13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Figure 5. Typical SEM images of fracture surfaces; (a), (b) As-received, σa=525MPa, Nf = 1.88×10 5 cycles, (c), (d) N15, σa=625MPa, Nf=2.31×10 5 cycles and (e), (f) N25, σ a=600MPa, Nf=2.96×10 5 cycles. In conclusion, the degree of increase in the surface hardness and the thickness of the hardened layer are dominant factors determining the fatigue limit level of the plasma nitrided specimens. Therefore, the fatigue limit levels of the N15 and the N25 specimens were almost the same and thickness of the hardened layers had almost the same value. In addition, the improvement by plasma nitriding process is small because the nitrided layer, particularly diffusion layer, is quite thin. It is believed that the increase of the depth of diffused zone could improve the fatigue strength in plasma nitride stainless steels. 4. Conclusions The following conclusion can be reached from the results of this study: (1)A hardened surface layer was formed at the SUS316 stainless steel surface by plasma nitriding. However, the hardened layer was quite shallow. The thickness of the hardened layer was less than 50 µm for both nitriding time of 15 and 25 hours. (2) The S-N curves represent a horizontal asymptote shape for all specimens. (3)The fatigue strength was improved by plasma nitriding in all life regimes. However, the improvement of the fatigue strength was small due to the small thickness of the nitrided layer. The nitriding time also had a small influence on the fatigue strength because the thickness of the nitrided layer was similar for both nitriding times. (4)Fatigue crack was initiated from specimen surface for all specimens. (5)The degree of increase in the surface hardness and the depth of the hardened layer are dominant factors determining the fatigue limit level of the plasma nitrided specimens. References [1] A.D. Wilson, A. Leyland, A. Matthews, A comparative study of the influence of plasma treatments, PVD coatings and ion implantation on the tribological performance of Ti–6Al–4V, Surf. Coat. Technol., 114 (1999) 70–80. [2] A. Wilson, A. Matthews, J. Housden, R. Turner and, B. Garside, A comparison of the wear and fatigue properties of plasma-assisted physical vapour deposition TiN, CrN and duplex coatings on Ti-6A1-4V, Surf. Coat. Technol., 62 (1993) 600–607.
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