The Influence of Applied Stress Ratio on Fatigue Strength of TiN-coated Carbon Steel

The influence of applied stress ratio on the fatigue strength of carbon steel coated with TiN was studied on the basis of measurement of crack initiation with the D. C. potential method. Fatigue tests were performed under the stress ratios of R=0 and -1 in air using the round notched specimens of O.37%C steel, JIS S35C, normalized and coated with TiN by physical vapor deposition (PVD) and chemical vapor deposition (CVD). From the experimental results, increase in fatigue strength was observed in TiN-coated specimens under test conditions of R=-1, as compared with that of uncoated specimens. However, the fatigue life of specimens coated by PVD was decreased under the tests of R=0, except for the region of low stress amplitude. Also, the fatigue life of specimens coated by CVD under test conditions of R=0 was smaller than that of uncoated specimens. The difference in fatigue life with applied stress ratio is explained by the fracture behavior of coating film on the specimen surface