniques such as polarisation and electrochemical impedance spectroscopy (EIS). Definition of wear, types of wear, wear mechanisms and wear tests are introduced. It also presents a review of thermal spray coating techniques and focuses on HVOF. Laser theory, types of lasers and laser beam interaction with materials have been discussed. Previous work on corrosion and wear performance of WC-Co HVOF sprayed coatings has been reviewed. Also previous work of laser surface treatment of HVOF coatings for improvement of corrosion and wear properties has been presented. 2.2 Corrosion of Metals Corrosion is defined as The destructive result of chemical reaction between a metal or a metal alloy and its environment [14]. Most metals after being reduced from their ores are thermodynamically unstable, and tend to change their state to more stable state. Furthermore, almost all metals are found as a combination with other elements such as sulphur or oxygen. In order to change it to pure metal, it is essential to provide a large amount of energy to the system. This energy will be stored in the pure metal. However, from the laws of thermodynamics, a higher energy state of material tends to transform into a lower energy state. This energy which is stored in the metallic system must be restored by turning back to its oxidized state by recombination with the environment. In general, the more energy absorbed by the metal, the more readily the metal corrodes [15]. 34

35 CHAPTER 2 LITERATURE REVIEW Fundamentals of Electrochemical Corrosion Corrosion process of a metal is electrochemical reactions. A corrosion cell (figure 2.1) is essential for corrosion to take place, containing anode, cathode, electrolyte and metallic path. Almost all corrosion processes concern transfer of charges in aqueous solutions (electrolyte). The mechanism of corrosion is based on anodic and cathodic reactions in an electrolyte. The anodic reaction (oxidation) releases electrons, while the cathodic reaction (reduction) consumes electrons. Moreover, there are three general cathodic reactions, namely: oxygen reduction, hydrogen evolution from water, and hydrogen evolution from acid [16]. Current Flow by Ionic Conduction Electrolyte Anode Cathode Current Flow by Electron Conduction Figure 2.1 Current flow in corrosion cell [17] Anode: Anode is an electrode in an electrolyte cell, at which oxidation occurs producing electrons. Moreover, it is the zone where the metal dissolves into the solution. This reaction is called an anodic reaction. Metal ion formation in the solution may occur by one of a number of ways: Production of metal cations The sample M represents a metal atom having n valence electrons, and converted into an ion M n+. Oxide-formation 35

36 CHAPTER 2 LITERATURE REVIEW Hydroxide formation Formation of an insoluble salt Cathode The second element in the electrolyte cell is the cathode. Cathode reactions involve electron consuming and are essential in controlling the rate of corrosion which takes place at the anodes. The cathode reactions are important to neutralize the electrons which are produced by anodic reactions [18]. The reaction in the anode cannot be at a high rate unless the electrons produced can be consumed at the cathode. The consumption or neutralization of the electrons could occur by different reactions, as follows: In alkaline and neutral aerated solutions, In acid solutions with high concentration of hydrogen ions: In acid solutions with dissolved oxygen: Total reduction of metal ions to metallic state: 36

37 CHAPTER 2 LITERATURE REVIEW Electrolyte The electrolyte is the third important element in the corrosion cell. The electrolyte is the solution which covers the metal and acts as a medium to form the electric circuit. Moreover, the corrosion rate depends on the conductivity of the electrolyte; high conductivity produces rapid corrosion. The electrolyte contains ions so as to be conducting, for example, in the case of seawater, the corrosion is rapid, where the seawater has a high conductivity [18]. External Circuit The anode and cathode are present on the metal surface. Furthermore, the metal itself here acts as the external circuit. In other words, the anode and cathode should be connected by an external circuit to complete the corrosion process, also conductivity for external circuit plays important role in the rate of corrosion. Corrosion in acid environment In acidic environments, even without oxygen, the metal can be attacked aggressively at the anode, while hydrogen ions in the electrolyte become hydrogen gas at the cathode. Furthermore, corrosion by acids can produce salt which slows down the reaction, because it is formed on the surface being attacked. One of the examples of these reactions is when iron is placed in dilute sulphuric acid. The iron can be attacked and hydrogen gas is produced as shown in equation This reaction can be considered as the sum of two different reactions occurring at different sites on the metal surfaces: Anodic reaction Cathodic reaction 37

38 CHAPTER 2 LITERATURE REVIEW The electrode potential When metal M is in an electrolyte which contains ions of the metal M n+, the equilibrium stage will reached after electrode reactions take place at the surface of metal [19]: These reactions will create an electrical double layer (EDL) as shown in figure 2.2. This will result in a potential called Galvani potential E 1 which is different from solution potential E 2. EDL is associated with a polarized electrode, and the charge cannot transfer without imposing an external potential voltage source. M n+ M E + 1 E 2 _ + _ Figure 2.2 Metal in an aqueous solution containing metal ions; an electrical double layer has formed [19]. The difference E=E 1 E 2 in Galvani potential cannot be measured directly. However, this value can be measured by using a relative value, where a reference electrode such as saturated calomel electrode (SCE) is connected via an electrolyte to the test electrode, for which the electrode potential is to be determined (figure 2.3). The circuit contains a half cell, but with a potentiometer, the electromotive force ( E) of the electrochemical cell is produced and measured. These measurements are taken, where the flow of current should be as low as possible: Where, E is electromotive force of the electrochemical cell measured by a potentiometer. Furthermore, when this potential difference in the absence of an external potential source, it is called open circuit potential (OCP). E ref is the 38

39 CHAPTER 2 LITERATURE REVIEW electrode potential of the reference electrode (constant) and E test is the absolute value of the electrode potential of the test electrode [19]. Potentiometer Reference electrode _ - _ - _ - - _ Test electrode Electrolyte Figure 2.3 Determination of electrode potential with the aid of a reference electrode [19] Corrosion thermodynamics From the thermodynamic point of view, the tendency of a metal to corrode can be predicted. However, thermodynamics provides no information about the details of the reaction, such as indication of corrosion rate. The rate of corrosion can be indicated by kinetic theory. This tendency of the reaction can be measured by the Gibbs free-energy change, G. For a spontaneous reaction to take place, G must be negative, and the more negative the magnitude of G, the higher the tendency will be for reaction to occur [20]. The tendency for corrosion can be presented in terms of the electromotive force (EMF) of the corrosion cells by the following relation: F = Faraday s constant coulombs mole -1, Faraday s constant represents the charge transported by one mole of electrons. E is the electromotive force (EMF). The number n is the number of electrons transferred in the corrosion reaction [15]. 39

40 CHAPTER 2 LITERATURE REVIEW Kinetics of corrosion under polarisation As discussed in Section 2.2.3, the thermodynamics does not predict the corrosion rate. In order to understand how fast a corrosion reaction is, it is important to have a clear understanding of the corrosion kinetics that occur on an electrode surface in contact with an electrolyte. When a metal is exposed to an aqueous solution, two complementary reaction processes will take place on the surface simultaneously, i.e. an oxidation of the metal atoms to metal ions by losing electrons, and a reduction of metal ions by accepting electrons. These reactions at equilibrium can be explained as follows, The free energy of activation ( E) for the anodic and cathodic reactions in this case has the same level [16]. This equilibrium is dynamic, where the two reactions occur at equal rates in opposite directions. Any changes in the conditions such as temperature, pressure, and concentrations of the participants may move the reaction from its equilibrium state. The reaction rate in both directions (anodic and cathodic) can be expressed by current or current densities, for cathodic reaction and for anodic reaction, both can exist because of an exchange of electrons between the metal and the electrolyte. At equilibrium,, where called exchange current density, and it depends strongly on many parameters such as electrode material, electrolyte, temperature and ion concentrations in the electrolyte [21]. Furthermore, if there is current applying to the surface that will make, so that the applied cathodic current density is: The applied anodic current density is: When the current is applied to the surface, the potential electrode will change to, the change in electrode potential is called Polarisation. 40

41 CHAPTER 2 LITERATURE REVIEW As figure 2.4 shows, the polarisation of a cathode is negative, on the other hand the polarisation of an anode is positive [19]. Figure 2.4 Anodic and cathodic polarisation curves [19]. The polarisation can be classified into three groups: Concentration polarisation This type of polarisation is caused as result of the difference in concentration in an electrolyte near the electrode surface. This means either that there is deficiency of reactants near the electrode surface, or that an increase of reaction products takes place. For example [21], if the concentration of hydrogen ions in a solution is relativity low, the neutralization of hydrogen ions by the electrons will depend on the rate of hydrogen ions diffusing through the solution, and the number of hydrogen ions available. In this case, the corrosion will be controlled by the diffusion of hydrogen ions in the solution. However, in high concentrations of hydrogen ions, as in acid solutions, the electrochemical reactions go rapidly. Resistance (Ohmic) polarisation Metal surfaces might have high ohmic resistance. This is, for example, due to oxide films on aluminium, stainless steels etc. The resistance will decrease when the 41

42 CHAPTER 2 LITERATURE REVIEW current flows through the film. If the resistance of the film is R (Ω), the resistance polarisation can be presented by η = RI. Moreover, anodic resistance polarisation has an influence on surfaces which are passivated by oxides or other materials [21]. Activation polarisation This polarisation takes place by the resistance opposed to the reaction itself at the metal surface. For example, the rate of gradual development of hydrogen at the cathode is dependent on the rate of the electron transfer from the anode. Therefore, the rate of evolution hydrogen will change at the cathode as the anode varies in its ability to give up electrons. In general, the controlling factor in corrosion with high concentration of hydrogen ions as in strong acids is activation polarisation. However, at low concentration of hydrogen ions as in aerated water or salt solution, the controlling factor in corrosion is concentration polarisation [18]. The relationship between activation polarisation η, that can be definite as overpotential (because it is described potential changes from equilibrium corrosion potential E corr), and the rate of reaction, can be represented by current density i a or i c. For anodic polarisation: For cathodic polarisation: Where and are the anodic and cathodic Tafel constants. is positive so must be positive too, and is negative because is negative [14]. Equation (2.20) and (2.21) might be re-written in exponential form as follows, 42

43 CHAPTER 2 LITERATURE REVIEW By substitution into equation (2.17), we obtain: By rearrangement of equation (2.24) and differentiation, an expression of the polarisation resistance is Thus, the slope at the origin of the polarisation curve is inversely proportional to the corrosion rate [14], where: B is the proportionality constant Types of corrosion There are several types of corrosion, although all corrosion involves electrochemical reactions. Each type of corrosion has a specific features occurring in specific locations. Corrosion can be generally divided into two types, uniform corrosion and non-uniform (localised) corrosion Uniform corrosion Uniform or general, corrosion is a type of corrosion attack uniformly distributed over a metal surface [22]. In uniform corrosion, multiple anodes and cathodes are working on the metal surface at any time, and their areas continually move on the surface, causing uniform corrosion [23]. Uniform corrosion normally occurs when there is no high macroscopic concentration differences across the metal surface, the metal is nearly homogeneous, and the material has not significant passivation tendency in the particular environment [24]. Uniform corrosion can be prevented by using coatings or inhibitors, or proper material selection. It is usually treated by establishing a corrosion allowance into the structure of the substrate. However, this 43

44 CHAPTER 2 LITERATURE REVIEW type of corrosion is not very dangerous from a technical standpoint because the life time of equipment can be estimated by simple immersion tests which allow calculation of the weight (mass) loss, and the dissolving of the thickness as a function of time [23] Localised corrosion Localised corrosion is a corrosion phenomenon concentrated at some specific sites of a metal surface, which are very small relative to the surface of the metal that does not corrode [22]. In general, localised corrosion can be more severe than uniform corrosion because it can take place after a short time period of use or exposure without warning [25]. Some types of localised corrosion are described below. Galvanic corrosion Galvanic (dissimilar metals) corrosion occurs when two metals with different electrode potential are coupled together in a corrosive electrode environment. In general any metal or alloy has a unique corrosion potential E corr. When dissimilar metals are connected together in an electrolyte, a flow of electrons is produced between the two metals as a result of the potential difference. One of the dissimilar materials becomes the anode and corrodes faster than it would by itself; the other material becomes the cathode and corrodes slower than it would alone [26]. Galvanic corrosion can take place at either macroscopic or microscopic levels. Furthermore, different phases and microstructure at the microscopic level can be subjected to galvanic corrosion. The corrosion in anodic materials will be more rapid and more damaging when the potential difference between the two materials in the galvanic series increases and when the cathode area increases relative to the anodic area [19]. To control galvanic corrosion, metals closer to each other in the galvanic series should be used, or by isolating metals from each other. Crevice corrosion Crevice corrosion is a form of localised corrosion which takes place at narrow openings gaps between adjoining surfaces (figure 2.5). This type of corrosion 44

45 CHAPTER 2 LITERATURE REVIEW results from a concentration cell created between the solution within the crevice, that is oxygen starved, and the solution outside the crevice, where the oxygen concentration is high. Moreover, the material inside the crevice (less oxygen) acts as an anode, and the exterior material (high oxygen) becomes a cathode [23]. Figure 2.5 Crevice corrosion [27] Pitting corrosion Pitting corrosion is a form of localised dissolution of a metal resulting from the breakdown of the protective film on the metal surface (figure 2.6). Furthermore, small areas corrode leading to the creation of cavities or pits, and the bulk of the surface remains unattacked. The driving force for pitting corrosion is the reduction of oxygen around the small area (at the bottom of the pit). This area becomes an anode while the area (at the top of the pit) with excess of oxygen will become a cathode leading to localised galvanic corrosion. As a result of the pitting corrosion mechanism, the ph value at the bottom of the pit becomes extremely low, with increasing chloride concentration with time, the pit depth increases. [16]. Figure 2.6 Pitting corrosion [28] For this type of corrosion, it is difficult to predict the location and the degree of pitting corrosion attack. Also, pits are generally small and difficult to detect by 45

46 CHAPTER 2 LITERATURE REVIEW simple visual examination [23]. Furthermore, the shape of the pits depends on metallurgy of the surface alloy and chemistry of the environment, pits might be deep, subsurface, undercut, shallow, or elliptical [29]. Stress corrosion crack This type of corrosion results in cracking which is induced from the combined influence of tensile stress and a corrosive environment, especially in chloride-rich environment like sea water and at high temperature. There are two kinds of crack propagation: cracks that propagate across grains which are called Transgranular and those which follow grain boundaries are called Intergranular (figure 2.7). Figure 2.7 (a) Transgranular cracks in an austenitic steel produced in a chloride environment (100x); (b) Intergranular cracks in a ferritic stainless steel produced in a high temperature caustic environment (50x) [30] Polarisation curve Polarisation curves represent the relationship between the logarithm of current density and potential, which illustrates active and passive behaviour of a metal (figure 2.8). The polarisation curve has three main regions which are: active, passive, and transpassive. From figure 2.8, OD represents anodic behaviour, and AO represents the cathodic behaviour. The two dashed lines AB and CD represent Tafel lines. Furthermore, the anodic and cathodic polarisation curves intersect in the active region at point O, which is called corrosion potential E corr, the net current at this point is zero. The corrosion rate increases at potentials higher than the potential at O, and reaches a maximum at the primary passive potential E pp. Moreover, at potentials higher than E pp the passive film stays stable, and the growth 46

47 CHAPTER 2 LITERATURE REVIEW of the protective film causes the sudden fall in corrosion current density from i crit to i pass. The region from E to F called the passive region, this region shows the full passivity on the metal surface as a result of film formation [16]. At point F, the passive film breaks down, so pits grow and propagate. The potential at F is called the pitting potential E p. The region from F to G denotes transpassive corrosion. Potential (V) E p (+) Passive film breaks down I pass F G Passive E pp E corr (-) B O C E D I crit Active-passive transition Anodic reaction Cathodic reaction A Active I corr Log Current Density (ma/cm 2 ) Figure 2.8 Schematic diagram of the polarisation behaviour [16] Electrochemical impedance spectroscopy (EIS) The Electrochemical Impedance Spectroscopy (EIS) technique based on alternating current (AC), and can be used to understand corrosion mechanisms, and to evaluate the efficiency of corrosion control methods, for instance, coatings and inhibiters. Furthermore, EIS is used extensively as a tool for investigating complex and difficult systems [31]. The advantages of EIS over DC methods are: (1) it can measure solution resistance easily, so it can provide more precise information about polarisation resistance for high solution resistance. (2) having the complete information over the whole frequency range, can provide useful information about the corrosion behaviour. (3) during EIS studies, a very small amplitude AC signal is applied which causes very little perturbation to the system. (4) the measurements 47

48 CHAPTER 2 LITERATURE REVIEW of EIS can provide useful information on electrode capacitance and charge transfer resistance. In addition, EIS can measure changes in the porosity, and simultaneously monitor the corrosion rate of the coatings [32, 33] Principles of EIS The principle of the impedance method is to apply a small amplitude AC sinusoidal excitation signal to the system, and then measure the response. Generally, a small voltage signal is applied and the resulting current is measured. For direct current (DC), Ohm s law represents the relationship between voltage and current. Where R is resistance in ohms, V is voltage in volts, and I is current in amperes. Unfortunately, this relationship is limited to the ideal resistor. The ideal resistor has many simplifications such as: its resistance follows Ohm s law, the resistance value is independence of frequency, and the AC and voltage signals through a resistor are in phase with each other. In the real world, many systems show behaviour that is more complex. In this case, impedance is used in place of resistance, which is a measure of the ability of the circuit to resist (or impede) the alternating current. The relationship which is equivalent to Ohm s law is: Where, Z is the impedance of the circuit in ohm s (Ω). The impedance Z is almost always depends on frequency (f). For an AC signal, the frequency is measured in cycles per second (Hz) [31]. Equation (2.28), represents the impedance of the system at a certain frequency which can be defined by another two terms, related to the current and voltage. These terms are the amplitude (A) of the signal (AC), and the phase difference (ϕ) between the current and the voltage (see figure 2.9). The following figure illustrates a standard sine wave voltage (V) applied to an electrochemical cell and the current 48

49 CHAPTER 2 LITERATURE REVIEW response (I) is also sine wave. However, the two sine waves are not in phase. Moreover, the two sine waves will only have same phase and different amplitude in a purely resistive network [34]. V ϕ I o Time I Figure 2.9 AC wave forms for an applied potential and a resulting current The current sine wave can be represented by the equation: Where is instantaneous current, is the maximum amplitude, is the angular frequency in radians per second, and ( where, is frequency in Hertz. t is time in seconds and is the phase shift in radians. The voltage sine wave can also be expressed as a function of time, and has the form: is the voltage at time t, The impedance of the system which analogous to Ohm s law: is the amplitude. can be calculated by the following expression Where the ratio of the size of the voltage sine wave to that of the current sine wave ( is the magnitude or the size of the impedance in Ohms of this system. The 49

50 CHAPTER 2 LITERATURE REVIEW magnitude value is usually written as:. As can seen from equation (2.31), the impedance can be expressed in terms of a magnitude,, and a phase shift. From Euler relationship Where is a complex number ( [35]. It is possible to represent the impedance as a complex function. The current can be described as, and the voltage can be expressed as in the following equation, The total impedance of the system can represented as a complex number, The impedance of the system can be written in vector form as a complex number: and (2.37) Where, and are the real and imaginary parts for the impedance respectively. Equation (2.36) may be plotted in polar coordinates as shown in the following graph (figure 2.10): 50

51 CHAPTER 2 LITERATURE REVIEW o θ Figure 2.10 Complex plane showing impedance vector Z [36]. Data Presentation There are two common methods to present the data, which are, Nyquist and Bode plots. In a Nyquist plot which is used in this research, it is common to plot on the Y axis against on X axis of the chart (figure 2.11). In this plot the Y-axis is negative and each point on the plot represent the impedance at one frequency. As shown in figure 2.11, the low frequency data are on the right side of the plot and higher frequencies are on the left. Furthermore, the impedance can be represented as a vector with magnitude. The impedance usually reduces with increasing frequency. However, this behaviour is not true for all circuits. The phase angle: and the magnitude of Z: Figure 2.11 Nyquist plot with impedance vector The Nyquist plot has one limitation, which is that the frequency does not appear in the plot. The other popular presentation method is the Bode plot. The Bode plot for 51

52 CHAPTER 2 LITERATURE REVIEW a system consists of two graphs: one showing the magnitude of the impedance as a function of frequency; the other showing the phase angle as a function of frequency. The scales used for the axes can vary but the most popular format is to plot and phase angle against log frequency, as shown in figure Figure 2.12 Bode and phase angle plots [32]. Equivalent circuit elements Very often it is difficult to know everything about the process and the mechanism in the electrochemical study, because of the complexity of the process where several processes could contribute to the system s EIS spectra. These factors include: Electrode double layer capacitance, electrode kinetics, diffusion layer and solution resistance. However, by using EIS and fitting the practical data to equivalent circuit models, it is possible to understand and get a clear picture of the corrosion behaviour and other useful information, such as solution resistance and porosity in the coating. The equivalent circuit should be as simple as possible, and it should provide the best possible match between the measured (Real) impedance and the model s impedance. Usually, the system depends on more than one cell element. The common method used to investigate the EIS spectra is predicting the process and the mechanism, and describing it by electrical components (resistors, capacitors and inductors). Building these components into logical series and parallel combinations is very important to simulate the real electrochemical process. 52

53 CHAPTER 2 LITERATURE REVIEW The impedance of the system will depend on the value of the impedance of the individual elements. The following table shows the equivalent circuit elements with their impedance equation: Table 2.1 Equivalent circuit elements [34]. Circuit Element Impedance Equation Resistor Capacitor Inductor Electrochemical systems such as coated surfaces or corroding metals can usually be described as simple electronic circuits. The Randles cell The Randles cell is the most common and simplest model of an electrochemical interface which can be used in bare metals [16]. Figure 2.13 shows the equivalent circuit and Nyquist plot for a Randle cell. In this circuit R s denotes the electrolyte resistance. C dl is the double-layer capacitance which is modelled in this circuit as a pure capacitance. R ct is the charge-transfer resistance. 53

54 CHAPTER 2 LITERATURE REVIEW Figure 2.13 Randle equivalent circuit (a) and Nyquist plot (b) As seen from figure 2.13 the Nyquist plot of a Randle cell is a semicircle. At high frequencies the impedance of the double-layer capacitance (C dl ) will become very low, so the evaluated impedance will be close to R s. Moreover, at very low frequencies the impedance of C dl becomes very high, so the measured impedance in this situation will be close to R ct + R s. The charge-transfer resistance is equal to the diameter of the semicircle. The Randle equivalent circuit is useful as a starting model to build up more complicated models for more complicated systems [33]. EIS of a coated substrate To understand the use of equivalent circuits in real electrochemical system, EIS of a coating system cell will be discussed. This equivalent circuit contains a combination of two capacitors and three resistors as shown in figure Figure 2.14 The Equivalent circuit for a coated substrate [37]. EIS can quantitatively determine resistances and capacitances in the electrochemical system and through these values, many properties of the coating 54

55 CHAPTER 2 LITERATURE REVIEW can be evaluated. By this technique, changes in a coating behaviour during exposure to an electrolyte can be detected [38]. The equivalent circuit shown in figure 2.14 is the one most commonly used for coating on metal substrates [37, 39, 40]. As the coating degrades with time during the exposure to an electrolyte, EIS can track changes which could occur in the capacitance of the coating (C coat ), and also evaluates changes in the porosity of the coating through the pore resistance (R pore ). Furthermore, EIS simultaneously monitors the rate of the corrosion by tracking the charge transfer resistance (R ct ) value [37, 41]. Electrolyte resistance (R s ) The electrolyte resistance (R s ) is the resistance between the working electrode surface and the reference electrode. The values of R s usually are low (1-50Ω) and can be ignored because the electrolyte is very conductive. Coating capacitance (C coat ) The coating capacitance usually has a low value because most coatings are relatively thick. It is caused by dielectric properties of the coating, and physical and chemical properties of the coating that affect the capacitance. It can be described by the following relation: Where is the permittivity of vacuum ( ), is the dielectric constant of the coating, is the surface area of one plate and is the distance between two plates (coating thickness) [42, 43]. Pore resistance (Rp) Pore resistance is also known as coating resistance (R coat ). It is the resistance of the coating which changes during the exposure as a result of penetration of electrolyte into micro-pores of the coating. Moreover, it is usually inversely proportional to the exposure time and to the porosity of the coating. The higher the value of (R p ), the 55

56 CHAPTER 2 LITERATURE REVIEW lower the value of porosity and the higher the protective ability of the coating. However, R p usually increases after a long time of exposure as a result of corrosion products blocking the pores in the coating [38, 44]. Double layer capacitance (C dl ) An electrochemical double layer occurs at the interface between an electrode and the surrounding electrolyte. The double layer is created as a result of attraction between the negative ions (anions) and positive ions (cations), and repulsion between similar charged ions (see Section 2.2.2). When an equilibrium between the electrolyte and the metal is established, charges in the electrode are separated from the charges of the ions in the electrolyte. Furthermore, this separation is very small, and will create high capacitance. The electric field of double layer capacitance prevents easy charge transfer, which limits the electrochemical interactions at the surface. Many parameters could affect on the value of double layer capacitance, for instance: electrode potential, types of ions, oxide layers, temperature, electrode roughness, ionic concentrations and impurity adsorption [16, 29]. Charge transfer resistance (Rct) Charge transfer resistance (R ct ), also called polarisation (R p ), describes the rate of the corrosion reaction and is inversely proportional to the corrosion rate; this parameter can be represented by a Nyquist plot (figure 2.10b). Constant phase element (CPE) Constant phase elements have been used widely in equivalent circuits to fit the experiment data. Many research groups have shown that using CPE instead of pure capacitance produce better fitting [45]. In some systems the Nyquist plot has been expected to be an ideal semi-circle with the centre on the X-axis. However, in some real impedance experiments, the plot was not an ideal semi-circle, but an arc with the centre located somewhere below the X-axis. This depressed arcs behaviour has been investigated in terms of the nature of the system. In other words, the system will not follow the ideal capacitance behaviour if the system is not 56

57 CHAPTER 2 LITERATURE REVIEW homogeneous or has dispersion of the values of physical properties, such as roughness, electrode porosity and different current and potential distributions associated with electrode geometry. As shown in table 2.1, the impedance of a capacitor is: where is the inverse of the capacitance. is an exponent which equals 1 for a capacitor. For CPE, the exponent constant [42, 46, 47]. is less than one, and can be treated as an empirical 2.3 Wear Wear is gradual loss or deterioration of material surface caused by relative movement with respect to another substance [48]. In order to reduce wear, it is important to understand the mechanism of wear. Moreover, there are several types of wear with very complex mechanisms which do not lend themselves to precise theoretical treatment without serious simplifications. Furthermore, wear is system property dependent. For instance, it is dependent on material properties, environmental conditions and the dynamics of relative movement, and also, chemical reaction and/or phase transformations that may also be involved [49] Classifications and Types of Wear There are three ways of classifying wear. In the first case, wear can be classified in terms of the appearance of the wear track, such as, scratched, pitted, or scuffed. In the second case, wear is classified in terms of the physical mechanism which removes the material or produces damage to the surface, such as, adhesion, abrasion, and oxidative. The third classification of wear is in terms of conditions surrounding the wear location, such as, lubricated wear, unlubricated wear, and 57

58 CHAPTER 2 LITERATURE REVIEW high temperature metallic wear [48]. Descriptions of wear types by thephysical mechanism classification are presented in the following sections Abrasion wear This is the material removal and cutting action due to hard particles or hard protuberances forced against and moving along its solid surface. Furthermore, it takes place in mechanical operations such as grinding, cutting, and polishing [50]. Two-body abrasive wear takes place when one hard surface plastically cuts material away from other softer surface (figure 2.15). In most cases, this mechanism changes to three body abrasive wear, which occurs when an abrasive particles such as debris becomes entrained in the operation and where it is sufficiently hard, and it is acts between two solid bodies that are in relative motion [48]. Figure 2.15 Abrasion wear [51] Adhesion wear Adhesion wear is material removal and surface damage which can take place when two smooth surfaces slide against each other (figure 2.16). There is no perfectly smooth surface. The surfaces have some high spots where contact between the surfaces occurs. Because of high local pressure between these contact points, plastic deformation, surface damage and material removal could take place. Furthermore, the distance between the surfaces becomes of the same order as the inter-atomic spacing in the material, and where the adhesive forces are higher than the cohesive forces, shearing of welded junctions can take place in the material of lower cohesion [51, 52]. 58

59 CHAPTER 2 LITERATURE REVIEW Figure 2.16 Adhesion wear [51] Fatigue wear Fatigue of a surface can also cause loss of material. When materials are subjected to severe contact forces and repeated stress cycling, it can lead to cracking. The linked cracks result in the formation of loose wear particles [49] (figure 2.17). Figure 2.17 Fatigue wear [53] Corrosion wear Corrosion wear takes place when sliding occurs in a corrosive environment. In this situation, the sliding movement can cause wear on the passivation film, which has been created by the corrosion processes, which will expose the fresh surface to the corrosive environment. Usually, there is a combination of the mechanical processes operating separately or together, which produce wear [49] Wear tests Wear tests can be classified into two categories, which are, phenomenological wear tests and operational wear tests Phenomenological wear tests Phenomenological wear tests have a tendency to focus on the general type of wear situation, such as, sliding wear and low-stress cutting abrasion. Moreover, these 59

60 CHAPTER 2 LITERATURE REVIEW tests provide general information, and present either first or second order simulation, providing useful information to understand and characterise basic wear behaviour of materials. Typical phenomenological wear tests are [48]: Dry sand-rubber wheel abrasion test Wet sand-rubber wheel abrasion test Slurry abrasivity Erosion by solid particle impingement using gas jets Vibratory cavitation erosion test Block-on-Ring wear test using wear volume Crossed-Cylinder wear test Pin-on-Disc wear test (this type of test has been used in this research, and will be introduced in detail in next section) Rolling wear test Impact wear test Scratch wear test Operational wear tests Operational wear tests focus on particular applications, and usually the name of the test indicates the specific application, such as a wear test for brake liners. In these tests, simulation of the test is concerned with the application more than the wear mechanisms. However, wear mechanisms can be evaluated from the results of the test. This category of tests presents useful information about understanding wear behaviour of certain applications. Typical operational wear tests are [48]: Cylindrical abrasive test Coin wear test Brake material wear tests Engine wear tests Drill wear tests Bearing tests 60

61 CHAPTER 2 LITERATURE REVIEW Pin-on-Disc wear test The Pin-on-Disc wear test has been used widely to investigate and evaluate wear rate. A stationary pin or ball (usually made of cemented carbide) is used against a rotating disc of the material to be tested (figure 2.18). This standard test can be used to measure friction and wear resistance of dry and lubricated surfaces of bulk materials and coatings. In most cases, users can choose the normal load, rotational speed of the disc, wear track diameter, and the duration of the test. The ASTM standard for this test is ASTM G99, which indicates the use of a rounded pin. However, this standard does not specify certain values for the parameters, but users can choose those parameters to provide simulation of an application. The ASTM standard allows researchers to measure the wear rate by geometrical or mass loss methods. In the case of the mass loss method, the volume loss can be evaluated by dividing the mass loss by the density. In a geometrical method, volume loss can be measured by converting a measured linear dimension, to a volume using suitable mathematical relations of the geometry of the wear track [48]. Load Specimen Rotating Disc Figure 2.18 Diagram of Pin-on-Disk configuration wear test [48]. 2.4 Thermal Spray Coatings The basic idea of thermal spray coating comes from Dr. Max Schoop, who invented thermal spraying in the early of 1900s. Due to the increasing demand for surfaces which can serve in aggressive environments, significant improvements of spray techniques were made between the 1920s and 1930s. The combustion powder and plasma powder processes were developed in the early 1960s, and since that time, thermal spray techniques have been developed very rapidly, developing to the 61

62 CHAPTER 2 LITERATURE REVIEW low pressure plasma spray and high velocity oxy fuel (HVOF) techniques. Nowadays, thermal spraying can be controlled by microprocessors to produce high quality coatings [3]. Thermal spray coatings have been used widely to improve corrosion resistance, conductivity, surface hardness, wear resistance, and repair surface damage of engineering components. These coatings are becoming important for a wide range of industrial applications such as aero engines, oil industry components, gas turbines, and automotive industry components [54]. Thermal spray coating technology uses different forms of materials as a coating such as: powders, wires, or rods. These stock materials are heated by different heating sources, depending on the used technology, like flames or plasma, in which molten or softened particles are accelerated before impacting on the substrate surface. Moreover, repeating the transition of the spray gun across the substrate will build up the coating layer, which has thickness from ten microns to hundreds of microns. Furthermore, the thermal spray technique is an effective, economical method for improving the surface of components facing aggressive environments. This technology can be divided into three main groups, which are: flame spray, electric arc spray, and plasma spray [55] Principles of Thermal Spray Coatings The thermal spray technique is a group of processes which start by feeding the coating material such as powder to the heating source, which is usually a thermal gun that generates the necessary heat by using combustible gases or an electric arc. When the coating material is heated, it changes to a molten or semi-molten state. These molten or semi-molten particles are rapidly accelerated by a compressed gas stream directed towards the substrate (figure 2.19). When the particles strike the prepared substrate, they will deform to thin platelets (splats) which adhere to irregularities of the prepared substrate and to each other. As the sprayed particles collide on the surface, they cool rapidly and construct splat over splat to form a laminar structure. Usually this structure has porosity, gaps, and oxide inclusions. The bond between the coating and the substrate is formed by mechanical interlocking. The properties of this type of coating will depend on the material used and the thermal spray parameters. 62

63 CHAPTER 2 LITERATURE REVIEW Figure 2.19 Basic principle of thermal spraying [55] Types of Thermal Spray Coatings Thermal spray processes can be classified into three main coating process groups: plasma arc, electric arc, and flame, along with another process which is referred to as cold spray. These processes have a number of subsets as can seen in figure Furthermore, each of these methods has its own unique characteristics as shown in table 2.2. Figure 2.20 Classification of thermal spray processes [56] 63

64 CHAPTER 2 LITERATURE REVIEW Table 2.2 Comparison of characteristics for various thermal spraying processes [3] Deposition Heat Propellant Material Spray Particle Coating Porosity technique source feed type gun velocity materials level temp. m / s vol. % ( C) Electric arc Plasma arc spraying Low pressure plasma spraying Spray & fuse Flame spraying Detonation gun spraying Highvelocity oxy-fuel (HVOF) Arc between electrodes Air Wire Ductile materials 8 15 Metallic, Plasma Inert gas Powder ceramic, 2-5 arc plastic, and compounds Metallic, Plasma Inert gas Powder ceramic, 5 arc plastic, and compounds Fusible - - Powder - - metals 0.5 Oxyacetyl ene/ Air Powder Metallic and oxyhydro ceramics gen Oxygen / acetylene/ Detonation Powder Metallic, nitrogen shock waves ceramic, gas plastic, and detonation compounds Oxypropy lene/ Combustion Powder/ Metallic and hydrogen/ jet wire ceramic propane/ LPG 64

65 CHAPTER 2 LITERATURE REVIEW High Velocity Oxy-Fuel Thermal Spraying This type of spraying differs from conventional flame spraying in that the combustion process is internal, and the gas pressure is much higher than that used in the atmospheric burning flame spraying processes. In this process the combustion fuel gas (hydrogen, propane, or propylene) and oxygen are fed to the spray gun together with the spray material. The combustion of gases will produce high temperature and high pressure in the spray gun (figure 2.21). The burning gas mixture will accelerated to supersonic speeds, and the spray powder is ejected to the hot gas stream. The spray powder is heated and accelerated by the hot gas stream and projected into the substrate, the bonding between the substrate and the coating is usually mechanical bonding (figure 2.22). Figure 2.21 Schematic diagram of the HVOF process [57]. Figure 2.22 Cross-section of T800 HVOF coating on stainless steel substrate Splat formation and structure of HVOF coating The splat is the base building block of the structure in HVOF coating. These splats are formed when a stream of molten and semi molten particles impacts on the substrate (figure 2.23). The molten droplets are in general spherical in shape before 65

66 CHAPTER 2 LITERATURE REVIEW impacting on the substrate surface, followed by flattening, rapid solidification and cooling processes [56]. Then continuous impacting of particles will increase the thickness of the coating. The produced microstructure of this thermal spray is a complex mixture of lamellas (figure 2.24). Figure 2.23 Schematic diagram of adhesion of a particle to a substrate asperity. (1) asperities, (2) particle in flight, (3) particle mechanically locked to the substrate, (4) substrate [55] Coating materials There are a large number of modern coating materials available and suitable for thermal spray technology. In fact, any material that has a well known melting point and which is not easily decomposed or evaporated when heated can be used in the thermal spray process [55]. These materials can be classified into three basic types [56]: Single-phase materials Metals: Most pure metals and metal alloys have been thermally sprayed, such as cobalt-based (Setllites and Tribaloys), nickel-based (Inconel 625), zinc, aluminium, tungsten, stainless steels, and NiCrBSi (self-fluxing) alloys. Ceramics: Most ceramics can be thermally sprayed, such as metallic oxides (Al 2 O 3, stabilized ZrO 2, TiO 2 ); carbides (Cr 3 C 2,TiC, WC, SiC), usually these carbides are used in a more ductile supporting metal matrix such as cobalt or NiCr. Intermetallics: Most of intermetallics are very sensitive to high temperature and to oxidation, so shielding gas must be used in the spraying process. The most common intermetallics are TiAl, Ti 3 Al, Ni 3 Al, and MoSi 2. 66

67 CHAPTER 2 LITERATURE REVIEW Metal matrix composite (MMC)materials Metal matrix composite material is a mixture of metal as a matrix with less than 80% of ceramic or another metal. They are fabricated to have the optimal properties of a ceramic, such as hardness and wear resistance, and the optimal properties of a metal, such as the ability to undergo plastic deformation. The metals most commonly used as a matrix in MMC are cobalt, nickel, and molybdenum. These metals are usually used as a matrix for an oxide, boride, or carbide. MMCs are used as binder coatings in thermal barrier systems to solve thermal mismatch between substrate and the oxide ceramic top coating. One of the most popular MMCs is tungsten carbide and cobalt (WC-Co) which is used widely to produce very dense, hard and wear resistant coatings [55, 58, 59]. However, cermets can be regarded as a special type of MMC, where in this case, ceramic particles are bonded together by a small proportion (less than 20%) of metallic phase [60] Microstructure of HVOF-Sprayed coatings The HVOF coating generally consists of lamellar splats (droplets of semi-molten or molten powders), unmelted particles, voids and oxidised particles (figure 2.24). The typically HVOF coating also contains certain defects, such as oxide inclusions in metallic coatings, which are usually seen as dark phase areas in the coating cross section. As a result of the low temperature, only mechanical bonding is produced between the coating and the substrate. Also, HVOF coatings may contain unmelted particles and pores, which produce an inhomogeneous structure. 67

68 CHAPTER 2 LITERATURE REVIEW Figure 2.24 Thermal spray coating microstructure showing common features [56] A typical cross section of HVOF sprayed coating is shown in figure The powder will fully melt or partially melt due to flame temperature which varies from 2500 o C to 3200 o C depending on the fuel, fuel/oxygen ratio and gas pressure. Use of the HVOF process has been growing rapidly specially for materials which are sensitive to phase transformations due to oxidation, decarburisation, or evaporation, because this process has relatively low flame temperatures and low exposure time in the flame. The high kinetic energy and low thermal energy will result in better adhesive bonding and high density coating with relatively low residual thermal stress which results in coating thicknesses between 100 m and 300 m [61]. Figure 2.25 Cross-section of T800-68WC HVOF coating on stainless steel substrate In recent years, there has been a significant growth in the use of this technique to spray cermets, metallics and ceramics. HVOF coatings have recently gained more popularity and are now extensively studied for protection against corrosion and wear [12, 58, 62-65]. Furthermore, HVOF technology has found many 68

69 CHAPTER 2 LITERATURE REVIEW applications such as automotive, aircraft, oil and other industries, the goal of this technique in all applications is to increase the lifetime of the surfaces as compared with uncoated substrate surfaces [66-68] Corrosion Performance of HVOF-Sprayed WC-Co- base coatings HVOF WC-Co MMC coatings are widely used to improve wear resistance. However, in some cases, the service environment is corrosive, so it is necessary to understand how the coatings behave in these corrosive environments. In this section a review of some work on corrosion of HVOF sprayed WC-Co coatings is presented. Galvanic Coupling between WC and Co From previous studies, it has been found that the corrosion mechanisms of HVOF WC-Co coatings are very complex, as a result of the heterogeneous microstructure and binder phase composition [69]. It is believed that the corrosion mechanism in this system is controlled by the galvanic coupling between the hard phase (WC) and the metal matrix [5, 9, 69-75]. For example, Perry et al [75] studied the corrosion and erosion characteristics of HVOF thermally sprayed 86WC-10Co-4Cr coatings on a stainless steel (UNS S31603) substrate, by examining these samples in artificial seawater under the influence of an impinging jet of liquid free from solids and containing solids. In static conditions, they noted micro-galvanic and/or crevice corrosion are concentrated at metallic/ceramic interfaces, and the corrosion attack occurred on metal phase, because the hard phase WC particles have a lower corrosion rate compared with the metal phase. The dissolution of metal phase led to extensive removal of carbide particles from the surface, as a result, surface roughness (R a ) increased from 10 ± 3 nm to 48 ± 5 nm. Monticelli et al [9] has studied the corrosion process of carbon steel coated by HVOF WC-12Co and WC- 17Co with different coating thickness in 3.5% NaCl solution. They showed the corrosion resistance of the coatings depended on the cermet layer porosity. Increasing the coating thickness from 0.05 to 0.2 mm in WC-17Co, offered the highest hindrance to the anodic oxidation of the substrate, where the access of the aggressive solution to the substrate was more difficult. They suggested the cobalt 69

70 CHAPTER 2 LITERATURE REVIEW and iron dissolution were enhanced by the WC coupling, where, a galvanic current was between steel or cobalt (anodic) and WC (cathodic). Cho et al [5] have studied the open circuit potentials between WC and different matrix metals in aerated 5 wt.% H 2 SO 4 solution, and showed considerable microgalvanic corrosion occurred, where the WC is cathodic and the binder material becomes anodic. They also assumed that the potentiodynamic polarisation behaviour of the WC coating layers is related to corrosion of the binder material in region I and to WC oxidation in region II as shown in figure Figure 2.26 Potentiodynamic polarisation curves of different coatings in the aerated 5 wt. H2SO4 solution [5]. Schnyder et al [73] have investigated the corrosion behaviour of WC-15Co in different environments (acidic, neutral, and alkaline solutions) in order to study the effect of ph on the corrosion behaviour. In their study, they concluded that WC-Co system showed high dissolution rate in all types of electrolyte, and they believed that WC is significantly noble and has much less corrosion rates than Co. Also, galvanic coupling between Co and WC increased the dissolution rate of the Co binder in the hard metal, and they noted very high dissolution rate of Co led to a loss of the WC particles. Lekatou et al [76] studied the electrochemical behaviour of HVOF WC-17Co with Ni-5Al bonding layer coatings on Al7075 substrate. The top surface was WC-17Co. Firstly; they investigated the corrosion behaviour of the coating in 3.5% NaCl solution, and they noted that during anodic polarisation, pseudopassivation (will be discussed in more detail in next section) occurred. Moreover, there was parallel active corrosion induced by the inhomogeneous binder composition at the same time as pseudopassivation. Also, they investigated 70

71 CHAPTER 2 LITERATURE REVIEW the same coating in 0.5 M H 2 SO 4 solution [7], and they noted that the corrosion behaviour of the WC-Co composite was affected by the galvanic effect of the WC- Co couple and by the galvanic effect between matrix regions with different content of WC and Co. Furthermore, they believed the dissolution of Co was greater at the WC/Co interface, at W-rich/W-poor matrix interface and along lamellae formed of well-melted particles. A previous study by Hochstrasser-Kurz et al [77] used scanning Kelvin probe force microscopy (SKPFM) under controlled humidity to measure the local electrochemical potential distribution of WC grains and Co binder, and showed for the first time, increasing the humidity allowed demonstration of coupling mechanisms of the two phases [77, 78]. As a result of their study, in humid environments the WC grains were the cathodes and the cobalt binder was active. Moreover, the electrochemical potential difference between the two phases was measured, to be around mv. The mechanisms of corrosion as a result of this galvanic coupling have been discussed in this study (figure 2.27) as follows. Co dissolves in neutral and acidic solutions, and is passivated in alkaline solutions, showing higher dissolution rates for decreasing of ph. However, WC shows dissolution in alkaline solution and continuous stabilisation with decreasing ph. The galvanic coupling causes accelerated anodic dissolution of the less noble phase (Co) as shown in the following equation: The reduction reaction then occurred on the WC phase, which can be one of the following reactions: The cathodic reaction from reactions 2.43 and 2.44 creates a local increase in the solution ph. Using inductively coupled plasma mass spectroscopy (ICP-MS) analysis, WC is instable in alkaline solution has been confirmed. In aerated conditions, the WC surface becomes covered by a tungsten oxide layer, which is 71

72 CHAPTER 2 LITERATURE REVIEW WO 3 as shown in figure This oxide can be dissolved in aqueous solution as shown in the following reaction: As seen from reaction (2.45), no electron takes part in this reaction, so it cannot be detected by electrochemical techniques. This oxide dissolution can decrease the local ph, and that can possibly initiate accelerated Co dissolution around the WC grains [77]. Figure 2.27 Schematic representation of the reaction processes taking place on the WC-Co surface as a consequence of the galvanic coupling between Co and WC [77]. The same process has been reported by Souza and Neville [79] in studies of the corrosion behaviour of 86WC-10Co-4Cr HVOF coating in 3.5% NaCl using in situ atomic force microscope (AFM) and ICP analysis. They concluded that, not only the matrix material underwent dissolution, but the hard phase WC had oxidation and dissolution at much higher potentials than Co, where, WC oxidizes to WO 3 then dissolves into WO 2-4. However, when the potential is high (0.7 V (SCE) ), the formation of WO 2-4 can also take place directly. Corrosion of Co in the solution takes place in the early stages of the process but is slowed down by the Cr 2 O 3 oxide layer, which is formed from the Cr in the matrix. The initial dissolution of Co creates conditions and interactions such as reduction of localized ph, which promotes oxidation/dissolution of WC particles. The WC particle can be removed from the surface if the surrounding area has been corroded. 72

73 CHAPTER 2 LITERATURE REVIEW Pseudopassivation behaviour Many studies [71, 72, 80] have confirmed that the reactions which occur on the hardmetal surface depend on the potential of the system. At low applied potentials or at open-circuit potential, the dissolution starts at the matrix phase, while only in the higher potential range does the dissolution of the WC phase occur. Some researchers have reported that the WC-Co system has a passive behaviour [81]. However, it was also reported that the system did not show any passivation, just active dissolution throughout the potential range studied [82]. Conversely, some researches confirmed that the passivation behaviour of this composite was kind of pseudo-passive state. The HVOF sprayed alloy coatings may not reach the perfect passive state as a result of defects, pores and oxidation inclusions [70, 83]. Bolelli et al [84] has studied the effect of heat treatment on corrosion properties of three different HVOF coatings: Co-28Mo-17Cr-3Si (T800), Ni-20Cr-10W-9Mo- 4Cu-1C-1B-1Fe (Diamalloy-4006) and Ni-32Mo-16Cr-3Si-2Co (T700). The corrosion tests were performed in 0.1 M HCl solution. A passive-like state (pseudopassive) has been observed in anodic polarisation curves. It has been confirmed that the anodic polarisation curves cannot be described as a complete true passivation, and the current density is too high for a perfect passivity state. However, T800 showed significant active corrosion, where the current density increased slowly over a large potential range (figure 2.28) [70]. The lack of full passivation has taken place because of defects, porosity and lamellae boundaries. Figure 2.28 Electrochemical polarisation curves of HVOF-sprayed coatings in 0.1 M HCl solution [70]. When the porosity defects due to imperfect interlamellar cohesion exist, these will be preferential sites for corrosion to take place, since the diffusion of oxygen into 73

74 CHAPTER 2 LITERATURE REVIEW these defects is much slower than that of small Cl - ions, which limits passivation occurring [70]. Sutthiruangwong and Mori [85] studied the corrosion properties of WC-Co in deaerated and aerated acidic solutions (H 2 SO 4 and HCl), the polarisation test with saturated calomel references electrode shows that dissolved oxygen has little influence on the anodic behaviour. Moreover, polarisation curves for Co and WC in 1N H 2 SO 4 solution have been discussed. Corrosion potential (E corr ) of Co was found to be about -460 mv, the current density (i corr ) was 0.68 ma/cm 2, the corrosion potential (E corr ) of WC was 110 mv, and the corrosion current density of WC was ma/cm 2 as shown in figure Figure 2.29 Potentiodynamic polarisation of Co and WC in aerated 1 N H 2 SO 4 [85]. Pseudopassive behaviour has been observed in their study, which concluded that the anodic film caused the current density reduction. The same conclusion was reached by Human et al [86]. Another explanation for pseudopassive behaviour is the Co 2+ ions taking a longer path through the porous tungsten carbide after dissolution of Co, where the diffusion for these ions through the WC skeleton is slower than free diffusion [85]. Furthermore, the stability of Co and WC in acidic environments has been discussed, as shown in the Pourbaix diagram of cobalt (figure 2.30); there is no stability of Co at anodic (between -390 and -330 mv) and pseudopassive (+200 mv) potentials and ph 0 where, the Co is dissolved as Co 2+ into the solution. 74

75 CHAPTER 2 LITERATURE REVIEW Figure 2.30 Potential-pH equilibrium diagram for the system cobalt-water, at 25 o C, potential with respect to standard hydrogen electrode [85]. However, tungsten exhibits a different behaviour, as shown in Pourbaix diagram of tungsten (figure 2.31); W is stable at the anodic potentials (between -390 and -330 mv) and ph 0. However, at the pseudopassive potential (+200 mv) and ph 0, it shows instability, and can be oxidized to oxide compounds (WO 2, W 2 O 5 and WO 3 ) [85]. Figure 2.31 Potential-pH equilibrium diagram for the system tungsten-water, at 25 o C, potential with respect to standard hydrogen electrode [85]. Many researchers have concluded that in H 2 SO 4 solution, at the low potentials, the pseudopassivation state (region I in figure 2.26) is closely related to the oxidation of Co and W [5, 81, 87] whereas, at the high pseudopassive potentials (region II in figure 2.26) it is related to the oxidation of W and C [5, 7, 71, 72, 88, 89]. 75

76 CHAPTER 2 LITERATURE REVIEW Effect of porosity In general, spraying unmelted/semi-melted powders leads to a low spraying efficiency and high porosity in the coating layers. Sufficient melting of the sprayed powders is important in order to create coatings with low porosity and excellent properties [90]. It has been reported by Guilemany et al [91] that, more micro cracks and porosity in coating layer will produce more corrosion, as a result of infiltration of the electrolyte into the coating layer/substrate interface. Godoy et al. [92] have reported that the pathways generated by interconnected porosity caused corrosion of WC-Co HVOF coatings due to formation of galvanic couples between the coating and substrate. Magnani et al [58] have sprayed WC-Co on aluminium alloy by the HVOF technique, and they noted that optimization of thermal spray parameters can produce low porosity and homogeneous coatings, which provide good corrosion resistance. Liu et al. [12] concluded that the existence of interconnected porosity and lamellar grains within the splat-structures with oxide inclusions at interlayer boundaries is the main reason for the failure of the coatings due to corrosion penetration into the interlayer which could cause de-bonding of the coatings. In general corrosion resistance can be improved by decreasing the porosity, avoiding cracks, and enhancing inter splat adhesion [91]. Effect of WC percentage and particle size in WC-Co alloy As discussed above, the chemical stability of WC is higher than that of Co in acidic media [71, 80]. Moreover, the current density which was measured in polarisation tests of WC-Co alloy was mostly from oxidation of binder phase, especially at low potentials. Consequently, it might be expected that as the binder content increased, the corrosion current density will increase proportionally. However, Human and Exner have concluded that the corrosion rate increased with increasing proportion of matrix phase because the fraction of the phase area was increased. However, the kinetics of the binder phase corrosion are not changed [72]. It has been proved that the electromechanical behaviour of WC-Co can be modelled using a simple linear rule of mixtures which was proposed by Stern [72, 80]. 76

77 CHAPTER 2 LITERATURE REVIEW Where is the electrochemical current density of the WC-Co composite, is the cross-sectional area fraction of the WC phase, is the current density of WC, is the cross-sectional area fraction of the binder phase and is the current density of a Co(W,C) alloy (matrix composition). It has been shown that the corrosion behaviour of WC-Co was similar to the corrosion behaviour of pure Co in 0.1 M NaCl solution, and strong dissolution of the Co binder may cause the loss of WC particles that might fall out of the surface [5, 73, 75, 93]. Sutthiruangwong et al [93] studied the influence of W on the corrosion behaviour of W-Co alloys in 1N H 2 SO 4 solution, it showed that (figure 2.32) the corrosion current density decreased with increasing tungsten content in the binder. Figure 2.32 Electrochemical behaviour of Co-W alloys in aerated 1 N H 2 SO 4 [93]. Many researchers reported that the size of the WC particle size does not result in any effect on the corrosion behaviour of WC-Co alloys [71, 72, 80]. However, Tomlinson and Ayerst [94] have reported a decrease in passive current density by reducing the grain size of WC Wear Performance of HVOF-Sprayed WC-Co-base coatings WC-Co cermets are extensively used as wear resistance materials as a result of their favourable combination of toughness and hardness. These composite coatings used on engineering components by the thermal spray processes, especially the HVOF technique, result in a dense structure that shows excellent resistance against conditions where erosion, abrasion and other forms of wear exist. The HVOF technique is performed at a lower temperature compared with other thermal 77

78 CHAPTER 2 LITERATURE REVIEW techniques such as plasma spray. For this reason it is suitable for spraying low melting point alloys and cermets. Furthermore, HVOF spray creates the highest mechanical properties and wear resistance compared with plasma spray or lower velocity combustion [95]. Due to the wide application of these coatings where good abrasion, erosion or sliding wear resistance is needed, there are a number of researchers concerned with the improvement of the wear resistance of these coating materials and studying the factors which could affect the tribological properties of the coating. Guilemany et al [96] studied the mechanism of wear on WC-12Co HVOF coating and indicated that the wear started by removing the binder phase which takes place mainly by abrasion (two body abrasive wear) and the fine debris cause three-body abrasive wear, followed by pull out the carbides. However, the three-body abrasive wear is a function of the quantity and hardness of the debris. The oxide layers which are produced on the surface will decrease the friction. But, will be removed by the abrasive effect, and the debris will cause an increase in the friction parameters again. Similar mechanisms of wear have also been reported by Shipway et al [97] in WC-17Co, Stewart et al [98] in WC-12Co and Kumari et al [99] in WC-10Co- 4Cr. It was found that the wear behaviour of WC-Co HVOF sprayed coatings is different from wear behaviour of bulk sintered WC-Co as a result of inhomogeneity and phase transformations which occur during the thermal spray process, and that has a significant effect on the tribological properties of the coatings. Liao et al [100] reported for WC-17Co HVOF coating that hardness and cohesion of the material are very important for resistance to abrasive wear in cermets such as WC- Co, where, in this type of material, more hard WC particles means increasing the average hardness. The cobalt matrix in this coating operates as a binder for carbide particles. So, to improve the wear resistance of these materials the cohesion between the carbides and cobalt matrix should be increased, which influences the microstructure and phases in the coatings at the interface between hard WC particles and the cobalt matrix. Appearance of M 6 C or M 12 C gives a good indication of improvement of cohesion between hard carbide and the cobalt matrix, which enhances the abrasive wear resistance [100]. Furthermore, it was found the 78

79 CHAPTER 2 LITERATURE REVIEW smaller the carbide size, the better the abrasive wear resistance, because pull out of single carbide particles produces less damage to the finer coating and the debris for the finer carbides have less influence as the third body abrasive [ ]. However, Guilemany et al [63] have observed that in WC-12Co coatings, the mixture of nanostructure and conventional coatings (which is called bimodal coating) has higher wear resistance than nanostructure or conventional coating individually. They suggest that this is because the mixture of nano-sized carbides hardens the matrix and improves the cohesion for hard micro-sized WC particles with the cobalt matrix, which creates a strong, tough surface which leads to improve the wear resistance. Optimization of thermal spray parameters has a significant influence on coating properties and wear resistance. Zhao et al [103] have reported that for WC-CoCr and under the experimental conditions, the particle velocity has more effect on coating properties than the particle temperature, and the total gas flow rate has a significant influence on wear resistance. Heat treatment has a significant effect on microstructure and coating properties. Stewart et al [104] have studied the influence of heat treatment on abrasive wear behaviour of WC-17Co HVOF coatings, and reported that the heat treatment reduced the residual stresses in the coatings. Low temperature treatment has improved the abrasive wear resistance. They also observed that heat treatment at temperature above 600 o C produced low carbon phase Co 6 W 6 C [65, 104]. However, Asl et al [64] found heat treatment of WC-17Co HVOF sprayed coatings at high temperature (1100 o C) decreased the wear resistance of the coatings. The decomposition of WC and subsequent decarburisation due to the hot gas jet in HVOF, typically WC to W 2 C has a negative effect on wear resistance of the coating. It has been reported that the wear resistance decreased due to brittle phases which are produced from decarburisation mainly WC W 2 C [98]. Karimi and Verdon [95] showed that in WC-CoCr coating, during the spray, some of the WC could decompose into W 2 C or interact with cobalt and form Co 3 W 3 C. It has been reported that adding Cr to this composite could have a positive effect on wear resistance, where Cr could inhibit the decomposition of WC to a large extent [95]. 79

80 CHAPTER 2 LITERATURE REVIEW Wood [105] reported that the tribological properties of the spray coatings are functions of a wide range of parameters such as: composition (figure 2.33), phases and their distribution, microstructure, residual stress and porosity. These parameters have a direct influence on the hardness of the coating, which are used as initial correlation factors to evaluate the wear resistance. Also another important factor which can influence the tribological performance of HVOF WC-Co coatings is the adhesion of the coating to the substrate. Figure 2.33 Total (pin and disc) specific wear rates for different HVOF coating composition under dry oscillating wear [105]. The inhomogeneous microstructure of WC-Co-Cr thermal sprayed coatings, especially the low fracture toughness in a direction parallel to the substrate, is the main factor which affects the nature of crack formation under erosion conditions. These cracks could start at voids, splat boundaries and interfacial inclusions [105]. However, brittle phases in the thermal spray coating can be avoided by employing suitable spraying parameters [106]. 2.5 Laser Surface Engineering Lasers In 1917, Albert Einstein illustrated the phenomenon of stimulated emission. This process is important for amplification of light, and for laser operation. During the period of time from 1920 to 1940, there were many discoveries in quantum mechanics physics, which made it possible to progress towards the establishment of the first laser. Moreover, during World War II experience was gained during the 80

81 CHAPTER 2 LITERATURE REVIEW improvement of radar, in particular, the important work in the field of microwave radiation. That work led scientists to analyse the conditions that are important for the laser process to succeed. In 1960 the first ruby laser was operated successfully by T. H. Maiman. After that many substances were discovered as the active laser medium, such as Helium-Neon gases, crystals and dye solutions [107]. Lasers have since been used in many applications in our life for instance in medical applications, communications, material processing, holography and isotope separation Basic Principle of Lasers Laser stands for Light Amplification by Stimulated Emission of Radiation. In every laser device three conditions should exist. Firstly, it has an active medium, which contains atoms, molecules, or ions. Secondly, it supports population inversion, this condition does not exist in nature, but it is created in laser by a process known as pumping. Finally, for a laser beam to be generated, there must be a pumping process to give energy to the system. The Active Medium The active medium is the main material, which is responsible for laser action. This material can also be considered as an atomic system. Every material in the world can be regarded as having energy levels, and the energy level contains atoms. The number of atoms in each energy level depends on the energy of this level [107]. The atoms can transit from one level to another level. Physics laws control these transitions. In the following paragraph, the relationships between energy levels and the important transitions are discussed briefly. Absorption and Emission Although not true of real materials, these phenomena can be described using a theoretical material which has just two energy levels (figure 2.34). The ground state with energy E 0, and the excited state with energy E 1. 81

82 CHAPTER 2 LITERATURE REVIEW E 1 E 1 E 0 E 0 Photon Emitted h (a) (b) E 1 (c) Photon Absorbed h E 0 h (d) h Figure 2.34 Two-level energy system (a), Spontaneous emission (b), absorption (c), Stimulated emission (d). If the atom in the upper level E 1 makes a transition to the lower level E 0, it will emit energy. This energy could be in the form of radiation. The frequency of this radiation is given by equation (2.47). This kind of transition is called spontaneous emission (figure 2.34 (b)). Where: E 1 Energy level for upper state. E 0 Energy level for lower state. Planck constant = J.s Frequency of the radiation. An atom initially at the lower energy level (ground state) E 0, must absorb energy to transit to the upper level E 1 (figure 2.34 (c)). The frequency of this energy is given by the above equation [108]. There is another kind of emission called stimulated emission. This type of emission is different from the spontaneous emission, which occurs randomly in time. However, the stimulated emission occurs when an atom in the higher energy state interacts with a photon, which stimulates it to go down to the lower state. In this case the result of stimulated emission is two photons (figure 2.34(d)), these photons are coherent [108]. 82

83 CHAPTER 2 LITERATURE REVIEW Population Inversion In normal conditions (Thermal Equilibrium), the population (number of atoms) in the lower level is greater than the population in the higher level. The distribution is known as a Boltzmann distribution, which can be expressed in this equation [108]: Where: Population of upper level. Population of lower level. Energy level for upper state. Energy level for lower state. Boltzmann constant = m 2 kg s -2 K -1 Absolute temperature (K). From equation (2.48) when E 1 > E 0 (normal condition) then N 0 > N 1. Any photons will be absorbed rather than causing stimulated emission. To make stimulated emission happen, it is necessary to increase the population of the upper energy level. This situation is known as a population inversion [108]. In fact population inversion can be achieved only in non-equilibrium conditions, where Boltzmann s law does not apply. Laser pumping Pumping is an important to reach a population inversion. Population inversion is obtained in a laser by exciting or pumping the atoms in the laser medium to a non thermal equilibrium state using an external source of energy [107]. The most well known methods of laser pumping are optical pumping to provide the system with energy; a flash lamp is fixed near the active medium (used especially in solid state lasers), electrical pumping uses an external high voltage as a source of energy for gas lasers. 83

84 CHAPTER 2 LITERATURE REVIEW Laser components Any laser device must contain an Active medium, a Pumping source and two mirrors (Resonator), one with 100% reflectivity and the other with partial reflectivity. Figure 2.35 shows the main parts for every laser device. Active Medium Laser R=100% R= Partially % Pumping Process Figure 2.35 Laser components How a laser works Population inversion is the important condition of the laser process, which is reached by pumping the active medium using an external source of energy. Stimulated emission is the following process, then immediately after that, the amplification for coherent radiation, which occurs by a resonator, where the mirrors will reflect the two photons into the active medium and these photons will stimulate more photons. Afterword, the new photons which have been reflected from the other mirror again pass into the active medium (figure 2.36). This process will continue until the radiation reaches a high enough power to emit from the partial reflectivity mirror as laser radiation. At the same time, the pumping process continues to give the system energy for population inversion [107]. 84

85 CHAPTER 2 LITERATURE REVIEW Active Medium R=100% Pumping R= Partially % Process Active Medium R=100% Pumping Process R= Partially % Active Medium Laser R=100% Pumping R= Partially % Process Figure 2.36 Amplification process Properties of laser radiation As a result of the unique properties of laser radiation, it is the perfect tool for many areas of industrial and research. Monochromaticity The word, Monochromatic, means laser has single wavelength. From Greek language monos means single and chroma means colour [107]. In fact, the laser light is emitted over a very narrow range of wavelength compared with conventional light sources. Ordinary white light is a mixture of many different wavelengths (colours). Directionality A laser beam is directional; it is nearly parallel when emitted from the resonator. This means it has very small divergence angle. This property comes from the fact 85

86 CHAPTER 2 LITERATURE REVIEW that laser radiation emits from the resonator cavity, and only the waves propagating along the optical axis stay in the cavity and are amplified [107]. Coherence Coherence is one of the unique properties of the laser. This fascinating radiation is totally coherent whereas, ordinary light sources are incoherent. This property comes from the stimulated emission process which produces the amplification. The emitted photons have a defined phase relation to each other. This coherence is both temporal and spatial mean that the photons at any point along the laser beam are in phase and remain in phase throughout the time that the laser is operating [108]. High power density A laser beam can be focused by special optical instruments such as lenses to a small spot area with high power density. This is important in laser material processing, which require high power density, like drilling, alloying, cutting and welding [107] Types of laser The different laser types developed to date have a wide range of physical and operating parameters. Moreover, lasers are classified according to the physical state of the active material, which are: solid state, liquid or gas lasers, depending on the active medium state [109]. The main laser types are as follows: Solid state lasers Ruby laser, Nd: YAG (Neodymium: Yttrium-Aluminium Garnet) laser and Semiconductor laser. In these lasers the active medium is solid material, for example, the Ruby laser which emits visible light at wavelength nm, and Nd:YAG laser emits infrared light at 1064 nm [109]. 86

87 CHAPTER 2 LITERATURE REVIEW Gas laser The most important gas lasers are Helium-Neon laser, Argon laser, Carbon dioxide laser, Excimer laser and Nitrogen laser. In gas lasers He-Ne (Helium-Neon) laser is the most common gas laser used in laser labs for beam alignment, which has a radiation of visible red light, and its usual operation wavelength is nm. Moreover, Carbon dioxide laser operates in the infrared at 10.6 m [109]. Liquid laser (Dye laser) Dye lasers use complex organic dyes. They are tuneable over a broad range of wavelengths [109]. The following table shows most types of lasers and their properties. Table 2.3 Main lasers and their properties [110]. Laser Wavelength Power Operation mode CO 2 laser 10.6 m 1 W 40 kw Continuous/ pulsed Excimer laser 193 nm/ 248 nm/ 308 nm 1 kw-100 MW Continuous/ pulsed He-Ne laser nm 1 mw-1w Continuous Argon laser 515 nm/ 458 nm 1 mw-1w Continuous Dye laser Between IR and UV 1 mw-1 W Continuous/ pulsed Nd:YAG laser 1064 nm 1 W-3 kw Continuous/ pulsed Ruby laser nm Several MW Pulsed Semiconductor laser From infrared to visible 1mW-100 mw Continuous/ pulsed 87

88 CHAPTER 2 LITERATURE REVIEW Semiconductor Laser A diode laser is made by forming a junction between (positive) and (negative) types materials in the same host material, known as a - junction. Figure 2.37 shows that the lower part of the conduction band of the n-type material is filled with electrons and in the p-type material, the top is filled by holes. Moreover, a high voltage needs to overcome the energy gap V. The n-type is connected to the negative part of a potential supply and the p-type is connected to the positive part of the potential supply, called forward bias. Under this condition, the electrons in the n-type region are driven towards the junction and combined with holes to produce photons. Furthermore, when the forward bias is increased, the photons emmision will be increased and intensity of light becomes higher. Figure 2.37 Semiconductor laser-junction region [111]. The most common semiconductor material used in semiconductor lasers is gallium arsenide, and there are other semi conductor materials used in this laser type such as indium phosphide, gallium antimonide, and gallium nitride. Moreover, in this type of laser the resonator is different from conventional lasers, where the resonator in this laser is affected by the semiconductor structure itself using the polished faces as mirrors. These lasers are pumped by applying an electrical current (figure 2.38). The energy gap for gallium arsenide is 1.4 ev and the wavelength emitted is 840 nm. 88

89 CHAPTER 2 LITERATURE REVIEW Figure 2.38 Semiconductor laser [111] In this system without stimulated emission conditions, it is possible for the electrons and holes to be near each other without combination. The time to remain without this interaction is called the recombination time (about one nanosecond). The stimulated emission occurs when a nearby photon with energy equal to the recombination energy can cause recombination, and this will generate other photons at the same frequency, direction, phase and polarisation as the original photon. Moreover, to complete the laser process, the stimulated photons are constrained by an optical cavity. The stimulated light passes through the cavity and is amplified by stimulated emission. However, there is some loss in light due to absorption. When the amplification is greater than the losses in the cavity, the laser is generating [111]. The wavelength of a diode laser is dependent on many parameters such as; band gap energy, cavity length and refractive index of the semiconductor. The following table shows the wavelengths of some semiconductor laser materials [112]. Table 2.4 Wavelengths of a selected range of diode laser materials [112] Materials AlGaAs InGaAs AlGaInP Wavelengths (nm)

90 CHAPTER 2 LITERATURE REVIEW Characteristics of high power diode laser Characteristics of high power diode lasers (HPDL) make them suitable for many material processes. HPDL usually works in continuous mode. However, by controlling of current, the output power can be switched off and on rapidly to generate a pulsed mode. HPDL has a high optical/electrical efficiency of 20-50%, which is higher than other high power lasers. Furthermore, HPDLs are smaller in size than other lasers of the same output power level [113]. The short output wavelength of most practical HPDLs (between 800 and 940 nm) can be absorbed by most metals compared with CO 2 and Nd-YAG lasers (figure 2.39). This reduces the need of surface preparation [114]. Figure 2.39 absorption as a function of wavelength for carbon steel and aluminium [114]. A single diode laser has a low power of less than 5 W. To increase the output power, practical HPDLs are based on several individual emitters which are arranged either in a one dimensional linear array, as a high power diode laser bar (30-50 W), or a two dimensional multi-layer array to produce a rectangular shape of laser beam profile [112]. The average output power of HPDLs after using this focusing technique reaches up to 6 kw [115]. However, HPDL have relatively poor beam quality. Consequently, it is not suitable for precision materials processing such as cutting and welding. Moreover, the beam profile of a HPDL generally is top-hat in the slow axis direction and Gaussian in the fast axis direction with a rectangular shaped spot, which is suitable for surface engineering. The HPDLs have high beam stability compared with other conventional lasers such as Nd-YAG and CO 2 lasers, where, the conventional lasers have a single beam which has intensity fluctuations, apparent as irregular spikes in beam profile, as a result of small variations during the excitement of the laser active medium. However, the 90

91 CHAPTER 2 LITERATURE REVIEW great number of laser beams of a HPDL system reduces fluctuations, and giving the combined beam an outstanding modal stability. Consequently, the beam profile of HPDL appears smoother and has very little variation in shape and peak power as shown in figure 2.40 [115, 116]. Figure 2.40 Time resolved intensity distribution within a 1.6 s interval (a) HPDL system (b) Nd-YAG laser [116]. As a result of these unique features, HPDLs provide better surface finish, fewer cracks, more uniform melt zones, have more repeatable results than other lasers and now increasingly used in surface treatment [112] Laser beam Interaction with Materials When a laser beam is incident on the surface of material, such as a metal (Opaque material), some of the energy is absorbed into the near-surface regions, and some reflected. In these materials, Absorptivity depends on the material, laser wavelength, temperature of material, and surface roughness [108]. The portion of the incident optical energy that is absorbed is of interest in material processing. The absorbed optical energy (photons) is immediately transformed to cause vibrational motion of electrons in the affected zone by the excitation of electrons from their equilibrium states to some excited states. These vibrations are transmitted into the structure of the material within an extremely short period of time. The optical energy is converted to heat, which gradually transfers to adjacent atoms. The temperature will increase as more photons are absorbed. Moreover, the temperature will rise rapidly, and the near-surface region undergoes extreme heating and cooling rates ( K/s), very high thermal gradient ( K/m) and a rapid 91

92 CHAPTER 2 LITERATURE REVIEW solidification velocity (1-30 m/s) [117]. The flow of heat in the material can be described by conventional heat equation Where: h Thermal diffusivity K Thermal conductivity Q Heat production per unit volume time And Cp is heat capacity, and is density of material The thermal diffusivity determines how rapidly a material will conduct thermal energy. The heat equation (2.49) is a non-linear partial differential equation that makes the solution difficult. However, the equation can be solved by numerical techniques such as finite difference or finite element [118] Laser Surface Treatment Lasers have been used in numerous applications, such as communication, medical and material processing. Laser surface treatment is used to change the surface microstructures to enhance locally some specific properties of the surface such as hardness, wear and corrosion resistance. This process includes surface heating, alloying and cladding. Figure 2.41 is schematic showing a simple classification of laser surface treatment. Figure 2.42 depicts the different techniques of laser surface treatment. 92

93 CHAPTER 2 LITERATURE REVIEW Figure 2.41 Laser surface processing [119] Laser Transformation Hardening When the laser beam heats the surface of material, the surface may be hardened by a solid state transformation mechanism. Following heating, the laser energy is reduced or switched off or the beam moved off to another area, and the cold bulk material then quenches the heated surface; this will cause extremely rapid cooling. Diffusion occurs at high temperatures and the material is unable to be transformed back to its origin phases on rapid cooling; causing martensite transformation [108]. The factors which will affect hardening are speed of scanning, intensity of the laser radiation and type of metal. Laser surface hardening will increase hardness, wear resistance, and fatigue life [108, 119] Laser Surface alloying Laser surface alloying is variation on the surface melting process by adding another material into the melt pool. This process produces a surface layer which is different from the substrate. Nowadays many researchers have reported this technique improves wear and corrosion resistance for some specific parts which are target at corrosive environments [108, 119, 120] Laser Cladding This method involves melting and adding another material as in the alloying process. However, dilution in this process is kept to minimum. This technique 93

94 CHAPTER 2 LITERATURE REVIEW creates metallurgical bonding of the clad layer with the substrate to improve the surface properties of a material which will be subjected to corrosion, erosion or wear [108]. Figure 2.42 laser surface treatment techniques; (a) laser transformation hardening, (b) laser surface melting, (c) laser alloying, (d) laser cladding Laser Surface Melting In this type of processing, the laser beam is used to melt a small region of the substrate surface. This processing does not change chemical compositions within the melted layer, but changes its microstructure, in terms of a refined and possibly metastable microstructure in small localised areas of base material. The features of this process are: flexibility resulting from possibilities of computerisation (software control) of the process and automation, and it has small thermal penetration and therefore minimum distortion. By this procedure fine homogeneous structure can be obtained due to rapid cooling rates, where the unaffected substrate acts as a heat 94

95 CHAPTER 2 LITERATURE REVIEW sink. When the beam traverses past, the solidification process starts from the solid/melt interface towards the surface Laser induced rapid solidification The microstructural features during solidification are controlled by the shape of the solid-liquid interface. The nature and stability of the produced solid-liquid interface depend on the thermal and compositional conditions in the melted layer. The main variables in determining the morphology of solidification are, the temperature gradient G in the liquid, the growth rate R, and the alloy composition. Depending on these conditions, the interface between liquid state and solid state, might grow by planar, cellular or dendrite mechanisms. The temperature gradient G and growth rate R are important in the combined forms GR (cooling rate) which influence the scale of the solidification microstructure and the ratio G/R determines the solidification morphology [119],[121]. Figure 2.43 summarises the effect of the temperature gradient G and the growth rate R on the solidification microstructure of alloys. Figure 2.43 Effect of temperature gradient G and growth rate R on the morphology and size of solidification microstructure [122] During laser surface treatment, cooling rates are very high between Ks -1 depend on treatment conditions. The solidification velocity is also high between ms -1, and the temperature gradients between K m -1 [119]. Rapid solidification induced by laser melting can produce materials with superior distinct 95

96 CHAPTER 2 LITERATURE REVIEW physical and mechanical properties. The cooling rate decreases from the maximum on the surface to a minimum at the bottom of the molten pool and also decreases from a maximum at the centreline to zero at the edge of the molten pool. This variation in cooling causes a variation of microstructure in the molten pool [108]. For high cooling rates, the liquid might under-cool which results in protrusions at the solidification front. These protrusions will develop as cellular or dendritic structures depending on cooling rate (figure 2.44). Figure 2.44 SEM morphologies of the laser clad Tribaloy T-800 on stainless steel (AISI 304), shows: (1) substrate; (2) planar crystallization region; (3) cellular growth zone; (4) fine dendritic microstructure; (5) overlap zone between tracks. power density 200 W/mm 2, scanning speed 240 mm/min and powder feed rate g/min [123]. In most cases in substrate-quenched alloys as in laser surface treatment, nucleation leads to columnar growth dendrites, which start at the surface in contact with the quenching substrate [124, 125]. Figures 2.45 and 2.46 show different microstructures which might be created by rapid solidification induced by laser treatment for different materials. 96

97 CHAPTER 2 LITERATURE REVIEW Figure 2.45 SEM morphologies of the laser clad Tribaloy T-800 on stainless steel (AISI 304) power density 200 W/mm 2 scanning speed 240 mm/min and powder feed rate g/min: (a) Cellular structure; (b) dendritic microstructure [123]. Figure 2.46 SEM morphologies of laser clad of the Stellite 6. The dark dendritic phase is the cobalt-based matrix and the white phase is formed by the M 7 C 3 eutectic carbides: (a) slow processing conditions (V b =1.67 mm s -1 ); (b) fast processing conditions (V b =167 mm s -1 ) [126] Effect of laser Surface treatment on microstructure and electrochemical properties of HVOF Coatings Laser treatment promotes homogenisation of the surface composition, and increases the density of the HVOF coating by eliminating porosity to provide a better barrier to the substrate [11]. This section summarises the previous work of the consideration of corrosion properties after surface treatment by various lasers. Heat treatment can be an effective way of enhancing the overall corrosion resistance of the Co alloy, as has been reported lately by Bolelli et al [84], where, the heat treatment for one hour at 600 o C of AISI 1040 steel plates coated by T800, T700, and other alloys HVOF coatings, showed enhancement of corrosion properties. It has been suggested the heat treatment improved the interlamellar cohesion which reducing the activation of corrosion through the interlamellar 97

98 CHAPTER 2 LITERATURE REVIEW boundaries. Marginean et al. [127] studied the oxidation behaviour of CoNiCrY HVOF coating after laser heat treatment. After treatment without protective inert gas, an oxide layer was formed on the surface. Also the oxide layer had poor adhesion to the surface and cracks. Zhang et al. [128] found that porosity of WC- CoCr HVOF coating was reduced after laser heat treatment by a factor of approximately five compared to HVOF coating. Neither Co nor CrCo was detected in the coating after HVOF process, suggesting that most of Co is retained in the nano-crystalline binder phase. However, WC and W 2 C were still present in the coating, and a new phase (Co 4 W 2 C) has appeared in the LH treatment. Furthermore, Argon gas was used for shrouding the laser treated region and no oxide formation has been detected in the HVOF coating before or after laser treatment. In another paper [129] recently the authors investigated the corrosion properties of HVOF sprayed WC-24%Cr 3 C 2-6%Ni before and after LH treatment. They observed that the performance against corrosion has been improved after heat treatment. Their polarisation test results are shown in figure It has been concluded that the improvement in corrosion resistance was as a result of decreasing the size and the number of porosity, and creating a compact interface between the coating and the substrate, compared with HVOF coating without LH treatment. They reported that the coating with LH treatment was more successful in hindering the solution from reaching to the interface between the coating and the substrate. Figure 2.47 Potentiodynamic polarisation curves of HVOF WC24Cr 3 C 2-6Ni coatings in 3.5% NaCl solution [129]. 98

99 CHAPTER 2 LITERATURE REVIEW Corrosion properties of HVOF sprayed Inconel 625 on mild steel using laser melting has been studied by Tuominen et al. [10]. The coatings obtained after laser melting are metallurgically denser than HVOF, and also crack- and porosity-free. In an immersion test in 3.5% NaCl solution for one week, it was observed for the HVOF samples, that the coating materials had corroded selectively. The corrosion started along splat boundaries and corroded the material nearby, and as a result of splat boundaries, interconnected pores, and micro-cracks of sprayed coatings, the electrolyte quickly reached to the substrate and caused corrosion at the interface. However, for laser melted samples in certain laser parameters, the surface after one week of immersion was still free from any corrosion products. For some laser parameters the electrolyte can reach the substrate through the interconnected pores. It has been supposed that the reason of these pores is atmospheric gases which dissolved in melting pool during laser treatment, and at the high speed laser scanning the air bubbles do not have time to escape before solidification. On the other hand low scanning speed could cause high dilution from the substrate, therefore, appropriate processing parameters should be chosen. In the polarisation test they observed improvement in corrosion resistance after laser treatment as shown in figure Figure 2.48 Polarisation curves of laser melting and HVOF sprayed Inconel 625 coatings [10]. The same researchers in another paper [11] studied laser melting of high chromium (53.5%) nickel-chromium HVOF coatings to improve corrosion behaviour. They conclude that the corrosion behaviour of the coating can be improved by laser melting, only if the appropriate laser parameters have been chosen. Parameters which create high energy densities cause high iron dilution which decreases pitting 99

100 CHAPTER 2 LITERATURE REVIEW corrosion resistance, and high cooling rates should be avoided by using low traverse speed because high chromium alloys are sensitive to cracking. The immersion test showed very positive results, after one week of immersion test in 3.5% NaCl solution. The HVOF sample showed corrosion at the interface whereas the laser melted coating was unaffected (figure 2.49). Figure 2.49 optical micrograph of (a) HVOF sprayed and (b) laser remelted high chromium nickel-chromium coating after one week immersion test in 3.5 wt.% NaCl [11]. Suutala et al. [62] reported producing coatings by a laser-assisted spraying process; also called laser hybrid spraying, where thermal spraying and laser melting processes were combined to get denser coatings of different Ni-base materials on low carbon steel substrate. Laser-hybrid Inconel 625 coatings showed poor corrosion behaviour, as a result of cracks and high iron dilution from the substrate. Furthermore, they reported that the Nd:YAG laser is not ideal for real applications due to the inhomogenity in its power intensity in beam profile. Corrosion properties in 3.5% NaCl solution of pure Inconel 625 HVOF coating, and Inconel 625 with WC as MMC HVOF coating on 316 L steel substrate has been studied before and after HPDL laser remelting by Liu et al. [12]. The results showed that laser treatments improved the corrosion resistance due to a reduction of compositional gradient between the WC and the matrix metallic, and also due to elimination of the discrete splat-structure and micro-crevices. Moreover, they reported that 22.3% of Fe caused by dilution within the treated surface layer did not affect significantly the corrosion behaviour. Recently Ahmed et al. [130] studied the corrosion properties in 0.5 M H 2 SO 4 solution of Inconel 625 as a bulk material and HVOF sprayed coating before and after HPDL remelting. They concluded that the difference between bulk and HVOF sprayed coating is due to galvanic 100

101 CHAPTER 2 LITERATURE REVIEW corrosion between Cr-depleted in resolidified regions and non-melted material with the original Cr in the coating. Furthermore, the non-interconnected porosity has a negative effect on the corrosion behaviour of the as sprayed coating. Laser treatment largely reduced the difference between the corrosion behaviour of HVOF as sprayed and bulk material by elimination of both porosity and regions of material depleted in Cr Effect of laser surface treatment on hardness and wear resistance of HVOF Coatings Laser treatment of HVOF sprayed coatings has been performed by some researchers in an attempt to improve their hardness and wear resistance. This section contains a review of this work. Micro-scale abrasive wear behaviour of HVOF sprayed and laser remelted WC-12Co HVOF coatings has been studied by Chen et al. [13]. They found that the wear resistance was decreased by laser remelting, due to reducing the carbide size and fracture of carbide during laser treatment. Zhang et al. [128] studied the effect of laser heating on the hardness of WC-CoCr HVOF sprayed coating. The results showed that the average value of the microhardness of the coating increased after laser heating from Hv to Hv (figure 2.50). Furthermore, the microhardness is increased with decreased porosity of the coating. Figure 2.50 Microhardness of cross-sections of HVOF WC-CoCr coatings before and after laser treatment [128] The same researchers recently studied the effect of laser heating on tribological performance of HVOF sprayed WC-24%Cr 3 C 2-6%Ni coating [129]. They found 101

102 CHAPTER 2 LITERATURE REVIEW after laser heating there was a reduction in roughness level, and an improvment in hardness. Moreover, remarkably reduction of wear rate and friction coefficient has been noted (figure 2.51). Figure 2.51 Friction coefficient and wear rate of the coatings [129] They proposed the following wear mechanism; for the HVOF sprayed coating, the wear started by severe deformation of Cr 3 C 2 and Ni phases (matrix) between WC particles. These deformed matrix phases then extruded, and microcracking and/or pull-out of WC particles when there is no support of WC particles from the matrix. These processes produce wear debris, and that caused more damage on both sides as a third-body abrasive. However, they noted laser heating improved wear rate by densifying the coating as a result of compressive thermal stress, and that is useful to form denser tribofilm and enhance the cohesion between the matrix and WC particles. Moreover, they reported that the thin oxide film which was created during the test has a significant influence on adhesive wear, and for laser treated surface, the wide area of oxides helps to reduce roughness, and increase the lubrication of sliding surface, and that led to a decrease in friction coefficient during the wear test [129]. Morimoto et al. [131] found that laser melting has improved both hardness and wear resistance of Cr 3 C 2 -NiCr cermet coatings. Furthermore, the laser melting improves the solid particle erosion resistance for this cermet coating by twice the amount. Liu et al. [12] has been studied the effect of laser remelting on the mechanical properties of Inconel 625 and Inconel 625 with WC as MMC HVOF 102

103 CHAPTER 2 LITERATURE REVIEW sprayed on 316L steel substrate. They observed that wear resistance increase by increasing the percentage of WC in the coating material. They explained the increase of wear resistance by elimination of the discrete splats and porosity. Furthermore, laser melting creates faceted dendritic WC phases, and increases the bonding between WC particles and the surrounding matrix. However, wear resistance reduced as a result of fully melted WC particle. 103

104 Chapter 3 Experimental Procedures 3.1 Introduction This chapter describes the materials (i.e. powders and coatings), the laser and the laser operating conditions applied in the project. Examination and assessment of the coating microstructures, in terms of surface morphology, porosity, cracks, chemical composition, phase constituents, was carried out using scanning electron microscopy (SEM)/energy-dispersive x-ray spectroscopy (EDX), electron probe micro-analysis (EPMA), optical interferometry, and x-ray diffraction (XRD). Images processing and analysis was performed to establish the extent of the coatings porosity. The corrosion performances of the coatings both before and after laser treatment were evaluated using electrochemical tests and immersion tests. The electrochemical tests included anodic polarisation and Electrochemical Impedance Spectroscopy (EIS) in 0.5M H 2 SO 4 solution. The immersion test was carried out using 3M H 2 SO 4 solution. The corrosion morphology of coatings was observed using Scanning Electron Microscope (SEM) and solution after the tests was analysed using Inductivity Coupled Plasma-optical emission spectrometer (ICP-OES) to understand the corrosion mechanisms. Wear resistance of the coatings were also conducted using pin-on-disk, along with the Vickers hardness test 3.2 Materials Powders The feedstock powders were produced by an agglomeration method. The four types of powders used are: Tribaloy 800 (T800) (29 wt. %Mo, 18 wt. %Cr, and 3.5 wt. %Si and Co based) 104

105 CHAPTER 3 EXPERIMENTAL PROCEDURES T800+21wt.%WC(tungstencarbide) T wt.% WC T wt.% WC Coatings All the coatings were applied onto 316L stainless steel substrate using the HVOF technique. The dimensions of the samples were (10 mm thickness 20 mm 15 mm). The coated samples were supplied by Bodycote Metallurgical Coatings Limited; UK using a JP5000 gun, with liquid kerosene as the fuel source. The coatings thicknesses were ~ 250 µm for T800 coating, ~ 230 µm for T800+21WC, ~ 220 µm for T800+43WC and ~ 165 µm for T800+68WC Specimen preparation for laser treatment Before laser processing, each sample was cut to the size of 20 mm 15 mm 10 mm (figure 3.1) using Accutom-5 (Struers) and Brillant 250 H machine with coolant. Figure 3.1 Schematic diagram of cutting process 3.3 Laser processing Laser System A high-powered diode Laser (HPDL) generated by a Laserline Diode Laser has been used for laser processing. This HPDL consists of a compact laser beam source (laser head), a mobile power supply unit, and a laser cooling. All the 105

106 CHAPTER 3 EXPERIMENTAL PROCEDURES operating and monitoring functions are on the control panel. A diode laser bar, on which a number of separate emitters of the size of µm 2 are aligned, is mounted on a dissipater. The power generated by this single diode laser is W. These separate components are stacked on top of each other. This allows the laser to increase the power per compact stack to several hundred Watts. The rectangular laser spot measures 3.5 mm 2.5 mm at focal position, with an output power of up to 1500 W. It works in continuous-wave mode and pulsed mode with wavelengths of 808 nm nm Laser processing setup The laser processing setup is shown in figures 3.2 and 3.3. The laser head was fixed, while the samples were clamped to a base which can be controlled (travelling speed and position) by a PC. All the laser processes were carried out in an argon gas environment to avoid/reduce oxidation caused during laser process in the interaction zone. The samples were placed in metal box covered by a Perspex sheet that allows penetration of the laser beam (figure 3.3). The box was purged with argon gas with flow rate 10 l/min for 5 min before laser processing and was also purged continuously during each process. Figure 3.2 Laser processing setup 106

107 CHAPTER 3 EXPERIMENTAL PROCEDURES Figure 3.3 Laser treatment experiment 3.4 Material characterisation Sample preparation The HVOF samples before and after laser treatment were characterised to investigate the chemical composition, microstructure, morphology and porosity. The samples were sectioned, mounted, ground and polished. The mounting process carried out by using a cold conductive resin in ratio 8:5 powder and liquid respectively (see figure 3.4). The grinding was performed with abrasive SiC grinding paper with increasing mesh number in the following order: 220, 230, 500, 800, 1000, 1200, and the polishing was performed on a rotating wheel covered by polishing cloth using 6 m, 3 m and finished in 1 m diamond paste. The laser treated samples (T800-WC) were etched to reveal the microstructure. The etching solution used is Murakami s reagent (10 g potassium ferricyanide + 10 g sodium hydroxide +100 ml distilled water) [132]. The sample surface was dipped into the etching solution for 1 min then removed and quickly rinsed with water to remove all traces of the solution, and then rinsed with distilled water and dried by means of cold air blower. 107

108 CHAPTER 3 EXPERIMENTAL PROCEDURES Figure 3.4 Cross-sectioned of SEM samples mounted using conducting resins Scanning Electron Microscope (SEM/EDS) Scanning electron microscopy (SEM) is an important tool for studying of surface morphology and chemical composition. Two electron microscopes were used in this project and each microscope is equipped with Energy Dispersive Spectroscopy (EDS) to carry out the chemical analysis. SEM Philips XL-30-FEG (Field Emission Gun) equipped with EDS and EBSD (Electron Back-Scattered Diffraction) patterning analysis system EISS EVO-50 SEM The resolution in SEM is close to the bonding distance of atoms, and that results in useful information at high magnifications which reach up to Moreover, another feature of SEM is its large depth of field in the sample surface that produces a three-dimensional appearance of an object. The main component parts of SEM are illustrated in figure 3.5. The source of electrons can be a tungsten (W) a filament, lanthanum hexaboride (LaB6) or Schottky emitter. The highest resolution in images can be produced by a field emission electron gun. 108

109 CHAPTER 3 EXPERIMENTAL PROCEDURES Figure 3.5 Schematic of scanning electron microscope [133] These electrons are focused using electron lenses to a beam with a range of diameter between nm which affects the resolution, and then the focused electrons are scanned across the sample by the deflecting coils. The cavity of electrons tracks is under vacuum because electrons can be absorbed by air. The electron beam spot size might be of the order of 7 nm from W or LaB6 filament guns, whereas spot sizes in order 1 to 2 nm are produced by field emission guns. There are two factors affecting the resolution: the minimum electron spot size that can be focused on the sample, and how much current in the electron beam to extract a clear measurable signal. These measured signals are the electrons which are emitted by interaction between the incident focused electrons and sample surface. There are two types of emitted electrons: (1) secondary electrons (SE) produced from inelastic scattering, which have low energy (less than 10 ev). These electrons are emitted from near-surface layers (which is about 50 Å) of the sample. This operation mode provides images of surface morphology. (2) Backscattered electrons (BSE) produced from elastic scattering have high energy, approaching the energy of incident electrons. BSE mode provides image contrast as a function of composition, where higher atomic number material appears brighter than low atomic number. The secondary electrons which are created near to the surface can 109

110 CHAPTER 3 EXPERIMENTAL PROCEDURES escape to the vacuum; usually the escaping electrons are created within a small depth less than 2 nm beneath the surface (figure 3.6). Therefore, the SE images are representative of the surface structure and morphology of the sample. On the other side, the backscattered electrons with kinetic energy have sufficient energy to escape from the solid to the vacuum (figure 3.6), the distance in this case is of the order of tens or hundreds of nanometres. Therefore, the BSE technique detect the differences in atomic numbers on and below the surface sample [133, 134]. Figure 3.6 Schematic of electron beam interaction with material [135] X-ray diffraction (XRD) X-ray techniques are used to investigate the structure for crystals and chemical composition of materials. When an X-ray beam with wavelength λ is incident on a crystalline material at angle θ, the constructive interference (diffraction) occurs only when the distance travelled by the X ray beam reflected from the material (electrons) differs by an integer number n of wavelengths (nλ). In 1912 W. L. Bragg observed important relation which is named Bragg s Law: n 2d sin (3.1) Where: d is the distance between similar atomic successive planes in the material (the interatomic spacing) measured in angstroms. is the angle of diffraction. For practical reasons, a diffractometer measures 2θ. λ is the wavelength of the incident X-ray radiation (figure 3.7). 110

111 CHAPTER 3 EXPERIMENTAL PROCEDURES Figure 3.7 Waves reflected from successive planes of a crystal. Based on this phenomena, a diffractometer can be used to obtain a diffraction pattern for crystalline solid material, and by a diffraction pattern, one can identify unknown phases, where the intensities of the individual patterns are proportional to the concentrations of the phases present [136]. In this project, phase analysis were undertaken to identify the phases for coating materials as a powder and HVOF coatings before and after laser treatment. Powders were examined by Philips X-ray diffraction machine with CuK radiation, and the applied voltage of 50 Kv with a 40 ma current. The range of 2θ was from 5 o to 85 o with a step size 0.05 o and a step time of 2 s. HVOF coatings before and after laser treatment were examined by an X-ray diffractometer (Philips x pert) to identify the phases produced during HVOF thermal spray and laser melting. The XRD for HVOF coatings before and after laser treatment was performed with CuK radiation and the applied voltage was 45 Kv with a 40 ma current. The range of 2θ was from 5 o to 85 o with a step size 0.05 o and a step time of 30 s Surface profile The MicroXAM is an optical interferometer using white light. The device measures fields of view from microns to millimetres depending on the objective lens used. It splits a beam of light, then reflecting it off both the sample and reference surface 111

112 CHAPTER 3 EXPERIMENTAL PROCEDURES (see figure 3.8). The lights then recombine and interference patterns are detected by a high resolution camera. The information can be interpreted via a 3D representation of the surface. By using computer software, it provides precise surface measurements with advanced image processing, in terms of surface roughness, surface area and texture of surface [137]. Figure 3.8 MicroXAM surface mapping microscope [137] A MicroXAM profilometer was used to measure surface roughness of the HVOF coatings before and after laser treatments. Additionally, Developed Interfacial Area Ratio (Sdr) has been calculated. Sdr is the ratio of the increment of the interfacial area of a surface over the measured area (flat) x-y plane. In this project, four different areas (163 m 124 m) for each sample were measured, and then the average values for roughness and Sdr were calculated Porosity measurements Pores within HVOF coatings before and after laser melting were observed and captured using SEM. The pores were then characterised using Image Processing and Analysis in Java (ImageJ 1.43u) software developed by National Institute of Health, USA. 112

113 CHAPTER 3 EXPERIMENTAL PROCEDURES Electron probe micro-analyser (EPMA) A Cameca SX100 EPMA has been used for quantitative precise microchemical analysis. This technique is based on wavelength dispersive X-ray spectroscopy (WDS). Unlike EDS, the specific X-rays in WDS, instead of using solid state detectors collects and counts all of the emitted X-rays. WDS analysis results in a spectral resolution and sensitivity which is an order of magnitude better than EDS analysis. In comparison to EDS, WDS offers quantitative analysis, especially for light elements. It also provides better resolution of overlapping X-rays peaks for element identification and quantification. 3.5 Corrosion tests Corrosion behaviour of HVOF thermal spray coatings before and after laser treatment has been studied by immersion tests and by using electrochemical methods. The results from both tests are compared to determine the effects of laser surface treatment on corrosion behaviour Immersion test Immersion tests were performed in 3M H 2 SO 4 at ph ~ 1.27 at room temperature for different periods of time, including 24, 48, 72, 96 and 168 hours. The test was performed for all the HVOF coatings and for selective samples after laser treatment. The HVOF samples were washed in acetone using an ultrasonic bath and then dried. In order to make around 1 cm 2 of the surface area exposed to the H 2 SO 4 solution, the rest of the area was masked by lacquer then by wax to ensure good insulation as shown in figure 3.9. Each specimen was placed in a beaker containing 500 ml of H 2 SO 4. During the test, the level of the solution was monitored, and distilled water was added to replenish the evaporated solution as required. 113

114 CHAPTER 3 EXPERIMENTAL PROCEDURES Figure 3.9 Schematic diagram showing sample prepared for the immersion test After the test the specimens were washed in distilled water and then dried. For SEM observation, the specimens were sectioned, mounted in conductive resin, and then ground and polished by diamond paste up to 1 m Chemical analysis Inductivity Coupled Plasma-optical emission spectrometer (ICP-OES) technique has been used to analyse the H 2 SO 4 solution after 7 days of immersion testing for T800+43WC HVOF coating before and after laser treatment. The instrument used in this analysis is Perkin Elmer Optima 3300DV by Intertek ASG Sample preparation for polarisation and EIS tests Samples to be tested had copper wire welded to their sides by spark welding machine, to ensure good electric connection between the sample and the working electrode. The samples were cleaned with ethanol and dried before applying lacquer 45. By using a small brush, the sample was covered by lacquer 45, except for an area of ~ 1 cm 2 left exposed. Samples were kept in air to dry for about 3 hours, and then a subsequent layer was added to ensure better insulation. The copper wire was covered with plastic coating and exposed to hot air until the plastic adhered to the copper wire, such that there were no air gaps in order to avoid any contact between the solution and the wire (figure 3.10) 114

115 CHAPTER 3 EXPERIMENTAL PROCEDURES Figure 3.10 Sample prepared for polarisation or EIS test Polarisation The polarisation test was used to study the propensity of the HVOF to corrode both before and after laser treatment. This process has many advantages, for example only a few minutes or hours are required to perform a test, whereas the conventional methods such as weight loss measurements require days or more [29]. A ACM (Gill AC) potentiostat was used to apply a potential on a test sample to a selected value with respect to a reference electrode. The ACM instrument is computer-controlled, and the electrolyte solution is 0.5 M of H 2 SO 4 solution at ph ~ The solution was prepared by diluting the as-received 18 M sulphuric acid to obtain one litre (0.5 M concentration) of sulphuric acid, by using the dilution equation: One litre (0.5M) of sulphuric acid solution was prepared by 27.7 ml of 18 M concentrated sulphuric acid. The technique that was used in this test was cyclic polarisation, and before this test voltage and current versus time plots were produced for 1-hour to ascertain the open circuit potential (OCP) to stabilise the experiment. As shown in figures 3.12 and 3.13 the sweeping rate was 0.25 mv/s, and the scanned potential range was mv to mv. The complete electrochemical cell (figure 3.11) contains the test sample that is usually referred to as the working electrode, and the reference 115

116 CHAPTER 3 EXPERIMENTAL PROCEDURES electrode has constant electrochemical potential, and it is used as a reference point from which to measure the potential of a test sample. The reference electrode that was used in this test was a saturated calomel electrode (SCE). The other electrode used in the electrochemical cell is the auxiliary electrode (counter electrode), which is used to supply the required current and controlling the potential of the working electrode, where its main function is to ensure that current does not pass through the reference electrode. Figure 3.11 A schematic of experimental setup used in polarisation test In general, potentials measured in electrochemical tests represent a value of the driving force for the corrosion reaction to proceed, and the current provides a measure of rate at which reactions take place. The operation method of the potentiostat is that the potential difference between the reference and the sample is kept constant and equal to V c, and the instrument applies a current between the auxiliary electrode and the sample to fix the desired working electrode potential. The cell voltage or the driving voltage is the voltage difference between the auxiliary electrode and working electrode [138]. 116

117 CHAPTER 3 EXPERIMENTAL PROCEDURES Figure 3.12 Setup of open circuit potential Figure 3.13 Setup of polarisation experiment Electrochemical Impedance Spectroscopy (EIS) Test The EIS test has been performed using ACM (Gill AC) potentiostat. The experiments were performed at open circuit potential. The parameters for this test as shown in figure 3.14 were, a frequency range of Hz to Hz, with an amplitude of 10 mv, and reading per test of 50. These experiments were carried out every 1, 3, 6, 12, 24, 48 hours accordingly. A 0.5M H 2 SO 4 solution was used. 117

118 CHAPTER 3 EXPERIMENTAL PROCEDURES Figure 3.14 Setup of AC impedance experiment 3.6 Wear test Pin-On-Disc tester Pin-on-disc tests are used for evaluating sliding wear behaviour. The basic configuration of this test is shown in figure It consists of a pin in contact with a rotating disc. Either the pin or the disc can be the test piece of interest. The contact surface of the pin could be spherical, or flat. These experiments were conducted in accordance with ASTM G99, which specifies the use of a rounded pin, but does not specify specific values for the parameters, and therefore these are selected by the users to suit the application. The other variables of interest are the load, speed, materials, and size and shape for the pin [48]. Figure 3.15 Diagram of a pin on disc configuration wear test 118

119 CHAPTER 3 EXPERIMENTAL PROCEDURES The wear rate here has been measured using the pin on disc principle. The wear machine was provided by Teer Coatings Limited. The device consists of a 5 mm diameter WC-Co ball as a pin; the wear pin creates a circular wear track on the sample surface. As a result of sliding motion of the sample under the wear pin, a frictional force is generated and detected by the load cell, which is recorded by the computer wear machine. The load can be varied by adjusting the amount of weight been hanging at the end of the loading beam. The wear pin creates a circular wear track which can be controlled by offsetting the pin relative to the sample s centre of rotation. Loads can be changed from 10 N to 100 N, and the rotational speed and time of the test can be pre-selected. These input values are used to calculate the wear rate Wear test conditions The diameter of the wear track for all samples was 8 mm. The speed of rotation for all samples was 500 rpm. The load used for all samples was 40 N, and the testing time was varied from sample to sample, because of the different coatings that had been applied in each case (refer to table 3.2). Table 3.2 Conditions of wear test Sample Test time (min.) T800 (HVOF & Laser) 20 T WC (HVOF & Laser) 50 T WC (HVOF & Laser) 120 T WC (HVOF & Laser) 120 Parameters for all samples (Load= 40 N, Rotation speed= 500 rpm, diameter track= 8 mm) The temperature and relative humidity were at 22 o C and 40%, respectively. After the wear test is complete, the track dimensions (width (w) and depth (t)), were measured using an optical microscope (Polyvar MET). The width was measured 119

120 CHAPTER 3 EXPERIMENTAL PROCEDURES directly by the optical microscope, but to measure the depth of wear track, every wear scar was cut diagonally and then the cross section was ground and polished, for measurement using SEM. The worn volume and the wear rate were calculated as in Equations 3.3 and 3.4. Where: Number of revolutions = r.p.m. testing time in minutes w is track width, t is wear depth, and d is the diameter of the track (see figure 16). Figure 3.16 Parameters of wear track (a) track diameter and track width, (b) track depth 3.7 Microhardness measurement To measure the microhardness of the cross sections for both HVOF and laser treated samples, the preparation of the samples were prepared as follows. The 120

121 CHAPTER 3 EXPERIMENTAL PROCEDURES samples for HVOF and after laser treated were cut along the plane passing perpendicular to the sample surface. The cut was made using an automatic cutting machine (Struers Accutom-5). The specimens were mounted and ground and polished in the conventional way. After polishing the HVOF coating and the laser treated pool sections can be seen by the naked eye. Profiles of microhardness along the coating depth and laser treated depth were obtained using a Buehler tester, using a load of 100 g for all measurements. 121

122 Chapter 4 Optimisation of Laser Operating Conditions 4.1 Introduction This chapter presents the investigation of the influence of laser operating conditions on melting of various T800, T800+21WC, T800+43WC and T800+68WC HVOF coatings. It is aimed to establish laser operation windows to achieve crack-free, porosity-free HVOF coating melting layers. 4.2 Definition of the terms The laser operating conditions were determined by appropriately selecting laser power and scanning speed conditions that allow: A suitable melt depth within the HVOF coating to achieve partial melting of the HVOF coating (melt depth < HVOF coating thickness); or full melting of the HVOF coating (melt depth HVOF coating thickness) as shown in the schematic in figures 4.1 and 4.5. Melted layers to be free from porosity and cracks. Minimal dilution in the fully melted HVOF coating. Control of the melting of the hard carbide (WC) inside the melting pool, see figure

123 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS (a) Partial-melting of HVOF coating The HVOF coating in this state was partially melted as shown in the schematic in figure 4.1. The depth of laser melted layer < coating thickness. In this case, the interface between the coating and substrate is not affected (mechanical bonding). Figure 4.1 Schematic shown partially laser melting of HVOF coating Figure 4.2 shows surface views of a laser melted T800+21WC HVOF coating. The surface became smoother after laser treatment, and no cracks or porosity were observed on the surface under this magnification. From the observing of the melted surface, it is clear that a small amount of melting of the HVOF coating occurred. Figure 4.2 Overlapping tracks of laser treated T800+21WC HVOF coating showing surface of partially melting of HVOF coating Figures 4.3 and 4.4 show partially melted HVOF coating for T800 and T800+21WC respectively. Table 4.2 shows the laser parameters used in each case. 123

124 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS Figure 4.3 SEM cross-section image of HVOF T800 (a) partially laser melting T800 HVOF coating (b) high magnification of melted T800 HVOF layer Figure 4.4 SEM cross-section image of HVOF T800+21WC (a) partial laser melting of T800+21WC HVOF coating (b) high magnification of melted T800+21WC HVOF layer (b) Full-melting of HVOF coating Where the HVOF coating melted completely, the melting depth the coating thickness. The full melting of the HVOF coating could be with very small dilution if the interface between the coating and the substrate has been melted or without dilution if there is no melting of the substrate or the interface between the HVOF coating and the substrate, as demonstrated in the schematic in figure 4.5 (a). Also, full melting could be with dilution by the melting of the interface between the HVOF coating and substrate and some of the substrate. In this case the type of bonding between the coating and interface will change from mechanical bonding to fusion bonding. The full melting eliminates the defects in the interface between the coating and substrate as shown in the schematic in figure 4.5 (b), and figures 4.7,

125 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS (a) (b) Figure 4.5 Schematic shown fully laser melting of HVOF coating (a) melting of HVOF coating only (no dilution) (b) melting of substrate (dilution) Figure 4.6 shows extensive melting of the HVOF coating. No oxidation has been observed on the surface after laser melting due to Ar gas shielding during the laser melting process. Figure 4.6 Overlapping tracks of the laser-treated the T800+43WC HVOF coating, showing surface of large melting of HVOF+43WC coating In this research full melted of HVOF coating with small dilution was used in T800+43WC and T800+68WC HVOF coatings. 125

126 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS Figure 4.7 SEM cross-section image of HVOF coating T800+43WC fully laser melted (a) without dilution (b) with dilution Figure 4.8 SEM cross-section image of HVOF coating T800+68WC fully laser melted (a) without dilution (b) with dilution (c) Partial-melting of WC The results of the partial melting of the WC particles by the laser, is shown in the schematic in figure 4.9. Figure 4.9 Schematic illustration of partially and full melting of WC Figure 4.10 shows the cross-section of the as received T800+21WC HVOF coating. The carbide particles have a bright white colour and are polygonal in shape. Due to the lower melting point of T800 ~ 1352 o C [139] compared to the melting point of 126

127 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS WC ~ 2785 o C [140], under some conditions the T800 can be melted while the WC particles remain unmelted or partially melted. When the melt pool temperature reaches around the melting point of WC (T~T m, WC ), the T800 can be melted fully and WC melted partially. The melting of WC starts from the particle edges. See figures 4.11 to Figure 4.10 SEM cross-section image of HVOF T800+21WC (a) HVOF coating (b) high magnification of the coating Figure 4.11 SEM cross-section image of HVOF T800+21WC (a) partially laser melted WC near the surface (b) partially melted WC particles and fully melted T

128 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS Figure 4.12 SEM cross-section image of HVOF T800+43WC (a) partially laser melted WC (b) partially melted WC particles and fully melted T800 Figure 4.13 SEM cross-section image of HVOF T800+68WC (a) partially laser melted WC (b) partially melted WC particles and fully melted T800 To obtain optimum operating conditions for the laser treatment, various different laser conditions have been used by changing scanning speed and power (see table 4.1). Inappropriate laser parameters in processing can lead to defects in laser melted zone such as porosity, cracks and high dilution from the substrate, which in turn reduce the beneficial properties and the function of the HVOF coating. 4.3 Formation of porosity within laser melted surface Porosity is the presence of gas pores or pockets in the melted zone. It is caused by the gases in the melted pool which are released during cooling due to reduced solubility in solids. This is caused mostly when the gases have amounts more than their solubility limit in the solid [141], and when there is not sufficient time for air bubbles to escape from melted pool. The source of the gas could be from porosity present in the HVOF coating or from the reaction of metallic oxides with carbon to form CO and CO 2 [142, 143]. In this work, as shown in figure 4.14, there was a 128

129 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS substantial amount of large diameter porosity formed within the laser melted layer of T800+43WC and T800+68WC HVOF coating. When laser treatment was carried out using relatively high scanning speed (figure 4.14 (a)), vertical cracks were observed in the melting layer as well as large diameter porosity near to the surface. In lower scan speed (20 mm/s) the melting depth was higher, however still there was large diameter porosity (figure 4.14 (b)). The formation of porosity might be due to the faster cooling rates resulting from these high scanning speeds (interaction time is very low), which results in less time for the release of the gases within the HVOF coatings. Consequently, these become trapped in the melt zone instead. Furthermore, the dissolved carbides have an effect of hindering the escape of bubbles to the surface of the molten pool [144]. Thus, the air bubbles are captured as a result of dissolved carbides in T800+43WC and T800+68WC. However, at the same power (800W), and with reducing the scanning speed to given level (high interaction time), there would be sufficient time to reduce the porosity. Figure 4.15 shows a low porosity in the laser-melted layer of T800+43WC and T800+68WC HVOF coating after using low scanning speeds of 5 mm/s and 3 mm/s respectively at 800 W. In T800 HVOF and T800+21WC laser treated, the porosity was lower, and it has occurred only in relatively high speed scans (figure 4.16). The same conclusion has been reached in previous work by Tuominen et al. [10]. The authors have studied the corrosion behaviour of HVOF sprayed Inconel 625 by using a high power Nd:YAG laser. Their study showed that the inappropriate laser parameters cause porosity in laser melted layer. Previous work [145] has studied the effects of laser parameters, such as scanning speed on the porosity of HVOF thermally sprayed WC-CrC-Ni coatings. The results of this work suggested a decrease in porosity of the laser treated coating was a result of the laser scanning speed reduction. Goswami et al. [146] studied laser surface alloying chromium on mild steel and molybdenum on stainless steel surfaces, using a Nd:YAG laser. This work has yielded a similar result with a higher scan speed producing a higher cooling rate, with greater porosity. 129

130 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS Figure 4.14 porosity in laser treated area (a) porosity near the surface and a vertical crack in T800+43WC HVOF laser-treated (800 W, 40 mm/sec) (b) porosity in T800+68WC HVOF laser treated (800 W, 20 mm/sec) Porosity can be prevented or reduced by proper selection of scanning speeds and power density; very high scanning speeds should be avoided [10, 121]. In addition, high power should be avoided to avoid a large melting depth and a large amount of dilution from the substrate (as shown in figure 4.17). Figure 4.15 low porosity in the laser treated area (a) T800+43WC HVOF, lasertreated (800 W, 5 mm/s) (b) T800+68WC HVOF, laser treated (800 W, 3 mm/s) 130

131 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS Figure 4.16 porosity in the laser treated area (a) T800 HVOF, laser treated area (600 W, 20 mm/s) (b) T800+21WC HVOF, laser treated (800 W, 40 mm/s) Figure 4.17 low porosity in the laser treated area (a) T800+43WC HVOF, laser treated (550 W, 3 mm/s) (b) T800+68WC HVOF, laser treated (500 W, 3 mm/s) 4.4 Formation of cracks within laser melted surface T800 has a large volume fraction (~ 60%) of a hard, intermetallic Laves phase in a much softer matrix (figure 4.18). T800 is also hard, but has little capacity for plastic flow and has low fracture strength due to the hard brittle nature of the Laves phase [139, ]. The laser parameters (scanning speed and power) were optimized for each type of HVOF coating to minimise cracks within the treated zone in the T800 HVOF coating, and to produce crack-free laser-melted layer for T800+WC HVOF coatings. The T800 HVOF coating showed high sensitivity to cracking, particularly in high scanning speed (figure 4.19(b)). T800 HVOF had some vertical cracks after the application of a relatively low power and scan speed (as shown in figure 4.19(a)). T800 and T800+21WC HVOF show higher sensitivity to cracks than T800+43WC and T800+68WC HVOF. Additionally, it has been 131

132 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS observed that more cracks were produced by increasing the melt depth. The sensitivity of T800 HVOF to cracking can be seen in figure Figures 4.24 to 4.27 show that clearly T800 alloy has the narrowest range of laser permissible operation window. This behaviour might because of high percentage of Laves phase in these alloys. Figure 4.18 SEM microstructure of the Laves phase in thet800 HVOF coating after laser treatment Figure 4.19 vertical crack in T800 HVOF laser treated (a) small crack at (600W, 5 mm/s) (b) big crack with high dilution at (800W, 10 mm/s) It was found that the optimal laser parameters for T800 are a power of 500W and at scanning speed of 3mm/s; however, these parameters sometimes could cause very small cracks as can be seen in figure

133 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS Figure 4.20 a small vertical crack in T800 HVOF laser treated with a partiallymelted coating (500W, 3 mm/s) The other T800+WC HVOF coatings are less sensitive to cracks, in particular T800+43WC and T800+86WC as a result of reduction of a brittle phase (Laves). It can be observed in figures (21-23) that cracks in the treated zone in all HVOF coatings increase with an increase in the scanning speed. Figure 4.21 a vertical crack in a T800+21WC HVOF laser-treated surface (a) a small crack at (800W, 5 mm/s) (b) a large crack and porosity at laser parameters (600W, 40 mm/s) Figure 4.22 a vertical crack and porosity in a T800+43WC HVOF laser-treated surface, with a partially-melted coating (800W, 80 mm/s) 133

134 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS Figure 4.23 a vertical crack and porosity in a T800+68WC HVOF laser-treated surface with a partially-melted coating (800W, 20 mm/s) The main factor responsible for cracking during the solidification phase is thermal stresses. High thermal gradients and high scan speeds could cause higher thermal stress as a result of the larger cooling and solidifying rate [150]. The stresses will build up during the cooling stage, and if the thermal stress is higher than the strength of the coating material, cracks will occur in the melted layer [151]. It is reported [146, 152] that the misfit between the HVOF coating and the substrate during the thermal expansion or thermal shrinkage in the melted pool could cause residual stresses in the melted layer. The results in this research further demonstrate that the integrity of the laser treated zone is largely dependent on laser scan speed. In all HVOF coatings (except T800 which still contains small cracks), a crack-free zone could be achieved at the lower scanning speed of 3 mm/s, and the faster scanning speed the greater the number and the size of the cracks and pores created. Accordingly, to decrease the number of cracks, slower scan speed has been used, but concurrently, the laser power must be relatively low to avoid high melting depth and consequently high dilution between the coating and substrate. The same behaviour has been described by Pokhmurs Ka [153] using a CO 2 laser to treat electric arc spray Fe-Cr-B-Al thermal coatings. Also Tuominen et al. [154] have studied laser-remelted nickel-chromium HVOF by a Nd-YAG laser. It has been concluded that laser parameters which produce high specific energy should be avoided; hence iron dilution from the substrate has a negative effect on the corrosion properties of the coating. Additionally, it has been observed that high scanning speeds should be avoided in laser remelting to avoid high solidification rates, which cause more cracks in the melted layer. 134

135 Ratio (melt depth/coating thickness) CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS 4.5 Operating windows for laser treatment The figures 4.24 to 4.27 show the variation of melt depth ratio (melt depth/coating thickness) with scanning speed at various levels of laser powers. The melt depths range from 30 m m. It is clear from the graphs, as expected, that the melt depth for all samples are a function of the scanning speed of the laser. This is where, high scanning speed result in low melt depths due to a short interaction time between the laser and the surface, and the melt depths increased by decreasing the scanning speed T800 No cracks- No porosity area 300 W 350 W 400 W 600 W 800 W 1000 W Cracks Porosity Scanning speed (mm/sec) Figure 4.24 The variation of the ratio of melting depth/coating thickness with scanning speed for different laser powers for a T800 HVOF coating 135

136 Ratio (melt depth/coating thickness) Ratio (melt depth/coating thickness) CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS T WC No cracks- No porosity area 300 W 350 W 400 W 600 W 800 W 1000 W Scanning speed (mm/sec) Figure 4.25 The variation of the ratio of melting depth/coating thickness with scanning speed for different laser powers for a T800+21WC HVOF coating T WC No cracks- No porosity area 350 w 400 w 600 w 800 w 1000 w Scanning speed (mm/sec) Figure 4.26 The variation of the ratio of melting depth/coating thickness with scanning speed for different laser powers for a T800+43WC HVOF coating 136

137 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS Figure 4.27 The variation of the ratio of melting depth/coating thickness with scanning speed for different laser powers for a T800+68WC HVOF coating The previous figures illustrate, as expected, for that a fixed scanning speed, higher laser powers result in large melt depths. Furthermore, it is have been demonstrated that the narrowest operating window was for the T800 HVOF coating; this is may be due to a high percentage of very brittle Laves phase present in this coating, as discussed in the previous section. 137

138 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS Table 4.1 A laser operating conditions used for various HVOF coating with observations T800 Track no. Power (W) Scanning speed (mm/s) Surface and cross-section observations Track Low-melting, no cracks, no porosity Track Low-melting, small cracks, no porosity Track Low-melting, small cracks, no porosity Track No-melting Track Partially-melting, one crack, no porosity Track Low-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity Track Low-melting, cracks, no porosity Track No-melting Track Partially-melting, one crack, no porosity Track Partially-melting, one crack, no porosity Track Low-melting, no cracks, no porosity Track Fully -melting, one crack, no porosity Track Partially-melting, no cracks, no porosity Track Partially-melting, no cracks, no porosity Track Fully -melting, no cracks, no porosity Track Fully -melting, cracks, no porosity Track Partially-melting, cracks, no porosity Track Partially-melting, many cracks, no porosity Track Partially-melting, many cracks, no porosity Track Low-melting, many cracks, no porosity Track Low-melting, many cracks, no porosity Track Fully -melting, no cracks, no porosity Track Fully -melting, cracks, no porosity Track Fully -melting, cracks, no porosity Track Partially-melting, many cracks, no porosity Track Fully -melting, cracks, no porosity Track Fully-melting, cracks, no porosity Track Partially-melting, many cracks, no porosity Track Partially-melting, many cracks, no porosity 138

139 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS T WC Track no. Power (W) Scanning speed (mm/s) Surface and cross-section observation Track Low-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity Track Low-melting, cracks, no porosity Track No melting Track partially-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity Track Low-melting, cracks, no porosity Track No melting Track partially-melting, no cracks, no porosity Track partially-melting, small cracks, no porosity Track Low-melting, small cracks, no porosity Track Low-melting, small cracks, no porosity Track Low-melting, small cracks, no porosity Track partially-melting, no cracks, no porosity Track Fully melting, small cracks, no porosity Track partially-melting, no cracks, no porosity Track Fully melting, small cracks, no porosity Track Fully melting, small cracks, no porosity Track Oxidation problem in the sample Track partially-melting, small cracks, no porosity Track Fully melting, small cracks, no porosity Track partially-melting, no cracks, no porosity Track (repeated) Fully melting, one crack, no porosity Track Partially-melting, cracks, no porosity Track Low-melting, cracks, no porosity Track Low-melting, cracks, no porosity Track Fully-melting, cracks, no porosity Track Fully-melting, cracks, no porosity Track Fully-melting, cracks, no porosity Track Fully-melting, cracks, no porosity Track Partially-melting, cracks, no porosity Track Low-melting, cracks, no porosity Track Fully-melting, small cracks, no porosity Track Fully-melting, cracks, no porosity Track Oxidation problem in the sample Track Partially-melting, cracks, no porosity Track Partially-melting, cracks, no porosity Track (repeated) Fully-melting, cracks, no porosity Track Fully-melting, cracks, no porosity 139

140 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS T WC Track no. Power (W) Scanning speed (mm/s) Surface and cross-section observation Track Low-melting, no cracks, no porosity and lowmelting of WC Track Low-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity Track Partially-melting, no cracks, no porosity and partially melting of WC Track Low-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity Track Low-melting, small cracks, no porosity Track Fully-melting, no cracks, no porosity and partially- melting of WC Track Fully-melting, no cracks, no porosity and partially- melting of WC Track Fully-melting, no cracks, no porosity and fullymelting of WC Track Fully-melting, no cracks, no porosity and partially-melting of WC Track Partially-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity Track Low-melting, cracks, no porosity Track Fully-melting, cracks, no porosity and fullymelting of WC Track Fully-melting, cracks, no porosity Track Fully-melting, cracks, no porosity Track Partially-melting, cracks, no porosity Track Low-melting, cracks, no porosity Track Low-melting, cracks, no porosity Track Low-melting, cracks, no porosity Track Fully-melting, cracks, no porosity Track Problem in laser operation Track (repeated) Fully-melting, cracks, no porosity Track Fully-melting, cracks, no porosity Track Partially-melting, cracks, no porosity Track Partially-melting, cracks, no porosity Track Partially-melting, cracks, no porosity 140

141 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS T WC Track no. Power (W) Scanning speed (mm/s) Surface and cross-section observation Track Low-melting, no cracks, no porosity and lowmelting of WC Track Low-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity and lowmelting of WC Track Low-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity Track Partially-melting, no cracks, no porosity and partially-melting of WC Track Low-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity Track Fully-melting, no cracks, no porosity and partially-melting of WC Track Fully-melting, no cracks, no porosity and fullymelting of WC Track Fully-melting, no cracks, no porosity Track Problem in Laser Track Low-melting, no cracks, no porosity Track (repeated) Fully-melting, no cracks, no porosity Track Low-melting, no cracks, no porosity Track Fully-melting, no cracks, no porosity Track Fully-melting, cracks, no porosity Track Fully-melting, cracks, no porosity Track Problem in Laser Track (repeated) Partially-melting, cracks, porosity Track Low-melting, cracks, porosity Track Low-melting, cracks, porosity Track Low-melting, cracks, porosity Track Low-melting, cracks, porosity Track Fully-melting, no cracks, no porosity Track Fully-melting, no cracks, no porosity Track Fully-melting, cracks, no porosity Track Problem in Laser Track (repeated) Fully-melting, cracks, porosity Track Partially-melting, cracks, porosity Track (repeated) Fully-melting, cracks, porosity Track (repeated) Partially-melting, cracks, porosity Track Partially-melting, cracks, porosity Track Low-melting, cracks, porosity Track Low-melting, cracks, porosity 141

142 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS Table 4.2 summarises the laser operating conditions for the laser-treated coatings that were selected to represent partially-melting, fully-melting without defects for further characterisation and various tests in this project. For T800+43%WC and T800+68%WC, the coatings were fully-melted and crack-free with low dilution. For T800 and T800+21WC, the coatings were partially-melted and crack-free. Table 4.2 Laser operating conditions and coating characteristics of the selected coatings Coating Power Scanning Beam size (W) speed (mm/s) (mm mm) T T WC Treated coating characteristics Partially melted HVOF, some cracks, (No dilution) Partially melted HVOF, Partially melted WC, Cracks free, (No dilution) T WC T WC Fully melted HVOF, Partially melted WC, Cracks free, (No dilution) Fully melted HVOF, Partially melted WC, Cracks free, (No dilution) In conclusion, laser operating windows for various HVOF coatings have been established, by formation of porosity-free and crack-free coatings in specific laser operation parameters. The formation of cracks under some laser processing conditions was due to thermal stresses. T800 coating was highly prone to cracks due to the hard and brittle nature of the Laves phase. Furthermore, formation of porosity under some laser processing conditions was due to trapped gas in the melted zones that often occurred at high scanning velocities that provided less time for gas to release. Different coating materials presented different optimised laser conditions to produce laser melted layers free from defects, when the laser beam spot size was fixed as 3.5 mm 2.5 mm at the focal position. 142

143 CHAPTER 4 OPTIMISATION OF LASER OPERATING CONDITIONS 4.6 Summary Laser operating windows for various HVOF coatings have been established, indicating the optimised laser operating conditions for formation of porosity-free and crack-free coatings. A melt depth of 100 m to 210 m for various HVOF coatings can be obtained by controlling laser operating conditions. Low dilution of the full laser treated HVOF coatings can be also achieved. Formation of cracks under some laser processing conditions was due to thermal stresses. T800 coating was highly prone to cracks due to the hard and brittle nature of the Laves phase. Formation of porosity under some laser processing conditions was due to trapped gas in the melted zones that often occurred at high scanning velocities that provided less time for the gas to release. Different coating materials presented different optimised laser conditions to produce laser melted layers free from defects, when the laser beam spot size was fixed as 3.5 mm 2.5 mm at the focal position. Due to the time limitation of the project, only the following laser processing parameters were selected to produce laser-melted coatings which were crack-free, porosity-free, and with or without low dilution. These coatings were further characterised and evaluated in terms of corrosion and wear performance: T800 (Partial melting of HVOF coating): 500 W, 3 mm/s. T WC (Partial melting of HVOF coating): 450 W, 2 mm/s. T WC (Full melting of HVOF coating): 550 W, 3 mm/s. T WC (Full melting of HVOF coating); 500W, 3 mm/s. 143

144 Chapter 5 Materials Characterisation 5.1 Introduction This chapter presents the materials characterisation of the HVOF coatings before and after laser treatment, in terms of morphology, phases and elemental distribution. Optical microscopy, scanning electron microscopy (SEM)/energydispersive x-ray spectroscopy (EDX), electron probe micro-analysis (EPMA), X- ray diffraction (XRD), atomic force microscopy (AFM), and white-light interferometery have been applied for the above characterisation. 5.2 Powders Various powders containing Tribaloy 800 (T800) and T800 with variable percentages of WC have been used for the production of HVOF coatings Morphological observation The T800 powder is almost spherical in shape, with a smooth surface, as shown in Figure 5.1. The range of the powder size is from 10 to 40 m. SEM micrographs of the T WC powders are shown in Figure 5.2, presenting the porous nature of an agglomerated powder. This morphology was formed during the powder manufacturing process. It was believed [58] that such a porous nature promotes more homogeneity of thermal sprayed coatings, because it provides better heat distribution inside the powder particles during the spraying process. Figure 5.2 also shows two different types of powders consisting of WC particles and T800, respectively. One is smooth T800 powder particles while the other is porous WC powder particles with binder. The difference makes them easy to distinguish. 144

145 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.1 SEM morphology of T800 powder Figure 5.2 SEM micrographs of the T WC powder 145

146 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.3 illustrates the characteristic of an agglomerated T WC and T WC powder mixtures. The size range of WC particles is within m, while the size range of T800 particles is within m. Figure 5.3 SEM micrographs of (a) T WC (b) T WC powders 146

147 CHAPTER 5 MATERIAL CHARACTERISATION T800 T800 WC (a) (b) T800 WC WC (c) (d) T800 Figure 5.4 Mapping images: (a) T800 powder (b) T800+21WC powder (c) T800+43WC powder (d) T800+68WC powder XRD analysis Figure 5.5 illustrates the X-ray diffraction pattern of the T800 feedstock powder. The detected phases are Co 3 Mo 2 Si, Co 2 MoCr and Co. The Co 3 Mo 2 Si phase represents Laves phase, in which the Si replaced the Cr in its original structure of Laves phase of Co 2 MoCr. Furthermore, the Co phase in fcc form represents the solid-solution-of-cobalt-[155]. 147

148 Intensity (arb) Intensity (arb) CHAPTER 5 MATERIAL CHARACTERISATION T800 powder Co Co 3 Mo 2 Si Co 2 MoCr Angle Figure 5.5 XRD pattern of the T800 stock powder Figures 5.6 to 5.8 show X-ray diffraction patterns of T800+21WC, T800+43WC, and T800+68WC stock powders respectively. The analysis confirmed the existence of four phases in all the powders. T800+21WC and T800+43WC have almost the same phases which are WC, Co, W 2 Co 4 C and Co 3 Mo 2 Si. Figures 5.6 and 5.7 show that the WC phase has higher peak intensity in T800+43WC compared with T800+21WC, reflecting the percentage difference of WC in these powders T800+21WC powder WC Co W 2 Co 4 C Co3 Mo2 Si Angle(2 ) Figure 5.6 XRD pattern of the T800+21WC stock powder 148

149 Intensity (arb) Intensity (arab) CHAPTER 5 MATERIAL CHARACTERISATION T800+43WC powder WC Co W 2 Co 4 C Co 3 Mo 2 Si Angle (2 ) Figure 5.7 XRD pattern of the T800+43WC stock powder Due to a low volume fraction of a hard, intermetallic laves phase (Co 3 Mo 2 Si), this phase was not detected in the X-ray diffraction pattern of the T800+68WC powder as shown in figure 5.8. However, the other three phases of WC, Co, W 2 Co 4 C can be seen T800+68WC powder WC Co W 2 Co 4 C Angle (2 ) Figure 5.8 XRD pattern of the T800+68WC stock powder Furthermore, the intensity of Co peaks become lower, while the intensity of WC peaks becomes higher, as a result of increased percentage of WC. 149

150 CHAPTER 5 MATERIAL CHARACTERISATION 5.3 As-sprayed coating The HVOF coatings are deposited by melting, partially melting and unmelting of powder particles which are sprayed at high velocity on the substrate (316L stainless steel), using a JP5000 gun. Lamellar microstructure is built-up by subsequent impact and rapid solidification of those molten or semi-molten particles. As a result of the process, pores are inevitably formed and oxide inclusions are probably present in typical HVOF coatings [156]. Characteristics of the coatings in terms of density and porosity strongly depend on coating material and spraying parameters [157, 158] Surface views of HVOF coatings Figures 5.9 to 5.12 illustrate the surface view of the HVOF coatings with various WC contents. Figure 5.9 (a) shows the presence of micro-cracks on the T800 coating surface. In addition, there are pores and gaps between powder particles, and the coating surface displays high roughness. Figure 5.9 SEM micrographs of surface view of T800 HVOF coatings with (a) low magnification (b) high magnification 150

151 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.10 SEM micrographs of surface view of T800+21WC HVOF coatings with (a) low magnification and (b) high magnification Figure 5.11 SEM micrographs of surface view of T800+43WC HVOF coatings with (a) low magnification and (b) high magnification Figure 5.12 SEM micrographs of surface view of T800+68WC HVOF coatings with (a) low magnification and (b) high magnification In order to investigate the surface morphology in more detail, the microxam mapping was used to obtain (2D, 3D) profiles of the coating surfaces. In addition, based on the profiles, the surface roughness (Ra) and surface area ratio (Sdr) are calculated. The surface area ratio (Sdr) is the ratio of the surface (taking the z 151

152 CHAPTER 5 MATERIAL CHARACTERISATION height into account) and the area of the flat x, y plane [159]. Figures 5.13 (a) and (b) show 2D and 3D profiles for different areas of the T800 HVOF surface. It shows that different areas of the same sample present significantly different surface profiles indicating the high inhomogeneity of the T800 HVOF surface. Table 5.1 displays the values of average roughness (Ra) and the average of surface area ratio (Sdr) based on four measurements. Similarly, 2D and 3D surface profiles of the other three coatings with WC are shown in Figures The values of surface roughness and surface area ratios are presented in Table 5.1. In addition, the comparison of surface roughness and surface area ratios of various HVOF coatings are presented in Figures 5.17 and The results show that the surface roughness and surface area ratios of T800+21WC coating are similar to those of T800 coating. However, the increase in the content of WC results in less rough coatings. The T800+68WC shows the lowest roughness of Ra ~ 4.49 µm, and the lowest surface area ratio of Sdr ~ 153%. 152

153 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.13 Surface profile (a) 2D and (b) 3D profiles of four different areas on T800 HVOF surface 153

154 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.14 Surface profile (a) 2D and (b) 3D profiles of four different areas on T800+21WC HVOF surface 154

155 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.15 Surface profile (a) 2D and (b) 3D profiles of four different areas on T800+43WC HVOF surface 155

156 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.16 Surface profile (a) 2D and (b) 3D profiles of four different areas on T800+68WC HVOF surface 156

157 S dr % R a ( m) CHAPTER 5 MATERIAL CHARACTERISATION Table 5.1 Surface roughness and surface area ratios of various HVOF coatings. HVOF coating Average roughness R a, µm Average surface area ratio S dr, % T T800+21WC T800+43WC T800+68WC T800 T800+21WC T800+43WC T800+68WC Figure 5.17 Comparison of surface roughness of various HVOF coatings T800 T800+21WC T800+43WC T800+68WC Figure 5.18 Comparison of surface area ratios of various HVOF coatings 157

158 CHAPTER 5 MATERIAL CHARACTERISATION Cross sectional view of HVOF coatings Figure 5.19 shows a typical cross sectional view of T800 HVOF coating, indicating the presence of porosity. The details of the coating microstructure can be seen in figure It shows that the T800 HVOF coating can be characterised as a lamellar structure with splats and distinct splat boundaries. The interface between the coating and substrate is mechanical bonding with some defects like pores appearing at the interface. The coating structure contains Laves phase (white phase) surrounded by the cobalt solid solution (gray phase). Figure 5.19 SEM micrograph of cross section of T800 HVOF coating Figure 5.20 SEM micrographs of T800 HVOF coating at the interface between the coating and substrate (a) and typical microstructure (b) 158

159 CHAPTER 5 MATERIAL CHARACTERISATION Table 5.2 presents the EDX chemical analysis of the two areas of S1 and S2, as marked in Figure 5.21, of the T800 HVOF coating. It appears that the region close to the surface contains higher oxygen than the inner region. Figure 5.21 SEM image of cross-section of T800 HVOF Table 5.2 EDX chemical analysis of T800 HVOF coating, wt.%. Element O Si Cr Fe Co Mo Area S Area S Average SEM micrograph with higher magnification of T800 HVOF coating is shown in figure 5.22, further indicating the presence of splats and splats boundaries that are of importance to its corrosion behaviour to be discussed in Chapter

160 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.22 SEM micrograph of T800 HVOF microstructure Chemical compositions of the phases by EDX, marked in T800 HVOF coating are shown in Figure It confirms that phase 1 is Laves and phase 2 is cobalt based solid solution surrounding the Laves phase. 160

161 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.23 EDX results of various phases in T800 HVOF coating (1) Laves phase (2) Eutectic Laves phase + Co solid solution (3) Co solid solution Figures 5.24 to 5.31 show the SEM micrographs of cross sections of T800 coatings with various WC contents. From these micrographs, it can be seen that there are some different phases presented in the coatings, including white phase, light-gray phase and dark-gray phase. In addition, the defects can be seen for all the coatings in the high magnification images. The coating thickness has been measured on the SEM micrographs. Average values of three measurements were obtained for each type of coating and are shown in Table 5.3. Figure 5.24 SEM micrograph of cross section of T800+21WC HVOF coating. From Figures 5.25 (a), 5.29 (a) and 5.31 (a), the mechanical bonding between the HVOF coating and the substrate can be clearly seen. Some other defects along the interfaces are also evident. The black voids at the interface could be formed during the deposition process due to surface contaminants that are burnt to become gases and trapped between the coating and the substrate [160]. Islands of sharp corners of crushed carbides have appeared in the T800+WC HVOF coatings (figures 5.24 to 161

162 CHAPTER 5 MATERIAL CHARACTERISATION 5.31), suggesting that the WC are not fully melted while only the binder alloys are melted during spraying process. The size of the carbides varies between 100 nm and 2 µm (Figure 5.27). Figure 5.25 SEM micrographs of T800+21WC HVOF coating at the interface between coating and substrate (a) and with high magnification (b) Figure 5.26 SEM micrograph of cross section of T800+21WC HVOF coating with high magnification Figure 5.27 SEM micrograph of cross section of T800+43WC HVOF coating with high magnification 162

163 CHAPTER 5 MATERIAL CHARACTERISATION The higher magnification SEM micrographs of the cross sections (Figures 5.25 (b) and 5.29 (b)) displayed some typical defects of thermal sprayed coatings, such as porosity and splat boundary separation. Figure 5.28 SEM micrograph of cross section of T800+43WC HVOF coating Figure 5.29 SEM micrographs of T800+43WC HVOF coatings at interface between coating and substrate (a) and with high ma.gnification (b) Figure 5.30 SEM micrograph of cross section for T800+68WC HVOF coating 163

164 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.31 SEM micrographs of T800+68WC HVOF coating at interface between coating and substrate (a) and with high magnification (b). Table 5.3 Coating thickness measured Coating Thickness (µm) Standard deviation T800 ~ T800+21WC ~ T800+43WC ~ T800+68WC ~ Figure 5.32 presents the results of EDX analysis of chemical composition of different phases, as described earlier. From the results, it is believed that the white phase is WC particles, the dark phase is T800 matrix and the gray phase is CoCr binder surrounding the WC particles. 164

165 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.32 EDX results of various phases in T800+WC HVOF coating (1) White phase, (2) dark matrix and (3) grey phase Further chemical analysis by EDX line scan was preformed crossing two clusters of the WC particles. The line profiles obtained for T800+68WC HVOF coating are displayed in figure 5.33 (b). Figure 5.33 (a) SEM micrograph of T800+68WC coating and (b) EDX line scan analysis of the line marked in (a) In addition, EDX analysis of the splat boundary is illustrated in Figure 5.34 (1). Compared with the point close to splat boundary (Figure 5.34 (2)), it was noticed that there is a small increase in oxygen at the inter-splat boundary. Furthermore, the EDX line scan across the splat boundary as shown in Figure 5.35 confirms the increase of oxygen. This is believed to be the oxidation stringers, in which the molten alloy particles during flight are exposed to air, and oxidation takes place. Therefore, the oxidation products, i.e. oxide inclusions, are formed between the splats. 165

166 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.34 EDX analyses at splat boundary (1) and near splat boundary (2) for T800 HVOF coating. Figure 5.35 Line scan for oxygen through splat boundary for T800 HVOF coating In order to further investigate the elemental distribution of the HVOF coatings, EPMA was used to obtain elemental mapping. Figure 5.36 illustrates the elemental mapping images of T800 HVOF coating, showing that oxygen is rich along the 166

167 CHAPTER 5 MATERIAL CHARACTERISATION inter-splat boundaries. In addition, oxygen is also enriched in specific regions at the coating/substrate interface. Figure 5.36 EPMA mapping of T800 HVOF coating Figure 5.37 shows the elemental mapping of the T800+43WC HVOF coating. As expected, the coating has a non-uniform composition in which various elements are concentrated at specific locations. Three different regions were observed including 1) the matrix area (T800) contains Co, Mo and Cr, 2) the binder areas surrounding the WC particles, are Co- and Cr-rich, and 3) WC particles. In addition, the T800+43WC HVOF coating has shown the presence of oxygen, especially along the inter-splat boundaries and at the interface between the coating and the substrate. Figure 5.37 EPMA mapping of T800+43WC HVOF coating 167

168 Intensity (arb) CHAPTER 5 MATERIAL CHARACTERISATION XRD analysis of HVOF coatings The XRD patterns of various HVOF coatings are shown in figures 5.38 to Compared with the same coating materials in powder form presented earlier in figures 5.5 to 5.8, the same phases were found in each individual coating, indicating that there was no detectable phase transformation during the spraying process. The presence of W 2 C in HVOF coatings due to undesired decarburisation which has been reported elsewhere [161, 162], was not observed in the coatings studied in this work. The occurrence of decarburisation of WC has been reported to cause loss of hard phase and result in metal matrix embrittlement, which consequently decreases the mechanical properties and corrosion resistance [161]. In addition the free carbon which is created by the decarburisation process may interact with oxygen to produce CO 2 gas that can be trapped in the coating to form porosity [162] T800 HVOF Co Co 3 Mo 2 Si Co 2 MoCr Angle Figure 5.38 XRD pattern of T800 HVOF coating 168

169 Intensity (arb) CHAPTER 5 MATERIAL CHARACTERISATION 6000 T800+21WC HVOF WC Co W 2 Co 4 C Co3 Mo2 Si Angle Figure 5.39 XRD pattern of T800+21WC HVOF coating T800+43WC HVOF WC Co W 2 Co 4 C Co 3 Mo 2 Si Figure 5.40 XRD pattern of T800+43WC HVOF coating 169

170 Intensity (arb) CHAPTER 5 MATERIAL CHARACTERISATION T800+68WC HVOF WC Co W 2 Co 4 C Angle (2 ) Figure 5.41 XRD pattern of T800+68W HVOF coating Porosity measurements of HVOF coatings Table 5.4 shows the results of porosity measured and calculated for various HVOF coatings. It is clear that the T800 HVOF coating has the highest porosity of 4.8%. Addition of WC reduces the porosity and the porosity decreases slightly with the increasing WC content. Table 5.4 Measurement of porosity of various HVOF coatings. HVOF Coating Porosity % T T WC 2.8 T WC 2.5 T WC

171 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.42 Porosity of T800 HVOF coating Figure 5.43 Porosity of T800+21WC HVOF coating Figure 5.44 Porosity of T800+43WC HVOF coating 171

172 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.45 Porosity of T800+68WC HVOF coating. Figures 5.42 to 5.45 illustrate various HVOF coatings with porosity coloured in red within the coatings. Apart from the porosity within the coatings, the porosity is also present at the interface between the coating and substrate. 5.4 Laser surface treated HVOF coatings High power diode laser (HPDL) has been used for laser surface treatment of various HVOF coatings, the surface became smoother and no obvious cracks, porosity, and oxidation were observed on the surface treated at the optimum laser operating conditions (full laser conditions showed in Chapter 4). Table 5.5 Optimum parameters obtained in laser processing and coatings features Coating Power (W) Scanning Speed (mm/sec) Beam size (mm mm) Coatings features T Partially melted HVOF T WC T WC T WC Partially melted HVOF, Partially melted WC Fully melted HVOF, Partially melted WC Fully melted HVOF, Partially melted WC 172

173 CHAPTER 5 MATERIAL CHARACTERISATION Surface views of laser melted HVOF coatings Figures 5.46 to 5.49 show the surface morphology of various HVOF coatings after laser treatment under the laser operating conditions that result in either partial melting or full melting, as described in Chapter 4, with formation of different microstructures. Figure 5.46 (b) shows the microstructure of the laser-melted surface of T800 coating at high magnification, containing eutectic Laves phase. For laser partial melting of T800 coatings with various WC contents, it can be seen from Figures 5.47, 5.48 and 5.49 that the WC particles on the surface are partially melted, which is confirmed by EDX analysis presented later. Figure 5.46 SEM micrographs of laser treated surface view of T800 HVOF coating at low magnification (a) and high magnification (b) Figure 5.47 SEM micrographs of laser treated surface of T800+21WC HVOF coating at low magnification (a) and high magnification (b) 173

174 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.48 SEM micrographs of laser treated surface of T800+43WC HVOF coatings at low magnification (a) and high magnification (b) Figure 5.49 SEM images of surface of laser treated T800+68WC HVOF coating at low magnification (a) and high magnification (b). The details of surface morphology after laser treatment have also been investigated by microxam mapping. The surface parameters such as roughness (Ra), surface area ratio (Sdr) have been calculated. Figure 5.50 illustrates 2D surface profiles and 3D surface profiles of different areas on T800 HVOF surface after laser melting. As shown in Table 5.5, the average roughness (Ra) of the T800 HVOF after laser melting is 0.18 µm. Compared with the roughness of the T800 HVOF before laser treatment (Table 5.1), the average roughness is reduced by 96%. The surface area ratio after laser surface melting is 3.4%, which is a 98% reduction compared with T800 HVOF coating before laser surface melting. 174

175 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.50 Surface profile (a) 2D and (b) 3D profiles of four different areas on laser treated T800 HVOF coating Figures 5.51 to 5.53 show surface profiles of T800 HVOF coatings with various WC contents after laser treatment using MicroXam 3D mapping and MicroXam 2D mapping. The values of average roughness (Ra) and surface area ratios (Sdr) of the coatings after laser treatment are shown in Table 5.5. Compared with those values before laser treatment as shown in Table 5.1, it can be seen that the laser treatment significantly reduces the surface roughness, as well as surface area ratios. For example, the surface area ratio, Sdr, of T800+21WC HVOF after laser treatment is 9.94%, while for the same HVOF coating, the Sdr is 223% (Table 5.1). The reduction of surface roughness and surface area ratios can be attributed to the melting of the top surface. 175

176 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.51 Surface profile (a) 2D and (b) 3D profiles of four different areas on laser treated T800+21WC HVOF coating 176

177 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.52 Surface profile (a) 2D and (b) 3D profiles of four different areas on laser treated T800+43WC HVOF coating 177

178 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.53 Surface profile (a) 2D and (b) 3D profiles of four different areas on laser treated T800+68WC HVOF coating 178

179 S dr % R a ( m) CHAPTER 5 MATERIAL CHARACTERISATION Table 5.6 Surface roughness parameters of laser treated surface Laser treated HVOF coating Average roughness R a (µm) Average of surface area ratio S dr (%) T T800+21WC T800+43WC T800+68WC T800 T800+21WC T800+43WC T800+68WC Figure 5.54 Surface roughness of various laser treated HVOF coatings T800 T800+21WC T800+43WC T800+68WC Figure 5.55 Average of surface area ratio of laser treated HVOF coatings 179

180 CHAPTER 5 MATERIAL CHARACTERISATION Cross section of laser treated HVOF coatings The cross section of T800 HVOF coatings after laser partial melting is shown in Figure 5.56, demonstrating that only the top part of the coating layer was melted, and the melted region showed a smooth surface with no visible cracks or porosity. Below the melted top-layer, there is a heat treated region, i.e. the temperature of the region was not sufficient to reach the melting temperature of T800. Therefore, the splat boundaries and small gaps still appear in this region. Furthermore, the interface between the HVOF coating and the substrate is not affected by the laser treatment. Figure 5.56 SEM micrograph of cross section of laser treated T800 HVOF coating. The backscattered SEM micrograph of the T800 HVOF coating after laser treatment is presented in figure 5.57, showing a typical solidification microstructure with dendritic features. The microstructure comprises a few phases including the white phase representing primary Laves phase, eutectic structure with white laves phase and the dark Co solid solution. Moreover, it is noted that no porosity or splat boundaries can be seen after laser melting. 180

181 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.57 SEM micrograph of laser melted T800 HVOF coating. The EDX spectrum in figure 5.58 (1) further confirms the primary Laves phase contains a higher percentage of Mo (43.18%) and Si (5.07%) and lower percentage of Cr (12.22%) and Co (39.53%) compared with other phases. The eutectic phase as shown in Figure 5.58 (2) has a higher percentage of Mo (22.78%) than the Co solid solution. The highest percentage of Co of 59.33% was in the Co solid solution phase (Figure 5.58 (3)). Figure 5.59 is a SEM micrograph of the cross section of T800+21WC HVOF coating after laser partial melting. The top part of the coating was melted, while the lower part of the HVOF coating remains unmelted. As can be seen in Figure 5.60 (a), the interface between the coating and the substrate remains mechanically interlocked. There is some small-scale porosity close to the surface and the WC particles were melted partially, where the carbides still appear within the upper part of the coating after laser treatment (Figure 5.60 (b)). 181

182 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.58 EDX analysis of various phases in laser melted T800 HVOF coating (1) Laves phase (2) Eutectic (Laves phase + Co solid solution) (3) Co solid solution Figure 5.59 SEM micrograph of cross section of laser treated T800+21WC of HVOF coating 182

183 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.60 SEM micrographs of laser treated T800+21WC HVOF coating showing (a) interface between the coating and substrate (b) microstructure in laser melted area The microstructure of partially melted T HVOF coating with partially melted of WC showing in Figure This complex microstructure was achieved after a rapid solidification process. Three typical phases can be seen in Figure 5.61, which are light-gray phase, dark phase and partially melted of WC particles. Figure 5.62 shows the EDX results of the three phases for laser partially melted of T800+21WC HVOF coating. The light gray phase (1) contains 41.91% of Co and 10.69% of W which was expected because W 2 Co 4 C was identified in the XRD results in Figure The dark phase (2) contains 41.11% Co, 29.46% Mo which is close to the composition of T800 phase. Figure 5.62 (3) illustrates the EDX result of partially melted of WC particles in T800+21WC HVOF coating. This phase, as expected, has the lowest percentage of Co (26.56%), and the highest percentage of W (31.55%). Figure 5.61 SEM micrograph of cross section for laser treated T800+21WC HVOF coating 183

184 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.62 EDX results of various phases in T800+21WC HVOF coating after laser treatment (1) Co-Cr binder phase (2) Co solid solution phase (3) partially melted WC Figure 5.63 shows a SEM micrograph of a cross section for fully melted T800+43WC HVOF coating. Figure 5.64 (a) illustrates a fusion bond at the interface between the coating and the substrate was created by laser treatment. Figure 5.65 shows the complex microstructure of the fully laser melted T800+43WC HVOF coating. It should be noted that this only indicates the full melting of the HVOF layer. Due to the large difference in melting temperatures between WC and the Co-matrix, WC particles were not fully melted, i.e. the temperature of the melt pool was between the melting temperatures of the two phases. Figure 5.66 shows the respective EDX results corresponding to the (1), (2), and (3) locations. In the EDX results, the dark phase (T800) (1) contains higher concentration of Co and Cr than the other two phases. On the other hand, the gray phase (2) has 25.7% of Co, 22.6% of W, and 1.05% of C, this indicates that the gray phase is composed of W 2 Co 4 C phase, consistent with the XRD result in Figure The white phase (partially melted carbides) (3) has the highest concentarion of W (80.24%). 184

185 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.63 SEM micrograph of cross section of laser treated T800+43WC HVOF coating showing partially melted WC Figure 5.64 SEM micrograph of laser treated T800+43WC HVOF coating showing (a) interface between the substrate and the coating (b) microstructure in laser melted area Figure 5.65 SEM micrograph of cross section of laser treated T800+43WC HVOF coating showing partially melted WC. 185

186 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.66 EDX results of phases (marked as 1, 2, 3 in Figure 5.65) in T800+43WC HVOF coating (1) T800 matrix (2) Co-Cr binder phase (3) partially melted WC. Figure 5.67(a) and (b) illustrate that the laser treated T800+68WC HVOF coating shows improved homogeneity, compared with inhomogeneity in the HVOF coating as seen in Figure 5.33 before laser treatment. This improvement in homogeneity might have a positive effect on the mechanical and the electrochemical properties of the HVOF coating. Figure 5.67 (a) SEM micrograph of laser treated T800+68WC (b) EDX line scan analysis on T800 and WC particles 186

187 CHAPTER 5 MATERIAL CHARACTERISATION Homogeneity of the HVOF coatings by laser treatment can also be observed by EPMA measurement. Figure 5.68 illustrates the EPMA elemental mapping of the laser partially melted T800 coating. The laser treated area (top area) has more Mo which indicates of the existence of Laves phase. Figure 5.69 shows EPMA results of the laser treated T800+43WC with partial melting of WC particles, indicating that the melting was mainly in the upper part of the coating. The blue areas in the Mo image shows the WC particles not completely melted in the coating. The oxygen distribution shows the presence of oxide inclusions in the splat boundaries and the interface between the coating and the substrate. Figure 5.68 EPMA mapping of laser treated T800 HVOF coating Figure 5.69 EPMA mapping of laser treated T800+43WC HVOF coating with partially melting of WC particles 187

188 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.70 illustrates the dilution changes of laser treated T800+43WC with partially melted WC particles. A3 in Figure 5.70 represents the bottom part of the laser melted layer and the EDX result in this area shows that the iron content in this area is 5.84%. The iron contents gradually reduce to 0.71% for the middle area A2 and then to 0.53% in the top area of the coating (A1). These results were also confirmed by EDX line scan of iron as shown in Figure Figure 5.70 EDX results of different areas of T800+43WC HVOF coating after laser treatment 188

189 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.71 EDX line scan of laser treated T800+43WC HVOF coating Laser treated T800+68WC HVOF coating with partially melted WC particles is shown in figure The laser melted layer has some defects including porosity as shown in Figure 5.73 (b). Fusion bonding was created by laser full melting between the coating and substrate (figure 5.73 (a)). The microstructure of the melted layer is shown in Figure 5.74 in which the white phase is partially melted WC particles, the light-gray phase is the partially melted binder phase, and the dark-gray phase is T800 (matrix). The T800+68WC HVOF coating shows more partially melted WC particles compared with T800+43WC (Figure 5.65). This might be due to the higher percentage of higher melting temperature phase, WC in T800+68WC, so that the melt pool temperature was not high enough to melt all WC particles. Although WC has lower thermal conductivity than the surrounding metals, it seems that the large difference in melting temperatures between the two phases plays a more important role than thermal conduction. It is also noted that the melting occurred along the edges of WC particles. 189

190 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.72 SEM micrograph of cross section of laser treated T800+68WC HVOF coating Figure 5.73 SEM micrographs of laser treated T800+68WC HVOF coating showing (a) interface between the coating and substrate and (b) microstructure of melted area. Figure 5.74 SEM micrograph of cross section of laser treated T800+68WC EDX results of different phases for laser treated T800+68WC HVOF coating are shown in Figure Binder phase (light-gray marked as 1) has higher percentage of Co (18.15%) compared with the other phases. The percentage of W in the binder 190

191 CHAPTER 5 MATERIAL CHARACTERISATION phase is 68.24%, indicating partially melting of WC particles. In addition, the T800 matrix shows a high percentage of W (81.51%), also suggesting the melting of WC particles. Figures 5.76 and 5.77 illustrate dilution behaviour in laser treated T HVOF coating with partially melted WC. As seen in Figure 5.76, Fe percentages in the upper (A1) and middle (A2) areas are around 0.6%. In the bottom of melted layer (A3) the percentage of Fe is 7.85%. As confirmed in Figure 5.77 by EDX line scan, a slightly higher dilution was found in the region of the coating adjoining the region of substrate melting. It suggested that the melt pool behaviour was not fully convective, and it seemed that the diffusion at the interface between the coating and substrate played a more important role. 191

192 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.75 EDX results in different areas of laser treated T800+68WC HVOF coating 192

193 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.76 EDX results in different areas of laser treated T800+68WC HVOF coating Figure 5.77 EDX results of line scan of laser treated T800+43WC HVOF coating showing Fe distribution along the coating thickness 193

194 CHAPTER 5 MATERIAL CHARACTERISATION XRD of laser treated HVOF coatings Figures illustrate the XRD patterns of various HVOF coatings after laser treatment. From the XRD result for the HVOF coating before laser treatment (Figure 5.38), it is clear that, before laser treatment, the HVOF coating consists of pure Co phase and various Co 2 MoCr and Co 3 Mo 2 Si phases. However, there is no Co 2 MoCr phase after laser treatment while the amount of Co 3 Mo 2 Si phase increases. The typical T800 alloy consists of a large percentage of hard intermetallic Laves phase distributed in a soft cobalt solid solution. The hard primary intermetallic phase is a compound of Co, Mo and Si with composition roughly of Co 3 Mo 2 Si [163]. The peaks of the crystal structures Co 3 Mo 2 Si and Co 2 MoCr have hexagonal symmetry [155, 164]. Comparing XRD patterns of T800+21WC and T800+43WC as HVOF coating (Figures 5.39 and 5.40) with those after laser treatment (Figures 5.79 and 5.80); the WC was a major phase before the laser treatment while W 2 Co 4 C became the major phase after laser treatment. The W 2 Co 4 C phase was formed by the reaction of Co with WC phase during laser treatment [162]. Cobalt-phase was detected in laser treated T800 coating only. On the other hand, all the spectra indicated the presence of WC phase. The Co 0.9 W 0.1 was only present in the spectra of laser-treated coatings which may be caused by the melting of WC and Co. Figure 5.81 shows the XRD pattern of T800+68WC HVOF coating after laser treatment, the pattern indicates the presence of W 2 Co 4 C and weak peaks of WC. The cobalt peak was very weak in the as received HVOF coating (figure 5.41) and not seen in laser treated spectra, suggesting that it has dissolved into the coating. 194

195 Intensity (arb) Intensity (arb) CHAPTER 5 MATERIAL CHARACTERISATION T800 Laser Co Co 3 Mo 2 Si Co 2 Si Angle Figure 5.78 XRD pattern of T800 HVOF coating after laser treatment 6000 T800+21WC Laser WC W 2 Co 4 C Co 0.9 W W Co 1.5 Si Angle (2 ) Figure 5.79 XRD pattern of T800+21WC HVOF coating after laser treatment 195

196 Intensity (arb) Intensity (arb) CHAPTER 5 MATERIAL CHARACTERISATION T800+43WC Laser WC W 2 Co 4 C Co 0.9 W Angle (2 ) Figure 5.80 XRD pattern of T800+43WC HVOF coating after laser treatment T800+68WC Laser WC W 2 Co 4 C Co 0.9 W Angle (2 ) Figure 5.81 XRD pattern of T800+68WC HVOF coating after laser treatment 196

197 CHAPTER 5 MATERIAL CHARACTERISATION Porosity measurements of laser treated HVOF coatings Table 5.7 along with Figures 5.82 to 5.85 illustrate the results of porosity measured for various HVOF coatings after laser treatment. Compared with the porosity before laser treatment (Table 5.4), there is significant reduction of porosity after laser treatment. T800 has the lowest porosity after laser treatment of 0.1 %, and T800+68WC has the highest porosity after laser treatment of 1 %. Furthermore, the significant porosity reduction is an important factor for protecting the substrate in corrosive environments as will discussed in electrochemical tests in Chapter 6. Table 5.7 Measurement of porosity of various HVOF coatings after laser treatment Laser treated HVOF coating Porosity % T T WC 0.3 T WC 0.3 T WC 1 Figure 5.82 Porosity of laser treated T800 HVOF coating 197

198 CHAPTER 5 MATERIAL CHARACTERISATION Figure 5.83 Porosity of laser treated T800+21WC HVOF coating Figure 5.84 Porosity of laser treated T800+43WC HVOF coating Figure 5.85 Porosity of laser treated T800+68WC HVOF coating 198

199 CHAPTER 5 MATERIAL CHARACTERISATION 5.5 Summary The surface of various HVOF coatings shows the presence of defects such as micro-cracks, micro-crevices, and pores. The surface roughness and surface area ratio slightly decreased by increasing the content of WC. The R a of T800 ~ 5.05 µm decreased to ~ 4.49 µm for T800+68WC. The S dr of T800 ~ 212 % decreased to ~153 % for T800+68WC. Cross sections of various HVOF coatings show lamellar, splat structure with high levels of defects such as porosity, and splats boundaries. The addition of WC reduced the porosity of the coatings. The values of porosity for T800, T800+21WC, T800+43WC and T800+63WC are 4.8%, 2.8%, 2.5% and 2.2% respectively. EDX and EPMA analysis showed the presence of oxide inclusions along the splat boundaries and at the interface between the HVOF coatings and the stainless steel substrate. The interface between the HVOF coatings and the stainless steel substrate exhibited-porosity-and-crevices. The phases observed by XRD of T800 in a powder form were Co, Co 3 Mo 2 Si, and Co 2 MoCr. The phases observed in T800+21WC and T800+43WC in a powder form were WC, Co, W 2 Co 4 C, and Co 3 Mo 2 Si. The phases observed in T800+68WC in a powder form were WC, Co, and W 2 Co 4 C. The HVOF coatings indicated the same phases as those in powder forms. 199

200 CHAPTER 5 MATERIAL CHARACTERISATION Laser surface melting of various HVOF coatings successfully eliminated various defects in the HVOF coatings, such as porosity, cracks, and splat structure. New microstructure has been achieved after laser treatment. T800 HVOF coating after laser partial melted showed elimination of the splat structures and boundaries with increase of eutectic and Laves phase percentages. T800+WC HVOF coatings after laser treatment showed partial melting of WC particles. After laser treatment there was significant reduction in the roughness (R a ) and surface area ratio (S dr ). The R a of T800 HVOF coating was reduced after laser treatment by 96%, and the S dr reduced from 212% for HVOF coating to 3.4%. The R a of T800+21WC HVOF coating was reduced after laser treatment by 65%, and the S dr reduced from 223% for HVOF coating to 9.94% after laser treatment. For T800+43WC the R a was reduced after laser treatment by 55%, and the S dr reduced from 212% for HVOF coating to 13.94% after laser treatment. EDX and EPMA results show improvement of elemental homogeneity after laser treatment. There was significant reduction of porosity after laser treatment. The value of porosity for T800 reduced from 4.8% of HVOF coating to 0.1 % after laser treatment, for T800+21WC reduced from 2.8% of HVOF coating to 0.3% after laser treatment, for T reduced from 2.5% of HVOF coating to 0.3% after laser treatment, and for T800+68WC reduced from 2.2% of HVOF coating to 1% after laser treatment. XRD results after laser treatment showed the changes in some phases. In T800 HVOF coating there was an increase in Co 3 Mo 2 Si phase indicating 200

201 CHAPTER 5 MATERIAL CHARACTERISATION an increase in Laves phase. W 2 Co 4 C became a major phase in T800+21WC and T800+43WC HVOF coatings after laser treatment suggesting the reaction between Co and WC. In T800+68WC HVOF coating after laser treatment, it mostly contained W 2 Co 4 C phase and a small amount of WC, while Cobalt solid solution was not seen in all the laser treated T800 with WC coatings. 201

202 Chapter 6 Corrosion Tests 6.1 Introduction This chapter presents the corrosion performance of various HVOF coatings before and after laser treatment in various corrosion tests, including immersion test, polarisation test, and electrochemical impedance spectroscopy (EIS) test. Corrosion morphologies of the coatings after the corrosion tests were characterised using SEM. Concentrations of the solutions after the immersion test were analysed by Inductivity coupled plasma- optical emission spectrometery (ICP-OES), to investigate the corrosion mechanisms. The complete laser melted conditions are shown in Table 5.5, where T800 HVOF, T800+21WC HVOF were partially melted coatings (figure 4.1), and T800+43WC, T800+68WC were fully melted coatings (figure 4.4), and partially melted WC. 6.2 Immersion test Samples of various as received and laser treated HVOF coatings were immersed in 3M H 2 SO 4 for 24, 48, 72 and 96 hours and observed using SEM. In general, there was no obvious weight loss after immersion testing for all the coatings. This is believed to be due to the difficulty of removing corrosion products that may be trapped at the interface between the coating and the substrate and inside the pores. In this section only the corrosion at the interface and improved substrate protection by laser treatment will be discussed. The corrosion which occurred at the surface and through the coating will be discussed in the following sections. 202

203 CHAPTER 6 CORROSION TESTS Tribaloy 800 (T800) coating From the SEM images of the cross section of the samples after immersion test, it is obvious that all the as received HVOF coatings had interconnected pores, so that the electrolyte can reach the substrate, causing corrosion at the interface between the coating and substrate. Due to the galvanic couple between the coating and stainless steel substrate, the substrate acted as an anode while the coating was cathode, leading to anodic dissolution. As shown in tables 5.4 and 5.7, the as received HVOF coatings had higher percentage of porosity than the laser treated HVOF coatings. The number of cracks and pores are higher for T800 (Table 5.4) which also contributed to the electrolyte penetration through the coatings Samples Immersed for 24 hours As shown in figure 6.1, after 24 hours of immersion, the as-received HVOF coating shows signs of corrosion attack at the interface between the coating and substrate, indicating the penetration of the electrolyte reaching the substrate. However, the laser-treated coating shows no sign of corrosion. HVOF Laser Treated (c) Figure 6.1 SEM cross-section images of T800 HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 24 h 203

204 CHAPTER 6 CORROSION TESTS Samples Immersed for 48 hours By increasing in the immersion time to 48 hours, the as-received HVOF coating shows more significant corrosion at the interface, as shown in figure 6.2 (a and c). For laser treated coatings, after 48 hours, still no sign of corrosion was found as shown in figure 6.2 (b) and (d). HVOF Laser Treated (a) (b) (c) (d) Figure 6.2 SEM cross-section images of T800 HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 48 h 204

205 CHAPTER 6 CORROSION TESTS Samples Immersed for 72 hours After 72 hour immersion, obviously, the as-received HVOF coating showed a much increased level of corrosion attack at the interface, but no sign of coating spallation, while slight corrosion was also observed at the interface for the lasertreated coating, as shown in figure 6.3. This suggested that a small amount of electrolyte penetrated through some defects existing in the laser-treated coatings. HVOF Laser Treated (a) (b) (c) (d) Figure 6.3 SEM cross-section images of T800 HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 72 h Samples Immersed for 96 hours After 96 hours, the as-received HVOF coating presented a much increased level of corrosion at the interface, leading to the spallation of the coating as shown in figure 6.4a and c. This means that the penetration of the electrolyte had resulted in a large amount of corrosion products that accumulated along the interface and volume expansion. However, following the treatment of the HVOF coating by the laser, the HVOF coatings show a much higher protection against the penetration of the 205

206 CHAPTER 6 CORROSION TESTS electrolyte through the coating that eliminated/reduced the corrosion at the interface. HVOF Laser Treated (a) (b) (c) (d) Figure 6.4 SEM cross-section images of T800 HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 96 h T800+21WC In general, the anodic dissolution of the substrate at the interface for T800 HVOF coating was more severe than that at the interface for T800+WC. This indicated that the coatings with WC had a denser structure than T800 coating, due to different operating parameters applied during the thermal spraying of different HVOF coatings [165]. The HVOF samples with WC did not show any corrosion at the interface after immersion for 24 hour. 206

207 CHAPTER 6 CORROSION TESTS Samples Immersed for 48 hours As seen in figure 6.5 (a) and (c), the electrolyte penetrated into the as-received HVOF coating and reached the interface between the coating and the substrate, resulting in corrosion attack at the surface. On the other hand, there was no obvious corrosion attack at the interface for the laser-treated coating as shown in figure 6.5 b and d. HVOF Laser Treated (a) (b) (c) (d) Figure 6.5 SEM cross-section images of T800+21WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 48 h 207

208 CHAPTER 6 CORROSION TESTS Samples Immersed for 72 hours As shown in figure 6.6 (a) and (c), longer immersion time resulted in more anodic dissolution of the substrate at the coating/substrate interface. After laser treatment, still no obvious corrosion attack was found at the interface as shown in figure 6.6 (b) and (d). HVOF Laser Treated (a) (b) (c) (d) Figure 6.6 SEM cross-section images of T800+21WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 72 h 208

209 CHAPTER 6 CORROSION TESTS Samples Immersed for 96 hours After 96 hour immersion, it was obvious that for the as-received HVOF coating, corrosion occurred at the interface as shown in figure 6.7 (a) and (c). For laser treated HVOF coating as shown in figure 6.7 (b) and (d), none or very low dissolution occurred at the interface, as a result of laser melting of the upper side of the coating which sealed the interconnected porosity. HVOF Laser Treated (a) (b) (c) (d) Figure 6.7 SEM cross-section images of T800+21WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 96 h 209

210 CHAPTER 6 CORROSION TESTS T800+43WC No corrosion at the interface was observed for the HVOF coatings with and without laser treatment after 24 hour immersion test Samples Immersed for 48 hours As shown in figure 6.8, the penetration of electrolyte in the as-received samples became obvious, causing anodic dissolution of the substrate after 48 hour, while no sign of corrosion was observed for the laser-treated coatings. HVOF Laser Treated (a) (b) (c) (d) Figure 6.8 SEM cross-section images of T800+43WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 48 h Samples Immersed for 72 hours After 72 hour immersion, the dissolution of the substrate in the as-received samples became more severe at the interface (figures 6.9). On the other hand, for laser treated samples, no corrosion was observed 210

211 CHAPTER 6 CORROSION TESTS HVOF Laser Treated (a) (b) (c) (d) Figure 6.9 SEM cross-section images of T800+43WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 72 h Samples Immersed for 96 hours Further increasing in the immersion time to 96 hours resulted in more significant corrosion at the interface in the as-received samples with serious dissolution of the substrate. However no coating spallation was observed, showing better corrosion protection than the T800 HVOF coating. Again, no corrosion was observed for the laser treated coating. 211

212 CHAPTER 6 CORROSION TESTS HVOF Laser Treated (a) (b) (c) (d) Figure 6.10 SEM cross-section images of T800+43WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 96 h T800+68WC In general, the T800+68WC HVOF coating presented the best protection with reduced level of electrolyte penetration compared with other HVOF coatings. This is consistent with the data presented in table 5.4 showing the lowest level of porosity. No corrosion was observed after 24 hour immersion Samples Immersed for 48 hours After 48 hour immersion, only a small amount of corrosion was observed at the interface for the HVOF coating as shown in figure 6.11 (a) and (c). After laser treatment (figure 6.11 (b) and (d)), no dissolution occurred at the interface, suggesting no electrolyte reached to the substrate as a result of sealing the 212

213 CHAPTER 6 CORROSION TESTS interconnected porosity by the laser. The relatively large pores presented in lasertreated coating were considered to be isolated. HVOF Laser Treated (a) (b) (c) (d) Figure 6.11 SEM cross-section images of T800+68WC HVOF coating: a, c as received HVOF coatings, b, d after laser treatment, all are immersed for 48 h Samples Immersed for 72 hours After 72 hour immersion, the as-received HVOF coating presented obvious corrosion attack at the interface as shown in figure 6.12 (a) and (c), revealing the dissolution of the substrate. However no corrosion was observed for laser partially melted samples as seen in figure 6.12 (b) and (d) 213

214 CHAPTER 6 CORROSION TESTS. HVOF Laser Treated (a) (b) (c) (d) Figure 6.12 SEM cross-section images of T800+68WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 72 h Samples Immersed for 96 hour Figure 6.13 (a) and (c) shows slightly increased corrosion at the interface. Compared with other HVOF coatings described earlier, much improved corrosion protection was found. On the other hand, no corrosion occurred at the interface for the laser treated samples (figure 6.13 (b) and (d)). 214

215 CHAPTER 6 CORROSION TESTS HVOF Laser Treated (a) (b) (c) (d) Figure 6.13 SEM cross-section images of T800+68WC HVOF coating: a and c as received HVOF coatings, b and d after laser treatment, all are immersed for 96 h Inductivity coupled plasma- optical emission spectrometer (ICP- OES) test ICP-OES was used to analyse the solutions after the immersion tests to gain information of the concentrations of a few representative elements like Co, W, Mo and Cr that were released from the samples to the solutions. Such information is related to the corrosion dissolution which occurred during the immersion test and aids understanding of the corrosion mechanisms involved in the HVOF coatings with and without laser treatment. The solutions after 7 days of immersion in 3M H 2 SO 4 acid for the T800+43WC coatings before and after laser treatment were selected to be analysed. Table 6.1 and figure 6.14 illustrate the mean concentrations of those elements released to the electrolyte after 7 days of immersion. 215

216 CHAPTER 6 CORROSION TESTS Table 6.1 Mean concentration of the elements (units of µg/ml) released into the electrolyte solution after 7 days of immersion of T800+43WC HVOF coating before and after laser treatment. Sample Fe Co W Mo Cr HVOF Laser treated Reference Standard g/cm HVOF coating Laser treated Fe Co W Mo Cr Figure 6.14 Mean concentration of the elements released in 3M H 2 SO 4 after 7 days of immersion of T800+43WC HVOF coating before and after laser treatment. For the T800+43WC coating, the results showed that relatively large amounts of the Cr and Fe dissolved in the electrolyte. The Fe was from the substrate while the Cr could be from both the coating and substrate, indicating severe dissolution of the substrate. This is consistent with the finding described in Section although the immersion time was 7 days. On the other hand, the Co, W and Mo were hardly dissolved, indicating a low level of corrosion occurred of the coating. A similar finding was also observed for Cr, W, and Mo in 5M H 2 SO 4 by Lee et al [166]. After laser treatment there was no significant change in the dissolution of Mo, Cr 216

217 CHAPTER 6 CORROSION TESTS and W. However, a large amount of the Co, 2.69 µg/ml, was observed in the solution showing the highest dissolution of the coating. On the other hand, the dissolution of Fe from the substrate significantly decreased, indicating a very low level of corrosion at the interface. These results were consistent with the observation using SEM as displayed in Section It can be explained by the observation described in Chapter 5. In summary, the HVOF coatings allowed the electrolyte to penetrate easily through the interconnected pores to the interface between the substrate and the coating, cause high anodic dissolution of the substrate. That was reflected by the high percentage of Fe in the solution. After laser partially melting of WC, the high concentration of the Co in the electrolyte showed the corrosion of the coating, while the electrolyte did not penetrate to the interface. 6.3 Polarisation test Figure 6.15 shows the polarisation curves obtained for the HVOF coatings in the aerated 0.5 M H 2 SO 4 solution at room temperature. Comparison between polarisation curves of the as-received HVOF coatings, all the coatings presented a similar value of E corr, but there is a slight decrease in the values of I corr with increasing the content of WC. A similar observation was also reported by Human [86], suggesting that addition of carbon and tungsten reduces the corrosion current density below that of pure cobalt. As seen in figure 6.15, the passivation range was wider in T800, but become narrower by adding WC due to the reduction of Cr content which is responsible for passive film formation. Moreover, cathodic regions of the polarisation curves in figure 6.15 for all the coatings are similar, implying that all the coatings presented a same cathodic reaction, which might be a simple hydrogen evolution progressed by the same mechanism [167]. 217

218 Potential (mv) CHAPTER 6 CORROSION TESTS HVOF T800 T800+21WC T800+43WC T800+68WC E-4 1E Current density (ma/cm 2 ) Figure 6.15 Polarization curves of the HVOF coatings in 0.5 M H 2 SO 4 solution. Figure 6.16 illustrates comparisons of the polarisation curves for various HVOF coatings before and after laser treatment. E corr, I corr and corresponding breakdown potentials (E b ) are summarised in table 6.3. Obviously there is an improvement in corrosion behaviour after laser treatment for all the coatings. As shown in figure 6.16 (a) the E corr of T800 HVOF coating was shifted in a positive direction from mv to -304 mv after laser treatment and corresponding corrosion current density reduced about 20 times from 0.67 ma/cm 2 to ma/cm 2. In addition, the breakdown potential increased from 907 mv to 1002 mv. For T800+21WC HVOF coating, the E corr of -312 mv was increased to be more positive after laser treatment to -243 mv. The current density reduced about 13 times after laser treatment. Furthermore, the current density reduced about 14 times after laser treatment of T800+43WC HVOF coating. The current density of T HVOF coating decreased after laser treatment about 5 times. 218

219 Potential (mv) Potensial (mv) Potential (mv) Potential (mv) CHAPTER 6 CORROSION TESTS Laser HVOF T Laser HVOF T800+21WC E-5 1E-4 1E Current Density (ma/cm 2 ) (a) 1E-4 1E Current Density (ma/cm 2 ) (b) 1500 T800+43WC 1500 T800+68WC 1000 Laser HVOF 1000 Laser HVOF E-5 1E-4 1E Current Density (ma/cm 2 ) (c) 1E-4 1E Current Density (ma/cm 2 ) (d) Figure 6.16 Polarization curves of HVOF coatings before and after laser treatment in 0.5 M H 2 SO 4 (a) T800 (b) T800+21WC (c) T800+43WC (d) T800+68WC 219

220 CHAPTER 6 CORROSION TESTS Table 6.2: Corrosion current density (I corr ) and corrosion potential (E corr ) of HVOF coatings before and after laser treatment in 0.5 M H 2 SO 4 solution Sample I corr (ma/cm 2 ) E corr (mv) E b (mv) T800 HVOF T800 Laser T800+21WC HVOF T800+21WC Laser T800+43WC HVOF T800+43WC Laser T800+68WC HVOF T800+68WC Laser As shown in figure 6.16 (a) the anodic polarisation curve of T800 HVOF coating show an initial active stage, followed by short stage of passivation which might be due to the formation of Cr 2 O 3 film [5], suggesting that the coating became pseudopassive, until the breakdown of the oxide laser at the potential of 907 mv. The laser treated T800 HVOF coating, started with an active stage followed by pseudopassive stage until breakdown potential reached. The same observations were also found for T800+21WC before and after laser treatment. The T800+43WC HVOF coating before laser treatment has the same observation. However, after laser treatment, the anodic polarisation curve shows firstly active stage followed by pseudo-passive stage. T800+68WC HVOF coating have the same observation. However, after laser treatment it shows initially active stage, then a reduction of current density might be due to the formation of Cr 2 O 3 and/or WO 3 oxide films [168] followed by pseudo-passive stage until breakdown potential. In theory, the T800 and T800+WC HVOF coatings should be able to passivate as a consequence of the presence of high amount of Cr (18%) as shown in table 5.2 in a metal matrix [161]. However, all the HVOF coatings did not present a true passive regime that 220

221 CHAPTER 6 CORROSION TESTS required much lower current density (below 10 µa/cm 2 ) and the behaviour was termed as pseudo-passive [70, 72]. The polarisation curves before and after laser treatment (figure 6.16) show different activation stages, indicating changes in the corrosion mechanism. The high defect density of HVOF coatings such as weak splat boundaries, oxide inclusions, etc. could provide many different corrosion initiation sites, which make the corrosion current density specially for HVOF coatings relatively high (10-4 A/cm 2 ). Similar results were also observed by Bolelli [161]. It reported that pseudo-passivation behaviour occurred as a result of increasing the length of cobalt diffusion path. Where after Co dissolution, Co 2+ should diffuse out through the porosity and gaps of the HVOF coatings and in the WC skeleton, which is slower than the free diffusion and much slower than convection. As a result of diffusion limitation of mass transport, current flow is decreased causing pseudo-passive behaviour [85]. It was found from the previous studies that the corrosion mechanism of HVOF WC-Co coatings is complex due to the heterogeneous microstructure and binder phase composition [69]. The cobalt binder phase is most susceptible to dissolution in acidic media [72]. Some previous studies [5, 79, 169] believed that there are two distinctive regions in the polarisation curve as shown in figure They assumed that region I at low potential, related to corrosion of the matrix material and leads to a rapid release of Co 2+, and region II at higher potential was related to WC oxidation. It was found that the hard phase WC had oxidation and dissolution at much higher potential than Co, and oxidize to WO 3 at low ph. The oxidation of WC increases with increasing ph [169]. Furthermore, the dissolution of Co takes place on the early stages of the process but slowed down by formation of oxide layer (Cr 2 O 3 ) [79, 168]. However, in this study, the same trend was noticed; where the polarisation curves for HVOF T800+WC coatings have two regions as shown in figure There is no evidence of dissolution of Co at low potential (region I) and WC at high potential (region II). However, the ICP results (table 6.1 and figure 6.14) showed evidence of Co and W in the electrolyte after 7 days of immersion for T800+43WC HVOF coating in 0.5 M H 2 SO 4 which indicated dissolution of Co and WC. 221

222 CHAPTER 6 CORROSION TESTS Figure 6.17 A typical polarization curve of T800+43WC HVOF coating. Figure 6.18 shows microcrevice corrosion occurred at the splat boundaries of T800+21WC. The existence of porosity in the HVOF coatings influenced the corrosion behaviour significantly, mainly due to the electrolyte reaching deeper areas and resulting in crevice corrosion as evident in figure Moreover, the corrosion was initiated preferentially around the splat boundaries, as a result of microcrevice corrosion mechanism [170]. The initiation of corrosion at splat boundaries of HVOF coatings was reported as a result of the presence of oxide stringers in the splat boundaries [171]. The crevice effects of porosity could be significant in the corrosion action especially at the top layers of the HVOF coating. Figure 6.18 Microcrevice corrosion of T800+21WC HVOF coating after immersion in 3 M H 2 SO 4 for 108 h. It was also found that the cobalt alloy matrix corroded more at the WC/Co interface. As shown in figure 6.19 the corrosion resulted in dissolution of the 222

223 CHAPTER 6 CORROSION TESTS matrix near the hard carbides due to micro-galvanic effects. From previous studies, as discussed in section 2.4.6, Co dissolves in neutral and acidic solutions, and shows higher dissolution rates with decreasing in ph. However, WC shows more stabilisation with decreasing in ph. The galvanic coupling between WC and surrounding metal matrix causes accelerated anodic dissolution of the less noble phase (Co) as follows. (6.1) The reduction reaction (cathodic) occurred on the WC phase, which can be one of the following reactions: The corrosion mechanism in this system is controlled by the galvanic coupling between the WC and the matrix [5, 9, 69-75]. Figure 6.19 Galvanic corrosion of T800+43WC HVOF coating after polarisation test. The improvement in corrosion behaviour for the coatings after laser treatment is believed to be due to the elimination of the defects such as porosity, microcracks and gaps which were found in the HVOF coatings as shown in table 5.7. In addition, decrease in the difference of electrochemical potentials between the WC 223

224 CHAPTER 6 CORROSION TESTS and the matrix was achieved by formation of interfacial phases (W 2 Co 4 C) between the two phases [12]. Figure 6.20 illustrates the T800 HVOF coating after polarisation test. It is obvious that there was a typical corrosion attack at splat boundaries in the upper part of the coating. Figure 6.21 shows the surface of laser treated for T800 HVOF coating after polarisation test. The dissolution of Co solid solution was confirmed by ICP results (figure 6.14) and can be also seen from figure 6.21, in which the Co solid solution was more anodic than Laves phase that contains a high percentage of Mo (43%) which was confirmed by EDX (figure 5.58). As shown in ICP results, Mo had a low dissolution in H 2 SO 4 solution. Some portions of the Laves phase pulled out of the surface showing a shape of flowers that was the original Laves phase when the around Co phase dissolved. As seen in figure 6.22, T800 HVOF coating after polarisation test the corrosion attack occurred at the splat boundaries and the Co solid solution inside the splat, and cracks have observed in specific areas within brittle splats that might be due to the oxide expansion around splat boundaries [172]. Figure 6.23 shows the corroded area of T800+21WC coating after polarisation test. The dissolution occurred on the surface at T800 matrix, the WC almost disappeared in the matrix. There are no obvious boundaries between T800 and WC. Corroded area Corrosion in the splat boundaries Figure 6.20 Cross section of T800 HVOF coating after polarization test in 0.5 M H 2 SO 4 solution. 224

225 CHAPTER 6 CORROSION TESTS Figure 6.21 Surface corrosion of laser treated T800 coating after polarization test in 0.5 M H 2 SO 4 Figure 6.22 Surface of laser treated T800 coating after polarization test in 0.5 M H 2 SO 4 solution. Figure 6.23 Cross section of laser treated T800+21WC coating after polarization test in 0.5 M H 2 SO 4 solution 225

226 CHAPTER 6 CORROSION TESTS As shown in figure 6.24 the dissolution in the T800 with WC HVOF coatings occurred at the T800 matrix (anodic), and at higher potentials some dissolution in WC occurred, which can be confirmed from ICP test (figure 6.14). After laser treatment the WC melted partially in T800+43WC and T800+68WC, and as can be seen in figure 6.25 for T800+43WC, the dissolution occurred around the partially melted carbides. Also there is dissolution in the not melted WC particles and phases that new phases such as W 2 Co 4 C, this dissolution confirmed by ICP test. Corroded area Corroded area in T800 matrix Figure 6.24 Cross section of T800+68WC HVOF coating after polarization test in 0.5 M H 2 SO 4 Corroded area Corroded area in matrix Figure 6.25 Cross section of T800+43WC laser treated coating after polarization test in 0.5 M H 2 SO 4 226

227 -Z" (ohm.cm 2 ) -Z" (ohm.cm 2 ) -Z" (ohm.cm 2 ) -Z" (ohm.cm 2 ) CHAPTER 6 CORROSION TESTS 6.4 Electrochemical Impedance Spectroscopy (EIS) Test Impedance diagrams obtained in 0.5 M H 2 SO 4 solution at various immersion times for HVOF coating and after laser treatment are presented in this section. Four impedance measurements were made for each sample to give an idea about reproducibility. Figure 6.26 shows Nyquist curves of HVOF coatings at various immersion times. For all HVOF coatings, after 1 h of exposure, only a large capacitive semi-circle is observed expanded with further immersion times T800 HVOF 1h 3h 6h 12h 24h 48h T800+21WC HVOF 1h 3h 6h 12h 24h 48h (a) Z ' (ohm.cm 2 ) T800+43WC HVOF 1h 3h 6h 12h 24h 48h 100 (b) Z ' (ohm.cm 2 ) T800+68WC HVOF 1h 3h 6h 12h 24h 48h (c) (d) Z ' (ohm.cm 2 ) Z ' (ohm.cm 2 ) Figure 6.26 Nyquist plots of HVOF coatings for different times (a) T800 (b) T800+21WC (c) T800+43WC and (d) T800+68WC As shown in figure 6.26 there was expansion of the semi-circle by increasing time due to the oxide layer of chromium oxide formed at the electrode surface. However, in Nyquist plot T800+68WC HVOF coating (figure 6.26 (d)) there was small decrease in the semi-circle, probably due to dissolution of matrix material. The same behaviour was noticed after laser treatment as can seen in figure By increasing the time of immersion there was expansion of the semi-circle of the total impedance as a result of created oxide layer. 227

228 -Z" (ohm.cm 2 ) -Z" (ohm.cm 2 ) -Z" (ohm.cm 2 ) -Z" (ohm.cm 2 ) CHAPTER 6 CORROSION TESTS (a) Z ' (ohm.cm 2 ) T800 Laser T800+43WC Laser 1h 3h 6h 12h 24h 48h 800 1h 600 3h 6h 12h h 48h 200 (c) Z ' (ohm.cm 2 ) T800+21WC Laser 1h 3h 6h 12h 24h 48h (b) Z ' (ohm.cm 2 ) T800+68WC Laser 1h 3h 6h 12h 24h 48h (d) Z ' (ohm.cm 2 ) Figure 6.27 Nyquist plots of laser treated HVOF coatings for different times (a) T800 (b) T800+21WC (c) T800+43WC and (d) T800+68WC The Nyquist plots (figures 6.29) showed clearly the different responses of EIS for the as received coatings (HVOF) and after laser treatment after 3 hours of immersion. The Nyquist plots obvious illustrated the positive effect of laser remelting of HVOF coating for all types of coating with different percentage of WC. The equivalent circuit model was proposed in figure 6.28 which was found to fit all the HVOF coatings before and after laser treatment. In this model, R S is the resistance of the solution, R P is the resistance of the coating that is directly linked to the coating defects such as porosity and microcracks, CPE P is the constant phase element that is used instead of pure capacitance where it showed better fitting. Because the real surface area which is exposed to the electrolyte is greater than the geometric one as shown in tables 5.1 and 5.6, and the roughness of the surface increases by increasing the exposure time, also due to splat boundaries, and porosity. R ct is the charge transfer resistance and CPE dl is associated with capacitor behaviour of the double layer. The corrosion resistance of the coatings can be evaluated by R ct. The higher the value of R ct, the less easily the charges transferred through the electrolyte/substrate interface and the higher the resistance to the 228

229 CHAPTER 6 CORROSION TESTS corrosion. Tables 6.3 and 6.4 summarise the electrochemical parameters of HVOF coatings before and after laser treatment respectively after 3 hours of immersion, using the circuit described in figure Table 6.3 shows that the values of R ct for all HVOF coatings after 3 hours of immersion were in the range of Ω cm 2, and decreased with the addition of WC. Also R P decreased slightly by addition of WC with the range Ω cm 2. However, R ct increased significantly after laser treatment for all the HVOF coating, which indicates improvement of corrosion resistance. In T800 HVOF coating, the laser treatment increased the value of R ct from to Ω cm 2. For T800+21WC R ct increased from 631 to Ω cm 2. The percentage of corrosion improvement after laser treatment for T800 and T800+21WC was 23% and 94% respectively. However, the improvement in corrosion resistance for T800+43WC and T800+68WC was 77% and 30% respectively. The higher percentage of WC the lower percentage of corrosion improvement. With increasing content of WC, more WC grains remained unmelted in the coating after laser treatment. Therefore, the reduction of microgalvanic activity between the two phases became less effective. Figure 6.28 Equivalent circuit proposed for the coating system 229

230 -Z" (ohm.cm 2 ) -Z" (ohm.cm 2 ) -Z" (ohm.cm 2 ) -Z" (ohm.cm 2 ) CHAPTER 6 CORROSION TESTS T800 after 3h HVOF Laser Hz T800+21WC after 3h Hz HVOF Laser Hz Hz (a) Hz Z ' (ohm.cm 2 ) 1000 (b) Hz Z ' (ohm.cm 2 ) T800+43WC after 3h Hz HVOF Laser T800+68WC after 3h (d) HVOF Laser Hz (c) Hz Z ' (ohm.cm 2 ) Hz Hz Hz Z ' (ohm.cm 2 ) Figure 6.29 Impedance spectra after 3 hours of immersion for various coatings before and after laser treatment: (a) T800, (b) T800+21WC, (c) T800+43WC, and (d) T800+68WC Table 6.3 Electrochemical parameters obtained from EIS spectra of HVOF coatings after 3 hours of immersion Electrochemical HVOF after 3h of immersion parameters T800 T800+21WC T800+43WC T800+68WC R s (Ω.cm 2 ) R p (Ω.cm 2 ) R ct (Ω.cm 2 ) CPE p (mf.cm -2 ) CPE dl (mf.cm -2 ) n p n dl

231 CHAPTER 6 CORROSION TESTS Table 6.4 Electrochemical parameters obtained from EIS spectra of laser coatings after 3 hours of immersion Electrochemical Laser / after 3h of immersion parameters T800 T800+21WC T800+43WC T800+68WC R s (Ω.cm 2 ) R p (Ω.cm 2 ) R ct (Ω.cm 2 ) CPE p (mf.cm -2 ) CPE dl (mf.cm -2 ) n p n dl Figure 6.30 illustrates the Nyquist plots of EIS spectra of various HVOF coatings before and after laser treatment after immersion time of 12 h in 0.5 M H 2 SO 4 solution. It is obvious there was improvement in corrosion resistance after laser treatment especially in T800 and T800+21WC HVOF coating as seen in figure 6.30 (a) and (b). Tables 6.5 and 6.6 show the electrochemical parameters using the equivalent circuit in figure Table 6.5 illustrates that the values of R ct for all HVOF coatings were in the rage of 2400 to Ω cm 2, and decreased with increasing WC. Also, R P reduced slightly by adding WC. However, after laser treatments, the value of R ct for T800 coating was increased from to Ω cm 2, which indicates an improvement of corrosion resistance of the coating after laser treatment. For T800+21WC coating, the R ct increased after laser treatment from 3750 to Ω cm 2, representing a significant improvement of corrosion resistance of the coating after laser treatment. Furthermore, the percentages of corrosion improvement of T800 and T800+21WC coatings after laser treatment were 42% and 75% respectively. There was no clear improvement in corrosion resistance for T800+43WC and T800+68WC HVOF coating after laser treatment. 231

232 -Z" (ohm.cm 2 ) -Z" (ohm.cm 2 ) -Z" (ohm.cm 2 ) -Z" (ohm.cm 2 ) CHAPTER 6 CORROSION TESTS T800 after 12h HVOF Laser Hz T800+21WC after 12h Hz HVOF Laser Hz Hz (a) Hz Z ' (ohm.cm 2 ) 1000 (b) Hz Z ' (ohm.cm 2 ) T800+43WC after 12h HVOF Laser T800+68WC after 12h HVOF Laser Hz Hz Hz Hz (c) Hz Z ' (ohm.cm 2 ) (d) Hz Z ' (ohm.cm 2 ) Figure 6.30 Impedance spectra after 12 hour of immersion for various coatings before and after laser treatment: (a) T800, (b) T800+21WC, (c) T800+43WC, and (d) T800+68WC Table 6.5 Electrochemical parameters obtained from EIS spectra of HVOF coatings after 12 hours of immersion Electrochemical HVOF after 12 hour of immersion parameters T800 T800+21WC T800+43WC T800+68WC R s (Ω.cm 2 ) R p (Ω.cm 2 ) R ct (Ω.cm 2 ) CPE p (mf.cm -2 ) CPE dl (mf.cm -2 ) n p n dl

233 CHAPTER 6 CORROSION TESTS Table 6.6 Electrochemical parameters obtained from EIS spectra of laser coatings after 12 hours of immersion Electrochemical parameters Laser / after 12h of immersion T800 T800+21WC T800+43WC T800+68WC R s (Ω.cm 2 ) R p (Ω.cm 2 ) R ct (Ω.cm 2 ) CPE p (mf.cm -2 ) CPE dl (mf.cm -2 ) n p n dl The decreasing of R ct with the addition of WC in the HVOF coatings might be associated with several reasons. First, the addition of WC increased the surface area of the T800 coating matrix by introduction of numerous interfaces between the matrix and carbides. Such interfaces can be subjected to dissolution governed by galvanic corrosion (figure 6.19) and form interconnected channels which are responsible for the electrolyte penetration through the coating and at the coating/substrate interface. Second, the value of R P decreased with an increase in the content of WC, indicating the increased porosity with WC during the test. These interconnected porosity provided channels allowing the electrolyte to penetrate through the coatings to reach the interfaces at the coating/substrate, which might leading to galvanic corrosion on the steel substrate. Third, for all HVOF coatings, the splat boundaries that might be sites of oxide inclusions could be the common sites for initiation of crevice corrosion [130]. Furthermore, the splat boundaries became less pronounced with increasing content of WC. It is believed that the overall corrosion resistance of the HVOF coatings could be considered as a combined effect. The addition of WC reduced the values of R ct by the first two mechanisms dominating the corrosion process, but increasing the content of WC 233

234 CHAPTER 6 CORROSION TESTS did not affect R ct significantly due to the third mechanism contributing to the overall process of corrosion [173]. After laser treatment as shown in figure 6.29 with tables 6.3 and 6.4 after 3 hours of immersion corrosion resistance improved for all HVOF coatings, where R ct increased after laser treatment for all HVOF coatings. Also, R p increased for all HVOF coatings which indicate reduction of porosity due to remelting of HVOF coating, same results were shown in porosity measurement (tables 5.4 and 5.7). However, after 12 hour of immersion the results show significant improvement in corrosion resistance in T800 and T800+21WC coatings by laser treatment. Also the results show significant reduction of porosity after laser treatment as R P increased. The improvement of corrosion resistance was due to reduction of porosity and splat boundaries, also due to the formation of a new phase of W 2 Co 4 C at the interface of WC/T800 matrix to reduce the microgalvanic activity. In T800+43WC and T800+68WC there is no obvious improvement in corrosion after 12 hour because of adding more WC which could cause more galvanic action and more WC grains remained unmelted in the coating after laser treatment. Therefore, the reduction of microgalvanic activity between the two phases becomes less effective. The following figures show the relation between the logarithm of coating resistance and charge transfer resistance with various immersion times (1, 3, 6, 12, 24, and 48 hour) for as-received HVOF coatings and after laser treatment. The full results of EIS for all immersion times (graphs and tables) for various HVOF coatings before and after laser treatment can be found in Appendices (A) and (B). It is observed from figures that the laser treated samples have different electrochemical impedance spectroscopy behaviour compared to as-received HVOF for all coatings. The general trend for almost all the HVOF coatings (as seen in figure 6.31) is that the coating resistance (R p ) decreased slightly by increasing time of immersion; this might be because of increased interconnected channels and defects in the HVOF coating. After laser treatment T800 and T800+21WC (figure 6.31 (a) and (b)) almost have the same values of R p which indicate no porosity increase by increasing immersion time in these coatings. However, in T800+43WC and T800+68WC coatings (figure 6.31 (c) and (d)) the values of R p increased by increasing immersion time, that might be as a result of 234

235 R P (ohm.cm 2 ) R P (ohm.cm 2 ) R P (ohm.cm 2 ) R P (ohm.cm 2 ) CHAPTER 6 CORROSION TESTS oxidation in the splat boundaries and corrosion products partially filling the gaps and impeding the electrolyte to penetrate through the coating surface. In figure 6.32 (a) and (b) there was improvement in corrosion resistance of T800 and T800+21WC after laser treatment during all immersion times. However, corrosion resistance was improved in T800+43WC (figure 6.32 (c)) before the first 10 hours then the HVOF coating might create oxidation layer (WO 3 ) on the WC [69, 169] that increase the values of R ct. T WC coating (figure 6.32 (d)) showed no improvement in corrosion resistance for this coating. As a general behaviour all T800 with WC HVOF coatings R ct increased gradually with immersion time which might because of creating of oxide layer (WO 3 ) [69] T T800+21WC HVOF Laser 1000 HVOF Laser HVOF Laser Immersion time (h) T800+43WC (a) Immersion time (h) HVOF Laser T800+68WC (b) (c) Immersion time (h) Immersion time (h) (d) Figure 6.31 Coating resistance R p before and after laser treatment for various time of immersion (a) T800 (b) T800+21WC (c) T800+43WC (d) T800+68WC 235

236 R ct (ohm.cm 2 ) R ct (ohm.cm 2 ) R ct (ohm.cm 2 ) R ct (ohm.cm 2 ) CHAPTER 6 CORROSION TESTS T800 HVOF Laser T800+21WC HVOF Laser (a) Immersion time (h) 1000 (b) Immersion time (h) T800+43WC HVOF Laser T800+68WC HVOF Laser (c) Immersion time (h) 1000 (d) Immersion time (h) Figure 6.32 Charge transfer resistance R ct before and after laser treatment for various time of immersion (a) T800 (b) T800+21WC (c) T800+43WC (d) T800+68WC Figure 6.33 (a) illustrates the cross section of T800 HVOF coating after EIS test at 12h immersion time. In figure 6.33 (b) it is clear the network of pores within the coating provides a path for electrolyte to penetrate through the coating and reach the interface causing corrosion of the steel substrate which is confirmed by ICP test (figure 6.14). Figure 6.34 (a) shows general view of T800 HVOF after EIS 12 h of immersion test. It was observed that splat boundary regions start to corrode preferentially. Also corrosion occurred within splats at Co solid solution (figure 6.34 (b)), where Laves phase contains high percentage of Mo which shows low dissolution in the electrolyte as confirmed by ICP test (figure 6.14). These splat boundaries have higher oxide contents as seen in EDX results (figures 5.34 and 5.35), and confirmed by EPMA mapping (figure 5.36). Moreover, lateral cracking within splats has been observed as shown in figure 6.34 (c), these cracks may have occurred because of oxide induced expansion in splat boundaries which initiate cracks in the very fragile splats material (T800) [172]. 236

237 CHAPTER 6 CORROSION TESTS Figure 6.33 Cross section of T800 HVOF coating after EIS test at 12h immersion time (a) cross section of the coating (b) interface Figure 6.34 Cross section of T800 HVOF coating after EIS test at 12h immersion time (a) surface of the coating (b) corrosion of Co solid solution within splats (c) cracks within splats Figure 6.35 (a) illustrates the cross sectional view of T800 HVOF coating after laser treatment following EIS test at 12h of immersion time in 0.5M H 2 SO 4. The coating was attacked slightly at the surface in the Co solid solution as can seen from figure 6.35 (b) and the difference can be noticed by comparison with assprayed condition in figure 6.34 (c). The interface was not attacked by corrosion, as 237

238 CHAPTER 6 CORROSION TESTS the laser treatment significantly reduced the porosity at the surface of the coating. The improvement of corrosion resistance was due the significantly reduced porosity (table 5.4 and table 5.7) which protected the substrate from corrosion. Also, the improvement of corrosion behaviour was as a result of reduced or eliminated the oxide inclusion around splat boundaries. Figure 6.35 Cross section of laser treated T800 HVOF coating after EIS test at 12h immersion time (a) cross section of the coating (b) zoom of the upper side of the coating Figure 6.36 illustrates cross section of T800+68WC HVOF coatings after EIS test at 12h of immersion time. After adding WC carbides, the corrosion occurred at the matrix (T800) regions as can see from figure 6.36 (c). The corrosion attacked the upper side of the coating as shown in figure 6.36 (a). The electrolyte penetrated through the interconnected pores to the interface and caused corrosion of substrate (figure 6.36 (b). The dissolution of T800 as a result of galvanic corrosion, where WC works as cathodic and T800 works as anodic. 238

239 CHAPTER 6 CORROSION TESTS Figure 6.36 Cross section of T800+68WC HVOF coating after EIS test at 12h immersion time (a) cross section of the coating (b) interface (c) zoom of upper side of the coating Figure 6.37 shows laser treated T WC HVOF coating after EIS test at 12h of immersion. Small corrosion at the interface has been noticed which indicates that laser treatment sealed the defects within the HVOF coating and protected the substrate from the corrosion (figure 6.37 (a)). However, as a result of existence of partially melted of WC, the coating still contains regions of WC and small regions of T800 (figure 5.74), this microstructure will create galvanic coupling between the wide area of WC particles and the small area of T800 (figure 6.37 (b)). Figure 6.37 Cross section of laser treated T800+68WC HVOF coating after EIS test at 12h immersion time (a) cross section of the coating (b) corrosion area 239

240 CHAPTER 6 CORROSION TESTS Cross section of T800 HVOF coating after EIS test at 48h of immersion time showed in figure 6.38 (a). The coating faced severe corrosion attack around the splat boundaries and within splats (figure 6.38 (b)), also the steel substrate corroded extensively, where the electrolyte found its way easily through the connected pores in the HVOF coating. After laser treatment (figure 6.39 (a)) the protection of substrate of electrolyte penetration has been significantly improved, even though there was a small amount of corrosion attack at the interface. Figure 6.39 (b) shows corrosion attack of Co solid solution, a very small amount of attack noticed at Laves phase, these results have been confirmed by ICP test (figure 6.14). The improvement in corrosion behaviour was clearly due to modification of the morphology of the coating and reduction in the defects within the coating. Figure 6.38 Cross section of T800 HVOF coating after EIS test at 48h immersion time (a) cross section of the coating (b) corrosion at the upper side of the coating Figure 6.39 Cross section of laser treated T800 HVOF coating after EIS test at 48h h immersion time (a) cross section of the coating (b) zoom at the upper side of the coating 240

241 CHAPTER 6 CORROSION TESTS Figure 6.40 illustrates cross section of T800+21WC HVOF coating after EIS test at 48h of immersion. The corrosion attacked the surface of the coating and as shown in figure 6.40 (a) galvanic coupling between WC and T800 control the corrosion action. The figure shows some WC particles moved from their positions due to corrosion of the surrounding T800. Also, there was corrosion attack at the interface where the electrolyte penetrated the coating as a result of interconnected pores in the coating (figure 6.40 (b)). However, after laser treatment T800+21WC HVOF coating did not show corrosion attack at the surface as can see in figure 6.41 (b). Moreover, a very small amount of corrosion attack was noticed at the interface (figure 6.41 (a)). The excellent improvement of corrosion behaviour of this coating was due to greater melting of WC particles which form a new phase of W 2 Co 4 C to reduced microgalvanic activity between carbides and the matrix. Figure 6.40 Cross section of T800+21WC HVOF coating after EIS test at 48h immersion time (a) upper side of the coating (b) interface Figure 6.41 Cross section of laser treated T800+21WC HVOF coating after EIS test at 48h immersion time (a) cross section of the coating (b) upper side of the coating 241

242 CHAPTER 6 CORROSION TESTS Figure 6.42 shows cross section of T800+68WC HVOF coating before and after laser treatment after EIS test at 48h of immersion test. As shown in figure 6.42 (a), in the as sprayed coating, the corrosion attacks the upper region of the surface, and the interface between the coating and substrate, which indicates there was penetration of the electrolyte to the interface through the interconnected porosity. After laser treatment as shown in figure 6.42 (b) there was corrosion at the surface. However, there is no indication of corrosion at the interface which confirms that laser treatment reduced the interconnected pores in the coating, making the coating more protective to the substrate against attack of the electrolyte. Figure 6.42 (c) illustrates that corrosion occurred on the upper regions of surface which are anodic (matrix). After laser treatment (figure 6.42 (d)) the corrosion attacks the small areas between the carbides (gray areas) which work as anodic, where the laser did not melt the carbides completely and still there is galvanic activity in the partially melted area. Figure 6.42 Cross section of T800+68WC HVOF coating after EIS test at 48h immersion time (a) cross section as HVOF coating (b) cross section after laser treated (c) upper side of HVOF coating (d) upper side of laser treated coating 242

243 CHAPTER 6 CORROSION TESTS In conclusion, laser surface treatment of HVOF sprayed Co based WC MMC coatings significantly reduced the microstructural defects such as porosity and splat boundaries. Laser surface treatment also resulted in the formation of new phase due to the interdiffusion and interaction between the WC and Co matrix. Immersion tests, polarisation tests, and electrochemical impedance spectroscopy analysis along with ICP test and the SEM examination of corrosion morphology confirmed that the densification of the coatings by laser treatment prevented the electrolyte penetration and subsequent corrosion attack of the steel substrate for laser treated HVOF coatings. For the laser treated T800+21WC coatings, EIS analysis showed significant improvement in the corrosion resistance due to the fully melting of the WC resulting in reduction of microgalvanic corrosion between the WC and Co matrix. However, for the T800+43WC and T800+68WC coatings, the laser treatment under the current processing conditions did not produce sufficient melting of the WC to reduce the microgalvanic activity. 6.5 Summary Immersion tests followed by ICP tests for the as-received HVOF coatings show that the electrolyte can penetrate easily through the interconnected pores of HVOF coating to reach the interface between the substrate and the coating, causing corrosion and dissolution of the substrate and delamination of the HVOF coatings. Immersion tests followed by ICP tests for the laser treated HVOF coatings show that after laser treatment the HVOF coatings with significant reduction of defects become more effective barrier coatings in protecting the substrate from corrosive attack. Polarisation tests in aerated 0.5M H 2 SO 4 solution show a positive shift of corrosion potentials and reduction of corrosion current density after laser treatment, indicating that the laser treatment improves corrosion resistance of HVOF coatings. 243

244 CHAPTER 6 CORROSION TESTS Electrochemical Impedance Spectroscopy (EIS) tests in aerated 0.5M H 2 SO 4 solution show that there is an oxide layer of chromium oxide formed at the electrode surface with increasing time of immersion. In general the Nyquist plots clearly illustrated the positive effect of laser remelting of HVOF coating for all types of coating with different percentage of WC. The degree of WC melting affect the corrosion behaviour of the MMC laser treated HVOF coatings, the higher the percentage of WC the lower the percentage of corrosion improvement. This is because, by increasing the content of WC, more WC grains remain umelted in the coating after laser treatment, so the reduction of micro-galvanic activity between the two phases became less effective. Also EIS showed significant reduction of porosity in HVOF coatings after laser treatment. The corrosion starts to attack the micro-crevices within splat boundaries, and the boundaries between the carbides and the matrix as a result of galvanic coupling between the WC and the T800 matrix. Laser surface treatment improved the corrosion properties of T800 and T800+WC HVOF coatings because of: 1) significantly reduction of defects such as porosity, micro-crevices, and splat boundaries. 2) high melting of WC formed new phase as a result of interaction between the WC and T800 matrix, which reduced the micro-galvanic coupling between the WC and the T

245 Chapter 7 Hardness and Wear Resistance Measurements 7.1 Introduction This Chapter presents the results of Vickers microhardness and wear rate tests to evaluate the wear performance of various HVOF coatings before and after laser treatment. SEM was used to investigate the worn areas to understand the wear mechanisms involved for the various coatings. 7.2 Microhardness Vickers microhardness under a 100 g load was measured on ground and polished cross-sections of various coatings from the top of the coating surface to the base stainless steel, to produce hardness profiles along the melt depth. The distance between indentations remained large enough to avoid interaction between the new indentation and any micro-cracks caused by the previous indentations. Figure 7.1 shows microhardness profiles for various HVOF coatings before and after laser treatment. It can be seen that for the T800 HVOF coating, there was an improvement in homogeneity after laser treatment. The improvement was due to the elimination of the defects from the HVOF coating such as splat boundaries, inhomogeneity, and porosity (as shown in Figures 5.20 and 5.42). However, the average values of the microhardness for T800 HVOF coatings before and after laser treatment were 909 HV 0.1 and 912 HV 0.1 respectively, indicating that laser treatment led to no significant change of microhardenss. The value of the hardness for the laser-treated coating was higher than the value of 850 HV 0.3 reported for a laser-clad layer of T800 coating on stainless steel (AISI 304) by Navas et al. [123]. Fig. 7.1(b) illustrates the microhardness for T800+21WC coating before and after laser treatment and the results show that the laser treated specimen presented 245

246 CHAPTER 7 HARDNESS AND WEAR RESISTANCE MEASUREMENTS a more homogenous microhardness distribution, and higher microhardness values in the laser treated area were found close to the surface. Within the HVOF coating, obviously the hardness of the WC cluster was up to 1254 HV 0.1, while the hardness of the T800 matrix was around 687 HV 0.1, giving the average value of microhardness of the HVOF coating of 729 HV 0.1. After laser treatment, the average hardness was increased to 955 HV 0.1. The inhomogenity in microhardness for HVOF can be explained from figure 7.2, where when the indentation was on WC phase as shown in figure 7.2 (b) the microhardness value was high. If the indentation was on T800 phase as shown in figure 7.2 (a) the microhardness was much lower. The high microhardness in some points on HVOF coating might be due to the presence of big WC particles in the untreated layer. As expected, the same behaviour has been observed in T800+43WC and T800+68WC coatings, in which only a small improvement in microhardness after laser treatment was obtained, while significant improvements in homogeneity have been achieved, as can be seen in figures 7.1, 7.2, and 7.3. The complete results of microhardness tests are shown in Figure 7.1 and Table 7.1. As shown in Table 7.1 the values of standard deviation indicate the inhomogeneity in the coating. The higher the standard deviation, the more the inhomogeneity in the coatings is. The results in the table also illustrate that the laser treatment improves homogenisation of the coatings which obviously reduces the standard deviation values after laser treatment. This improvement in homogenisation was due to the spread of the partially melted WC particles over a wider area in the coating as can be seen in figure 7.3. Moreover, the indentations on the coating after laser treatment have more symmetrical shape than the indentations on the HVOF coatings due to the homogeneity of the laser treated coating in figure

247 CHAPTER 7 HARDNESS AND WEAR RESISTANCE MEASUREMENTS Figure 7.1 Hardness profiles of various HVOF coatings before and after laser treatment (a) T800 (b) T800+21WC (c) T800+43WC (d) T WC Table 7.1 Microhardness values of various HVOF coatings before and after laser treatment T800 HVOF ± 99.2 * T800+21WC Laser HVOF Laser ± 39 * ± 364 * ± 99 * 247

248 CHAPTER 7 HARDNESS AND WEAR RESISTANCE MEASUREMENTS T800+43WC HVOF ± 157 * T800+68WC Laser HVOF Laser ± 61 * ± 146 * ± 21 * n * Standard deviation Figure 7.2 SEM micrographs of microhardness indentations on different phases of T800+43WC coating. 248

249 CHAPTER 7 HARDNESS AND WEAR RESISTANCE MEASUREMENTS Figure 7.3 SEM micrographs of microhardness indentations on different phases of laser treated T800+43WC coating. Figure 7.4 SEM micrographs of microhardness indentations (a) on as-received HVOF coating and (b) after laser treatment. 7.3 Wear test results Figure 7.5 illustrates the results for the wear test of various HVOF coatings before and after laser treatment. It is obvious that in T800 coating, there was a significant improvement in wear resistance, in which the wear rate of the HVOF coating was 0.43 m 3 /N m while after laser treatment it was reduced to m 3 /N m, indicating 25 times reduction in wear rate after laser treatment. Furthermore, the improvement in wear resistance was observed for the other coatings. Where, the wear rate in T800+21WC decreased by around 9 times after laser treatment. In T800+43WC and T800+68WC the wear rate decreased 3 times and 5 times after laser treatment respectively. Also from figure 7.1 it is clear that the coatings with higher percentage of WC, have higher wear resistance for both as-received HVOF 249

250 Wear rate ( m 3 /N m) CHAPTER 7 HARDNESS AND WEAR RESISTANCE MEASUREMENTS and laser treated. In previous work, Liu et al [12] used HPDL remelting for Iconel 625 with WC HVOF coatings. The results showed that wear volume loss has been decreased for Inconel wt.% WC by ~ 80% and by ~ 50% for Inconel Wt.% WC Wear rate for HVOF and after laser treatment 0.4 HVOF Laser T T800+21WC T800+43WC T800+68WC Figure 7.5 Wear rates for various coatings before and after laser treatment T800 In order to understand the improvement in wear resistance and wear mechanisms involved in various coatings before and after laser treatment, SEM images for the worn areas have been taken. Figures 7.6 and 7.7 show the surface morphology of the worn areas for T800 coating before and after laser treatment respectively. The T800 alloy is a very brittle material as reported in some previous studies [155, ]. As shown in Figure 5.9 in Chapter 5, the T800 HVOF coating contained gaps and micro-cracks between the splats. Surface fatigue led to mixture of severe plastic deformation, and fracture wear. During sliding cracks occurred in the wear track. Subsequently large wear layers tend to be produced during surface breakup. In the surface fracture wear the amount of damage is increased by increasing sliding distance (figure 7.6). In addition, the high wear rate can also be attributed to the high surface roughness of T800 HVOF coating (R a ~ 5µm) (Table 5.1). Where, the brittle protrusions on the T800 surface are easily removed by the abrading particles. 250

251 CHAPTER 7 HARDNESS AND WEAR RESISTANCE MEASUREMENTS On the other hand, the laser-treated T800 coating (figure 7.7) shows much denser layers, due to the elimination of the defects such as porosity and gaps. Therefore, the laser treatment improved the cohesion between the splats and the adhesion strength at the coating/substrate interface due to the formation of fusion bonds by laser treatment. The wear mechanism in this case might be mixed of adhesion and oxidation wear. Furthermore, harder phases precipitated after laser treatment such as eutectic (Laves phase + Co) had increased the hardness and positively affected wear resistance [178, 179]. Also the significant reduction in surface roughness in T800 after laser treatment (R a = 0.18 µm) (Table 5.6) also contributed to the reduction of the wear rate. Figure 7.6 SEM micrographs of worn surface of T800 HVOF coating. 251

252 CHAPTER 7 HARDNESS AND WEAR RESISTANCE MEASUREMENTS Figure 7.7 SEM micrographs of worn surface of laser-treated T800 HVOF coating T800+21WC Figure 7.5 shows an improvement in wear resistance for the T800+21WC coating after laser treatment. It has been observed from figures 7.8 and 7.9 there was abrasion wear before and after laser treatment. In general there are two important factors which affect the wear rate. One is hardness and the other is cohesion of material. For this MMC material, the hard WC particles increase the average hardness as shown in figure 7.1 and Table 7.1. The T800 matrix acts as a binder material between these hard particles. As shown in Figure 7.8, due to the nature of thermal spray processes, the cohesion between WC particles and the matrix is not strong, the wear process resulted in many fractured particles appearing in worn tracks (figure 7.8 (d)). It suggested that some material loss took place firstly by removal of the binder phase (matrix) due to fracture wear and cracks in the subsurface, followed by pullout of the carbides. The resultant fine debris from the matrix and fractured WC, caused further sharp cutting abrasive wear, promoting further removal of the matrix. The high surface roughness (R a = 5.47 µm) (Table 5.1) could be an important factor in the high wear rate. 252

253 CHAPTER 7 HARDNESS AND WEAR RESISTANCE MEASUREMENTS Figure 7.8 SEM micrographs of worn surface of T WC HVOF coating. Figure 7.9 shows the worn area of T800+21WC after laser treatment. The improvement of wear resistance after laser treatment as shown in figure 7.1 was mainly due to the combination of partially melted carbides and fully melted substrate that increased the cohesion between the carbide particles and the matrix. Also the elimination of porosity and gaps by laser treatment affected wear resistance positively. In this case the wear performance has the same mechanism as described for the HVOF coatings above. However, for laser-treated surface, pulling out the hard particles may take a longer time so that the harder particles fractured and remained in the matrix longer as shown in figure 7.9 (d), (e). And meanwhile, the laser re-melting reduced the lamellar nature of the structure, and increased the average hardness value, and reduced surface roughness to 1.9 µm (Table 5.6). These factors also contributed to the improvement of the wear resistance positively. 253

254 CHAPTER 7 HARDNESS AND WEAR RESISTANCE MEASUREMENTS Figure 7.9 SEM micrographs of worn surface of laser-treated T WC coating T800+43WC The same wear mechanism which occurred in T800+21WC coating before and after laser treatment can be observed on the T800+43WC coating. As shown in figures 7.10 and 7.11, it can be seen that some small WC particles (Figure 7.10d) after fracturing were pulled out the matrix. On the other hand, it is obvious that a higher percentage of WC particles in this case improved the wear resistance for both HVOF and laser-treated, compared with T800+21WC coating. This indicates that the WC particles are resistant to wear and are successfully retained by the matrix especially after laser partial melting. 254

255 CHAPTER 7 HARDNESS AND WEAR RESISTANCE MEASUREMENTS Figure 7.10 SEM micrographs of worn surface of T WC coating. Figure 7.11 SEM micrographs of worn surface of laser-treated T W coating T800+68WC Figure 7.12 shows the worn regions of the T800+68WC coating. The increased WC percentage in the matrix further improved the wear resistance. The wear mechanism in the HVOF coating had the same wear process as described for the lower WC content coatings (T800+21WC and T800+43WC) in which the loss of materials was due to fracture wear in T800 matrix caused by fracturing WC and pulling out the carbides (figure 7.12). However, after laser treatment (figure 7.13), the grooves running parallel to the slide direction became more obvious, indicating a mainly abrasive wear mechanism. Also, the small WC particles can be seen in the 255

256 CHAPTER 7 HARDNESS AND WEAR RESISTANCE MEASUREMENTS worn area which took a longer time to be pulled out due to the fusion bonding between the partially melted WC particles and the Co-matrix. Figure 7.12 SEM micrographs of worn surface of T WC coating. Figure 7.13 SEM micrographs of worn surface of laser-treated T WC coating. 7.4 Summary The microhardness distribution became uniform within the HVOF coatings, due to homogenisation of microstructures by laser remelting, and elimination of porosity and gaps from the coating. For T800 coating, the hardness was increased as a result of formation of Lavas phase; For T800+WC coatings, more uniform distribution of microhardness was observed. Wear test (pin on disk) showed that the wear process was dominated by the fracture wear mechanism in T800 HVOF coatings and in the matrix of T800+WC HVOF coating. There was significant improvement in wear 256