A study of rolling-contact fatigue of bearing steel coated with physical vapor deposition TIN films: Coating response to cyclic contact stress and physical mechanisms underlying coating effect on the fatigue life

The effect of physical vapor deposition (PVD) TiN coatings of different thicknesses on the rolling-contact fatigue (RCF) life of coated specimens is studied experimentally. Based on the results of conventional three-ball-on-rod testing, the optimum film thickness is found to be about 0.75 mu m. Thinner coatings do not significantly affect the fatigue life, while much thicker ones have a negative effect on it. These trends agree with the findings of prior studies. Using a progressive testing technique, the mode of localized damage experienced by thick TiN coatings under cyclic contact stress conditions is established. The fatigue life reduction observed with such coatings is explained by comparison of the coated surface roughness in the damaged and the intact portions of the rolling track. Progressive testing results also indicate that the stress-affected material volume plays an important role in near-surface initiated rolling-contact fatigue. By modifying fatigue test conditions, the fatigue life enhancement observed with some PVD TIN coatings is demonstrated to result from loading ball polishing by the hard coating. The present experimental observations agree with the predictions of a companion theoretical study. (C) 1998 Elsevier Science S.A.