FATIGUE CRACK GROWTH BEHAVIOR OF STAINLESS STEEL COATED WITH TiN FILM

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1 Advanced Materials Development and Performance (AMDP2011) International Journal of Modern Physics: Conference Series Vol. 6 (2012) World Scientific Publishing Company DOI: /S FATIGUE CRACK GROWTH BEHAVIOR OF STAINLESS STEEL COATED WITH TiN FILM SATOSHI FUKUI Department of Mechanical Engineering, Kagawa National College of Technology, 355 Chokush-cho Takamatsu, Kagawa , Japan fukui@t.kagawa-nct.ac.jp DAISUKE YONEKURA Department of Mechanical Engineering, The University of Tokushima, 2-1 Minamijosanjima-cho, Tokushima, Tokushima , Japan yonekura@me.tokushima-u.ac.jp RI-ICHI MURAKAMI Department of Mechanical Engineering, The University of Tokushima, 2-1 Minamijosanjima-cho, Tokushima, Tokushima , Japan murakami@me.tokushima-u.ac.jp In our previous study, we examined the influence of the fatigue properties of the stainless steel coated with TiN film and clarified the influence of TiN coating and the surface roughness on the fatigue property. In this study, the four point bending fatigue crack growth tests were carried out for martensitic stainless steel coated with TiN film deposited by arc ion plating method in order to investigate the effect of surface finishing on the fatigue crack behavior for film coated material. The fatigue crack growth behavior was evaluated using the replica method. As a result, the crack propagation rate of mirror polished specimens were lower than that of rough surface specimens. The crack propagation rate was especially decreased for TiN coatings deposited on the mirror polished substrate. The surface roughness near the crack initiation site increased after fatigue test. It concludes that the surface roughness of substrate influences crack propagation rate and the deposition of TiN film affected influenced crack propagation rate and fatigue strength when the surface roughness of substrate is small enough. Keywords: Crack growth, Stainless Steel, TiN, Film, Polish. 1. Introduction Many PVD coating method are developed along with the improvement of semi conductor related technology 1 3. Newly developed coating methods put film as TiN, CrN, and DLC to practical use. Thin hard films of materials such as TiN, CrN, and DLC are generally used for the improvement of wear resistance. Although PVD coating is a useful method, 282

2 Fatigue Crack Growth Behavior of Stainless Steel Coated with TiN Film 283 fatigue properties change along with tribological properties by a thin hard film coating 4 7 because of the films hardness, residual stress, and stiffness. Our previous reports 8,9 described that the fatigue strength of the martensitic stainless steel was improved by arc ion plating (AIP) coating of TiN under particular conditions. The fatigue strength of substrate was improved by applying the TiN thin film coating. But the crack growth rate of the coatings was almost equal for all samples coated at different coating conditions. The difference of fatigue strength is result in the difference of crack initiation life. This study, using samples that had been polished under different size of grind particle, examines the influence of pre-coating treatment on fatigue crack initiation and growth behaviors. 2. Material and Experimental Procedure Quenched and tempered martensitic stainless steel plate was used as a substrate. Table 1 presents its chemical composition. Table 2 presents its monotonic mechanical properties. The plate was machined to the shape of substrate illustrated in Fig.1. Then, surfaces of specimen were polished by lapping film sheet. This research concentrated on the surface treatment of the substrate before coating. Two kinds of surface conditions (A) and (B) of the substrate were prepared. Condition (A) is completely polished to mirror surface under 1 m of grind particle size. Condition (B) is polished under 2 m of grind particle size. Table 1. Chemical composition of the substrate (wt%). C Si Mn P S Cr Mo Fe Bal. Table 2. Monotonic properties. Tensile strength Elongation Vickers hardness 1749 MPa 5 % HV Fig. 1. Specimen configuration. A 2-µm-thick TiN film was deposited onto the substrate surface using AIP coating method. The TiN coating process is carried out under coating condition of Table 3. Figure 2 shows X-ray diffraction patterns of samples with TiN films.

3 284 S. Fukui, D. Yonekura & R. Murakami Table 3. Coating conditions. Arc Current Bias Voltage V B Base Pressure Cathode 60 A -160 V 10 mpa Ti Process Gas N 2 Film Thickness 2 µm Deposition Time 60 min Fig. 2. X-ray diffraction pattern of TiN film. Out-of-plane four-point bending load fatigue tests were performed using an electrohydraulic servo-controlled fatigue testing machine with loading frequency of 10 Hz and stress ratio R of 0.1. The fatigue crack growth behaviors of the samples were measured by a replica method under a few stress levels using a fatigue testing machine. The shape of fatigue cracks of replica sheets were observed by the optical microscope with CCD image sensor. The stress intensity factor range K and da/dn were estimated from the crack length by equation of follows: da dn ai ai Ni 1 Ni 1, K F A, A a (1) 2 Where F is the revised factor, a is a crack length, A is approximate area of crack. The revised factor F is decided 0.65 from the semi-ellipse crack model from Ref Experimental Results and Discussion 3.1. Fatigue rupture test Figure 3 shows the results of fatigue tests. The solid symbol plots are the results of uncoated specimen, and the open symbol plots are the results of TiN coated specimen. The result is divided into two gropes. The fatigue strength of polished condition (B) formed a line on a typical SN curve whether they are coated or not coated. But the fatigue

4 Fatigue Crack Growth Behavior of Stainless Steel Coated with TiN Film 285 strength of specimen polished condition (A) distributed randomly at the area of the upper side of the SN curve of the result of polished condition (B). Maximum Stress, max, MPa Fatigue crack propagate test Condition (A) Condition (B) Uncoated Coated Number of cycle to failure, Nf Fig. 3. Result of fatigue tests. In order to clarify the influence of the TiN coating and polished condition on the fatigue strength, crack propagation behavior was examined. Figure 4 is a typical example of an observed optical microscope image of the fatigue crack. Figure 5 is the fatigue crack growth behavior estimated by surface crack length from crack images like Figure 4. Figure 5 shows different crack growth behavior for each specimen. To make the difference clear, fatigue crack growth rate da/dn and stress intensity factor range K were calculated and shown in Figure m Fig. 4. Optical microscope photograph of the fatigue crack of uncoated specimen with polished condition (A), max=1500mpa, and N=1800. Figure 6 shows the crack propagation rate of the coatings was similar to that of the uncoated specimen. It indicates that the TiN films hardly affected the crack propagation behavior in this range, since the film is very thin in comparison with the crack length

5 286 S. Fukui, D. Yonekura & R. Murakami corresponding to the K in this study. On the other hand the difference of surface conditions of substrate leads deference of crack growth rate of same stress intensity factor range. Mirror polish of the substrate leads degrades the crack growth rate for coated and uncoated specimen. This result coincided with the fatigue rupture tests, since the fatigue strength was more strongly affected by surface condition of substrate than the existence of coating. Crack grouth rate, da/dn, m/cycle Crack length, l, mm Number of cycles, N 10 6 Fig. 5. Fatigue crack growth behavior of max=1500mpa Uncoated Coated Condition (A) Condition (B) Uncoated Coated Condition (A) Condition (B) Stress intensity factor range, K, MPa m 1/2 Fig. 6. Relationship between crack growth rate and stress intensity factor range of max=1500mpa Surface property In order to clarify the fatigue damage at the surface of the specimen, the surface roughness were measured before fatigue test and after fatigue test for several uncoated

6 Fatigue Crack Growth Behavior of Stainless Steel Coated with TiN Film 287 specimen. Table 4 shows the result of surface roughness measurement. Table 4 shows surface roughness for all substrate increased after fatigue test. It indicate a estimation of the fatigue damage accumulate on the surface of substrate. Fatigue crack initiate from substrate and propagate to the surface and under surface. This process is similarly produced in both uncoated specimen and coated specimen, though surface of TiN thin film is quite rough 9. Table 4. Result of surface roughness measurements for uncorted specimen [ m]. 4. Conclusions Polished condition (A) Polished condition (B) Ra Rz Ra Rz Before fatigue test After fatigue test The effect of TiN film prepared by AIP coating process and surface condition of substrate on fatigue cracking properties of high strength stainless steel was investigated by using 4 point bending fatigue test. (i) The TiN films hardly affected the crack propagation behavior, since the film is very thin in comparison with the crack length corresponding to the stress intensity range in this study. (ii) The fatigue strength was more strongly affected by surface condition of substrate than the existence of coating. (iii) The surface roughness of substrate increased after fatigue test. References 1. N. S. Athanasiou, F. Lanza, L. Paracchini and F. Brossa, Int. J. Modern Physics B (IJMPB), 12, 155 (1998). 2. J. Xu and W. Liu,Int. J. Modern Physics B (IJMPB), 12, 573 (2005). 3. Z. Liu and W. Gao, Int. J. Modern Physics B (IJMPB), 20, 4637 (2006). 4. D. Yonekura and R. Murakami, Int. J. Modern Physics B (IJMPB), 20, 3842 (2006). 5. D. Yonekura, A. Tsukuda, R. Murakami and K. Hanaguri, Int. J. Modern Physics B (IJMPB), 17, 1554 (2003). 6. C. M. Suh, B. W. Hwang and K. R. Kim, Int. J. Modern Physics B (IJMPB), 16, 181 (2002). 7. C. M. Suh, K. R. Kim, Y. G. Kang. D. Y. Suh and C. K. Kim, Int. J. Modern Physics B (IJMPB), 17, 1527 (2003). 8. S. Fukui, R. Murakami and D. Yonekura, Key Engineering Materials, 353, 1875 (2007). 9. S. Fukui,.D. Yonekura and R. Murakami, Int. J. Modern Physics B (IJMPB), 24, 3095 (2010). 10. Y. Murakami, Stress Intensity Factors Handbook, Soc. Materials Science Japan, 2, 822 (1987).