To improve the rolling–sliding contact fatigue strength of a case-carburized steel, the effect of the wear surface morphologies of manganese phosphate (MnP) coated steel and the growth and removal of phosphorus-containing tribofilm on surface-initiated crack formation was investigated. In order to modify the wear surface morphologies, two types of surface textures (ground and shot blasted) were prepared, followed by the MnP coating process. The tribological properties of the coated steel, tribofilm growth and removal, and surface-initiated crack formation were evaluated using a ball-on-disk tribometer with a rolling–sliding mode. The MnP coating on both the ground and shot blasted steel had nearly the same thickness and surface roughness. However, for the ground surface sample, the interface morphology between the coating and steel substrate was more irregular than the shot blasted surface sample, resulting in a larger number of exposed steel areas with smaller sizes after the MnP was almost worn away on tribological tests. During the running-in period, phosphorus-containing tribofilm growth and removal on the smaller exposed steel areas were observed. The surface-initiated crack formation on the smaller exposed steel areas was suppressed compared with larger exposed steel areas.
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