An analysis of the effect of hard coatings on near-surface rolling contact fatigue initiation induced by surface roughness

The effect of hard coatings on the fatigue life of rolling elements subject to cyclic concentrated contact loading (such as bearing races or gears) is studied theoretically. Approximate analytical formulae are derived that can be used to estimate the coating thickness required to protect the substrate effectively from the small-scale contact stress spikes produced by surface roughness, and thus prevent near-surface rolling contact fatigue initiation. The present model, although crude, takes into account the multiscale nature of real surface roughness, as well as interaction between different roughness components. The relative danger of near-surface fatigue initiation and the potential effect of a hard coating are determined by the interplay between the coating thickness, the typical size of defect responsible for fatigue crack initiation and the power spectrum of surface roughness. The model predictions agree qualitatively with the results of a companion experimental study. The analysis indicates that in order to be truly effective against rolling contact fatigue, a hard coating has to be relatively thick (say, ≥3 μm), adherent, and have the fine microstructure and the associated resistance to cohesive failure under cyclic contact loading currently found in thin PVD TiN coatings.