STUDY OF FAILURE PROCESSES IN SPECIMENS WITH DIFFERENT TYPE OF COATINGS

Transcription

1 Jan Siegl STUDY OF FAILURE PROCESSES IN SPECIMENS WITH DIFFERENT TYPE OF COATINGS Czech Technical University - Faculty of Nuclear Sci. and Physical Engineering, Department of Materials, Trojanova 13, Praha 2, Czech Republic Abstract Thermal spray is an advanced material-processing tool and is being used in a wider range of industries to solve increasingly challenging problems. Considerable advances in equipment and materials technology have greatly expanded the range of materials and applications for which thermal spray can be used. Thermal sprays have become such an important technology that, e.g., modern marine and jet aircraft engines would not be able to operate without it. These applications implicitly demand to ascertain how the coatings affect fatigue behaviour of components. This paper is concerned with an experimental study of fatigue behaviour and failure processes of flat steel specimens with thermally sprayed coatings. The three different types of coatings under study were : electric arc sprayed austenitic steel 316L, plasma sprayed molybdenum and alumina (Al 2 O 3 ). The results of fatigue bending tests are shown and discussed from the point of view of coating technology influence on fatigue lives. Fractographic analysis of fractured test specimens was carried out and main fractographic features of sprayed coatings were described. In-situ study of coated specimens in the vacuum chamber of SEM was used to specify fracture mechanisms of coatings. 1. INTRODUCTION The specific desired properties for structures and components working in critical environments cannot be fulfilled with only one material. A solution may be found by applying different types of composite materials. One of the perspective approaches is application of thermally sprayed coatings with appropriate properties to component surfaces. Thermal spraying processes are widely used techniques enabling production of different protective coatings that can be used as thermal-barrier, wear-resistant, and corrosion-resistant surface layers. Various component/coating combinations between metals, ceramics and plastics have to protect components against constantly increasing stresses and temperatures with weight savings at the same time [1]. The properties of coatings are quite different from those of bulk materials of the same composition, as a consequence of porosity, anisotropy and residual stress [2]. Thermally sprayed coatings can also influence the mechanical properties of substrate. Industrial and medical applications of coated components implicitly demand to ascertain failure processes taking place in coated bodies. Also the investigation of coatings influence on substrate fatigue behaviour has been gaining importance (e.g., [3, 4]). Therefore, a wide experimental programme was realised at the Department of Materials as a part of the research project Surface Engineering. The experiments were focused on studying fatigue properties of steel specimens with different thermally sprayed coatings [5, 6] : electric arc sprayed austenitic steel 316L, plasma sprayed molybdenum and alumina (Al 2 O 3 ). The main goals of these experiments were an evaluation of coatings influence on fatigue life of substrate and description of failure mechanisms taking place in coated steel specimens.

2 2. EXPERIMENTAL 2.1 Test specimens A sheet of thickness 2.5 mm produced from steel sheet ČSN was used as a substrate for fatigue test specimens (see Fig. 1). Three different types of coatings were applied on both sides of shot-peened specimens : Fig. 1: Fatigue test specimen (thickness = 2.50 mm). Mo coatings layer of thickness 0.25 mm/surface was plasma sprayed, using gas stabilised system in the laboratory of the Institute of Materials Engineering TU Brno. Plasma of temperature C was generated by electric current 400 A in argon (41 l/min) complemented with hydrogen (10 l/min). The feedstock material was Mo powder MOLI 200; distance of the plasma torch from specimen was 120 mm, and substrate temperature 120 C. 316L steel coatings layer of thickness 0.5 mm/surface was electric arc sprayed on the bond coat of Ni-5%Al of thickness 0.2 mm/surface in the laboratory of Škoda Research, Ltd. Plzeň. Deposition was carried out with a special device, TAFA Model 9000; the feedstock material was austenitic steel wire ( 1,6 mm); distance of the electric arc gun from specimen was 100 mm, and substrate was not preheated. Al 2 O 3 coatings layer of thickness 0.7 mm/surface was plasma sprayed, using water stabilised system in the laboratory of the Institute of Plasma Physics AS CR. The torch was operated around 160 kw (320 V, 500 A). The feedstock material was powder of brown corundum AH 230 (particle size µm); distance of the plasma torch from specimen was 450 mm, and temperature of substrate 100 C. 2.2 Fatigue tests All fatigue tests were performed on computer-controlled electromagnetic testing machine SF-Test developed at the Department of Materials. The specimens were loaded with reversible bend (as a cantilever beam - Fig. 2) at room temperature and with a loading frequency of (50 60) Hz. The constant loading level is determined by means of constant deflection of the free end of specimen. The control computer ensures maintaining constant deflection during the test: the input signal is deflection measured with an optical sensor; processing of input data enables generation of the required driving signal for electromagnetic coils. Detailed description of the testing machine SF-Test can be found in e.g., [5] and [6]. Signal frequency maintaining the constant deflection is recorded during each fatigue test. A systematic decrease of this frequency corresponds to reducing specimen cross-section, i.e., crack initiation and propagation. Analysis of this record enables to stop the fatigue test at an arbitrary selected stage of specimen damage. In the experiments presented, some fatigue tests were interrupted in case the decrease of loading frequency was 0.5 Hz. This value corresponded to approximately 15% reduction in specimen cross-section due to fatigue crack propagation.

3 All specimens were tested with 6 mm deflection (of the longitudinal axis of specimen). This deflection corresponds to the stress ~ 250 MPa on surface of steel substrate in the area of cracks initiation and propagation. In the course of fatigue experiment four specimens with each type of coatings were tested. Three specimens were tested to fracture, and the test of the last one was interrupted in case the loading frequency decrease was 0.5 Hz. Also, four uncoated specimens were tested as reference set. The results of fatigue tests have proved that thermally sprayed coatings have significant influence on the fatigue behaviour of the Fig. 2: Fatigue Test Machine SF-Test. specimens tested. The same results were found in our previous experiment [5, 6]. The average fatigue lives of coated specimens differ from average fatigue life of uncoated specimens : Specimens with Mo coatings average fatigue life is more than 10 times longer; Specimens with 316L steel coatings average fatigue life is about 25% shorter; Specimens with Al 2 O 3 coatings average fatigue life is more than 6 times longer. The comparison with our previous experiment [5, 6] indicates that the thermal spraying technique can have more significant influence on fatigue life than the coating material. The 316L steel sprayed by HVOF resulted in considerable increase of fatigue life (more than 3 times), while the same steel sprayed by electric arc led to decreased fatigue life. The main reason for this difference in coating effect is difference in residual stresses in specimens with HVOF coatings and in those with electric arc coatings [2, 4]. The effect of residual stresses in coated specimens is much more significant than a possible influence of bond Ni-5%Al coat used in the case of electric arc coatings. 2.3 Fractographic analysis The fractographic analyses were performed on SEM JSM 840A. The fractographic findings obtained when analysing failed specimens led to the following conclusions : a) The macroscopic character of all specimen fractures corresponds to loading by reverse bending. The number of fatigue cracks spontaneously initiated on the substrate surface under coatings, their propagation, and sequential coalescence resulted in specimens failure. b) The sprayed coatings do not influence the fatigue failure mechanism of substrate (steel sheet). Fatigue cracks initiate on substrate surface and grow by striation mechanism. c) The fractographic findings indicate that thermally sprayed coatings are failed by coalescence mechanisms of intersplats (interlamellar) pores and voids, and/or splats breaking. d) The character of fracture micromorphology of individual coatings is predetermined by the shape and size of splats, and also by the size and distribution of pores and voids, i.e., by coatings techniques (see micrographs in Fig. 3).

4 Plasma sprayed Mo Electric arc sprayed Ni-5%Al Plasma sprayed Al2O3 Electric arc sprayed 316L steel Fig. 3 : Fracture micromophology of studied coatings. Nevertheless, detailed fracture analysis of specimens fractured during fatigue tests does not offer sufficient information for reconstruction of failure history of coatings and substrate. Therefore, a special method for investigating incipiency of fatigue cracks propagation was developed [7, 8]. This method was applied to specimens whose fatigue tests were interrupted if loading frequency decrease was 0.5 Hz. The specimens were sequentially cut and then selected sections were metalographically ground and polished. Detailed observation of these specially prepared samples by optical microscope and SEM led to the following results : a) Fatigue cracks were found during observation of metallographic sections lying in planes perpendicular to the crack plane and specimen surface. b) These cracks initiated on substrate surface and propagated only in substrate, see Fig. 4. c) No long continuous cracks were found in coatings. These results imply that the fatigue failure of the specimens investigated originates on the surface of steel substrate, and failure of the thermally sprayed coatings is a consequence of substrate deformation related to fatigue cracks propagation. Similar fatigue fracture

5 behaviour was found during investigation aluminium and/or steel specimens HVOF sprayed with WC-Co coatings [3]. Plasma sprayed Mo Plasma sprayed Al 2 O 3 Electric arc sprayed 316L steel + Ni-5%Al Fig. 4: Fatigue cracks initiated on steel substrate surface and propagated into steel substrate. No cracks were found in coatings (Etched by Nital 5%) 2.4 In-situ study of failure processes Fractographic analyses do not offer sufficient information on failure processes taking place in thermally sprayed coatings. Therefore, the in-situ method was developed for direct SEM observation of failure processes of coatings [9]. These observations were performed for all the coatings investigated. Test samples were prepared from fatigue test specimens (thickness of substrate 2.5mm), by cutting out strips 3 x 20 mm in size. Each strip was sliced into two parts, and furthermore, steel substrate was ground until its thickness was only 0.3 mm or less. The final sample shape for in-situ observation was a thin bimetal strip (steel substrate with coating) with metalographically polished sides. This strip was slowly loaded by three-point bending in a special loading jig in the vacuum chamber of SEM JSM 50A (Fig. 5). The failure processes in coating can be observed on the polished side of samples and also on the top surface of coating (see Fig. 6). The individual stages of cracks formation and propagation can be monitored as a sequence of micrographs, corresponding to the changing level of loading.

6 Thin samples with all the three coatings investigated were studied. The slowly increasing bending load triggers off progressive cracks propagation in coating. Before observation in SEM, the samples with Al 2 O 3 coatings were evaporated with gold. Failure of Al 2 O 3 coating caused by loading of these samples led to cracking of evaporated Au layer, and to charging of samples, i.e., to a decrease in micrographs quality, see Figs. 7, and 8. Figure 5: Three point bend loading jig used Figure 6: Failure of tested sample (steel for in-situ observation in SEM, [9]. substrate with plasma sprayed Mo coating). Fig 7 : Two stages of cracking of Al 2 O 3 coating during in-situ test.

7 Two main mechanisms of crack propagation were observed during in-situ tests (see micrographs in Fig. 8) : a) Growth and coalescence of intersplats pores and voids (generally parallel with substrate surface). Crack growth along the boundary coating-substrate was also observed in case the sample deformation was sufficiently high. b) Splats breaking (often perpendicular to substrate surface). If the loading level was high enough, crack growth into substrate was observed. The developed method of in-situ observation offers very important information on failure processes of thermally sprayed coatings. The results obtained could be used for unambiguous interpretation of fractographic findings. Plasma sprayed Mo Plasma sprayed Al 2 O 3 Electric arc sprayed Ni-5%Al Electric arc sprayed 316L steel Fig. 8: Crack propagation in studied coatings monitored during in-situ test. Micrographs of Mo coating, Ni-5%Al bond coat, and 316L steel coating are in COMPO signal, micrograph of Al 2 O 3 coating is in SE signal.

8 3. CONCLUSIONS The main results of the investigation presented can be summarised as follows : a) Fatigue behaviour of specimen with coatings is considerably influenced by the techniques of thermal spraying. b) Plasma sprayed molybdenum and/or Al 2 O 3 coatings increase fatigue lives, while electric arc sprayed 316L steel coatings decreases fatigue lives of coated steel specimens. c) Fatigue failure of coated specimens originates on steel substrate surface, while failure of thermally sprayed coatings is caused by substrate deformation related fatigue cracks propagation. d) Cracks in coatings grow by the mechanisms of intersplats pores and voids coalescence, and/or of splats breaking. ACKNOWLEDGMENT This research has been supported by GA CR grant No. 106//97/S008 Surface Engineering. REFERENCES [1] LADRU, F. et all: Tailored Solutions for Off-Shore Applications by Plazjet Sprayed Coatings. In: Proc. Thermal Spray: A United Forum for Scientific and Technological Advances. ASM, Materials Park, Ohio 1997, pp ISBN [2] MATĚJÍČEK, J.: Processing Effects on Residual Stress and Related Properties of Thermally Sprayed Coatings. [PhD Thesis.] State University of New York, Stony Brook 1999, 154 pp. [3] IBRAHIM, A. BERNDT, C. C.: Fatigue Behaviour and Deformation of Aluminum and Steel HVOF Sprayed with WC-Co Coatings. In: Proc. Thermal Spray: A United Forum for Scientific and Technological Advances. ASM, Materials Park, Ohio 1997, pp ISBN [4] McGRANN, R.T.R. et all : Fatigue Life in Bending and Coatings Residual Stress in Tungsten Carbide Thermal Spray Coatings. In: Proc. Thermal Spray: A United Forum for Scientific and Technological Advances. ASM, Materials Park, Ohio 1997, pp ISBN [5] KANTOR, P.: Studium mechanismu únavového porušování funkčně gradientních vrstev a povlaků. [Diploma Thesis.] Praha: ČVUT-FJFI-KMAT 1999, 63 s. [6] KANTOR, P. SIEGL, J NEUFUSS, K. DUBSKÝ. J.: Fatigue test of Bodies with Different Types of Surface Coatings. In: Proc. Metal 99, Ostrava: Tanger 1999, Vol. III, pp ISBN [7] SIEGL, J. NOHAVA, J. KANTOR, P.: Fractographic Research of Fatigue Processes in Bodies with Surface Coatings. In In: Proc. Metal 99, Ostrava: Tanger 1999, Vol. III, pp ISBN [8] NOHAVA, J. SIEGL. J.: Fraktografické studium počátečních etap porušování zkušebních těles s žárovými nástřiky. [Reserch report V-KMAT-462/99.] Praha: ČVUT-FJFI- KMAT 1999, 21 p. [9] SIEGL, J. ADÁMEK, J. KOVÁŘÍK, O. KARLÍK, M.: In-situ study of failure processes of plasma spraying Mo coatings. Will be published.