Role of Surface Defect Density on the Corrosion Properties of PVD Coatings

Transcription

1 Role of Surface Defect Density on the Corrosion Properties of PVD Coatings Darja Kek Merl Department of Thin Films and Surfaces Jožef Stefan Institute Jamova 39, 1000 Ljubljana, Slovenia

2 Table of contents Motivation PVD Coatings Corrosion resistance of PVD coating Origin Growth defects in PVD coatings Stylus profilometry Focused ion beam Conclusions

3 Motivation BIOMATERIALS and IMPLANTS Metal, metal alloys Corrosion resistance? Biocompatibility? SURFACE R&D Solution? Coating(s) with better properties than substrate INCREASE OF LIFE TIME corrosion resistance tribological properties other functional properties (i.e.,biocompatibility, antibacterial)

4 Introduction PVD coatings Basic idea to protect the surface of the material while leaving the bulk intact unprotected material Type of coatings covered by a coating chemical methodes (organic coating, ceramic coatings..) electrochemical methodes surface engineering methods etc.

5 Surface engineering methods PROTECTIVE COATINGS SURFACE MODIFICATION FROM VAPOR PHASE PVD sputtering evaporation ion plating reactive deposition CVD classical lowpressure laser electron PACVD plasma CVD reactive pulsed plasma plasma polimerization FROM SOLID PHASE THERMAL DEPOSITION plasma spray low-pressure plasma explosion electric arc plasma arc WELDING fire electric arc plasma arc FROM SOLUTION electrochemical (galvanic) electroless MICROSTRUCTURAL IMPROVEMENT CHEMICAL IMPROVEMENT THERMAL induction quenching plasma quenching laser quenching quenching by electron beam MECHANICAL cold forming sand-blasting BY DIFFUSION cementing nitriding carbonitriding plasma nitriding borizing chromatizing BY INSERTION ion implantation ion mixing PVD physical vapor deposition CVD chemical vapor deposition PACVD plasma assisted chemical vapor deposition

6 PVD coatings: basics Sputtering anode Reactive sputtering substrate coating argon ions plasma Coatings are chemicaly identical to target material (i.e, Ti, Ti-Al,Cr) argon inlet sputtered material cathode (target) + plasma generator substrate coating argon ions cathode (target) plasma anode argon inlet reac. gas inlet sputtered material + plasma generator Coatings are nitrides, carbides, oxides of target material due to reaction with gas; N 2, O 2, C 2 H 2

7 PVD coatings: basics Properties of PVD coatings high hardness good wear resistance chemical inertness low coefficient of friction good biocompatibility Problems of coatings? adhesion defect density porosity? corrosion resistance? defect coating substrate

8 Table of contents Motivation PVD Coatings Corrosion resistance of PVD coating Origin Growth defects in PVD coatings Stylus profilometry Focused ion beam Conclusions

9 Types of corrosion Corrosion is defined as a chemical and/or electrochemical reaction of metal with the environment, where the solid, soluble or gaseous products are formed. homogenous corosion attack metal Oxide layer pit pit corrosion There are different mechanisms of corrosion: porous Cu narrow opening uniform corrosion that takes place across the exposed surface brass (Cu-Zn) local corrosion (e.g. pitting, crevise, selective), take place at specific places on the surface intercrystalline corrosion at grain boundaries stress corrosion cracking, observed on material exposed to a constant tensile stress in a corrosive environment selective corrosion intercrystalline attack intercrystalline corrosion load stress stress Crevise corrosion flow metal removal erosive corrosion cracks stress corrosion fatigue corrosion

10 Types of coatings Ideal case covered by a coating real cases More electroneg.coating then base material Less electroneg.coating then base material As deposited After exposed to corrosive media e.g. Zn on Fe e.g. CrN on Fe; Ag on Fe

11 Corrosion testing by electrochemical methodes 3-electrode corrosion cell; WE electrode is multilayer structure substrate/coating/electrolyte corrosive media; Hanks solution; bioapplication 0.1M NaCl solution; general corrosion Methodes: potentiodynamic measurements (PD) at de/dt = 1 mv/s electrochemical impedance spectroscopy at opent circuit potential

12 Potentiodynamic polarisation: measurement princip Voltage is applied on (WE) and the current response is measured that occurs as a result of electrochemical reactions between the electrode and corrosive media. de/dt = 1 mv/s E b transpassive regionafter corrosion products have been formed Data from PD measurement: E corr j corr E b Potential (E) / V E corr passive regionstable oxide film active regiondissolution cathodic region j corr log j

13 Potentiodynamic polarisation: results Case: SS316L/Ti/DLC corrosive media; simulated physiolagical solution (Hanks) at room T; de/dt = 1 mv/s E corr more nobel material E /V vs Ag/AgCl 2,0 1,5 1,0 0,5 DLC/Ti/SS Ti/SS i corr higher corrosion resistance 0,0 1 kv 2 kv 3 kv SS-316L -0,5 1E-11 1E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0,01 0,1 j/acm -2

14 Potentiodynamic polarisation: results PVD hard coatings: Cases: (CrN, CrCN,TiAlN)/MS 1.5 TiAlN corrosive media (general corrosion) 0.1M NaCl de/dt = 1 mv/s E corr more nobel material E/ V vs Ag/AgCl μm CrCN defect -0.5 coating CrN mild steel (MS) i corr higher corrosion resistance E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E j /Acm -2 substrate

15 Electrochemical Impedance spectroscopy (EIS): princip Principe of measurements: AC potential applied AC current responce current response Impedance= AC resistor=z(jω) applied potential 20μm substrate

16 Corrosion testing by EIS: results case:tialn/steel Nyquist diagram 120 R p better corrosion properties h 2h 4h 10h 24h 48h 96h defect time R p -Z Im /kω 60 20μm 40 coating R p i corr Z Re /kω substrate Rp

17 Corrosion resistance EIS: results Bode-Bode Diagram Case:TiAlN/steel Equvalent circuit - physical model C coat Z /Ω h 2h 4h 10h 24h 48h 96h substrate mild steel coating defect-pore electrolyte NaCl solution C dl R corr R coat R ele 20μm 80 coating -Theta ν/hz defect 1h 2h 4h 10h 24h 48h 96h 20 substrate ν/hz

18 Corrosion resistance EIS results Values of fitted parameters (R, C) from equivalent circuit Imersion time/ h E corr /mv Q coat μ/cm 2 s -n Ω n R coat / Ωcm 2 Q dl μ/cm 2 s -n Ω n R corr /Ωcm 2 R p /Ωcm 2 CrN/steel * * * * * * μm coating defect * * * *10 3 substrate

19 Corrosion resistance EIS: results Impedance response of 316L/Ti/DLC-films in Hank solution; Bode-Bode Plot Z /Ω substrate A-1h A-96h B-1h B-96h C-1h C-96h -Theta substrate A-1h A-96h B-1h B-96h C-1h C-96h ν/hz ν/hz

20 Corrosion resistance EIS: results Impedance response of 316L/Cr/DLC films in Hank solution Q DLC Q DLC/electroyte -Z Im / MΩ cm Z Im /Ω cm Z 2 Re /Ω cm 2 1kV-1h 1kV-96h 3kV-1h 3kV-96 2kV,N2-1h 2kV, N2-96h Z Re / MΩ cm 2 ε=7.5 R ele Q R DLC R DLC/electrolyte n n = C ( jω) n=1, Q reduces to an ideal capacitor

21 Corrosion resistance EIS results Impedance response of multilayer structure 316L/Ti/DLC/electrolyte Fitting with 2RQ term to determine resistance and capacitance properties of film, and film/electrolyte interface substrate Ti DLC DLC film electroyte R ele Q Q DLC R DLC n n = C ( jω) Q DLC/electroyte R DLC/electrolyte n=1, Q reduces to an ideal capacitor IJS, Institute Jozef Stefan

22 Corrosion resistance of PVD coatings PVD coating i.e. transitional metal (Cr, Ti)- nitrides bellong to the nobel materials, thefore they are chemical inert At the defects, pitting corrosion is formed due to galvanic cell between substrate and more nobel coatings Changeing the EIS spectra (reducing R, increasing icorr) with time is due to reaction 20μm take place at the substrate/electrolyte interface substrate

23 Corrosion resistance of PVD coating All PVD coatings are microporous. Pores and other defects in the coating limit their corrosion resistance. 20μm coating defect substrate

24 Corrosion resistance of PVD coating Corrosion resistance of hard coatings can be improved by reducing of surface defect density; therefore the origin of defects should be analysed. 20μm 2 μm 10 μm coating 2μm defect 1 µm 1 μm 1 μm

25 Table of contents Motivation PVD Coatings Corrosion resistance of PVD coating Origin of growth defects in PVD coatings Stylus profilometry Focused ion beam Conclusions

26 Introduction to growth defects Growth defect features conical growth poor adhesion to the matrix (delamination) decrease film properties TiAlN/CrN on hard metal 200 nm 200 nm

27 Intoduction to growth defects: defects origin substrate imperfections substrate contamination 20μm coating defect film contamination (flaking, dust, abrasion, arcing) defect removal and regrowth

28 Substrate imperfections Case: tool steel ASP30 + TiAlN coating after polishing after ion etching after deposition

29 Substrate imperfections valleys peaks 10 µm improper polishing AlTiN/TiN 1 µm pinhole formation HM 1 µm TiAlN carbide inclusions TiAlN D2 1 µm steel VC 500 nm

30 Substrate contamination 5 µm dust after etching shade effect voids CrN/TiAlN HM 1 µm CrN/TiAlN D2 1 µm

31 Film contamination 500 nm 1 µm 200 nm

32 Defect removal and regrowth 1 µm Growth defect study What is the concentration of growth defects? What is the structure of growth defects? 1 µm density 100 mm 2

33 Table of contents Motivation PVD Coatings Corrosion resistance of PVD coating Origin Growth defects in PVD coatings Stylus profilometry Focused ion beam Conclusions

34 Stylus profilometry: experimental A common technique for evaluation of roughness 0,5 stylus with a diamond tip measurement direction [µm] 0,4 0,3 0,2 sample 0, ,2 0,4 0,6 0,8 1 [mm]

35 Stylus profilometry: experimental Features: insensitive to material type or color slow problems in sharp edges sharp peak narrow trough Optical alternative: problem with dust size of defects comparable to λ

36 Stylus profilometry: experimental growth defect resolution x: 0.2 µm Y: 1 µm Z: few nm film substrate tip radius: 2 µm scanning area: 1 mm 1 mm scanning time: 1.5 h y growth defect film surface Acquired topography x

37 Stylus profilometry: results Statistics mm µm roughness height distribution of defects number of peaks, valleys density 100 mm mm

38 Table of contents Motivation PVD Coatings Corrosion resistance of PVD coating Origin Growth defects in PVD coatings Stylus profilometry Focused ion beam Conclusions

39 Focused ion beam: experimental A relatively new technique for observation of cross-section Usually integrated in a SEM untreated surface etching by Ga ions observation by electrons

40 Focused ion beam: experimental growth defect film substrate Sample ion etching growth defect film substr. 10 µm FIB trough etching growth defect film SEM analysis substrate seed

41 Focused ion beam: results CrN on Fe 5 µm

42 case 1 CrN / Al alloy Focused ion beam: results CrN CrN Ni Al alloy case 2 TiAlN / steel 5 µm 5 µm Fe TiAlN steel 2 µm 2 µm IJS, Institute Jozef Stefan

43 Focused ion beam: results TiAlN/on steel a 20 µm b 5 µm 2 µm 5 µm 2 µm c d 10 µm 5µm 5 µm 2 µm

44 Table of contents Motivation PVD Coatings Corrosion resistance of PVD coating Origin Growth defects in PVD coatings Stylus profilometry Focused ion beam Conclusions

45 Conclusions Corrosion resistance of PVD coating Potentiodynamic measurements Corrosion potential Corrosion current density(corrosion rate) Break down potential Destructive methode Electrochemical impedance spectroscopy Corrosion potential, corrosion rate Properties of coating/electrolyte and substrate/electrolyte interface Modeling neccessary Non-destructive methode Take-home message PVD coatings are microporous, pore and defects limit the corrosion resistance R corr can be improved by: interlayer, thickness, multilayer, nanocomposite etc.

46 Conclusions Growth Defects in PVD coating Stylus profilometry growth defect density statistical nature time consuming Focused ion beam analysis of one defect only complete structure of the growth defect EDS: chemical composition of the seed Take-home message do not underestimate the role of growth defect in PVD coatings carefully prepare the substrates and keep the lab hygene

47 Jožef Stefan Institute, Department of Thin Films and Surfaces Peter Panjan, Miha Čekada, Matjaž Panjan, Srečko Paskvale, Peter Gselman University of Maribor, Faculty of Mechanical Engineering Franc Zupanič, Tonica Bončina National Institute of Chemistry Gregor Kapun Acknowledgements Thank you for your attention!