Effect of Oil Additives on Wear Characteristics in Case-Carburized Steel Coated with Manganese Phosphate

This study examines the wear characteristics of case-carburized steel surfaces with manganese phosphate (MnPh) coating under commercial fully formulated oil (FFO). This combination can improve the steel’s tribological properties and rolling contact fatigue strength. The role of oil additives in FFO, which affect the wear characteristics through the formation of a wear-resistant tribofilm, on the MnPh coated steel was analyzed in detail. The process of chemical etching during the MnPh coating application makes the steel substrate rougher. Consequently, both MnPh-coated and MnPh-removed disks were analyzed to ensure comparability in terms of surface roughness and morphology. Under FFO lubrication, the MnPh-coated disks exhibited continuous wear of the steel peaks during the initial friction period, similar to that observed under base oil lubrication. This continuous wear occurs because the residual MnPh coating in the valleys of the roughness primarily supports the contact load during the initial friction period, preventing the formation of a wear-resistant tribofilm on the exposed steel peaks. The proposed wear mechanism for MnPh-coated steel aims to further improve the rolling contact fatigue strength.