Effect of Surface Morphological Changes in the Substrate Induced by Coating Manganese Phosphate on Surface Fatigue Crack Formation in Case-Carburized Steel

This study investigates the effect of changes in substrate morphology induced by coating manganese phosphate (MnPh) on surface fatigue crack formation, which can help clarify mechanisms for improving rolling contact fatigue (RCF) life under MnPh coating and establish design guidelines for friction surface textures that can suppress RCF damage.Surface fatigue crack formation trends of ground steel (Grinding) and steel from which only the MnPh coating was removed (MnPh-removed) were compared.After sufficient friction progression, the MnPh-removed sample, characterized by isolated asperities with small curvature radii induced by the MnPh coating process, exhibited fewer surface fatigue cracks than Grinding, even with increasing surface roughness.To better understand the delayed formation of surface fatigue cracks observed in the MnPh-removed sample, contact behavior under dry conditions and surface morphological changes during rolling-sliding conditions were analyzed.The MnPh-removed sample revealed an early reduction in the mean surface contact pressure, which was attributed to asperity deformation.Therefore, this proposed mechanism is unique to its surface morphology, which is specific to the MnPh treatment.In addition, it is expected to contribute to the suppression of macroscopic RCF damage such as pitting.