Comparative Study of the Tribological Properties of MoSe2 Coatings Under Dry and Oil-Lubricated Sliding Conditions

MoSe2 is considered one of the most promising low-friction coatings for tribological applications due to its exceptionally low sensitivity to air humidity. However, knowledge of its tribological performance, especially in combination with oil lubrication, is still very limited. In this study, the tribological properties of MoSe2 coatings deposited by magnetron sputtering were investigated using a reciprocating pin-on-flat tribometer against steel balls under both dry and PAO4-lubricated sliding conditions. The worn surfaces of the coatings and their counterparts were analyzed by profilometry, Raman spectroscopy, and scanning and transmission electron microscopy. Under dry lubrication, the coatings exhibited low friction (0.054), which was attributed to the combined effects of a lubricious transfer layer forming on the steel ball and a crystalline MoSe2 tribolayer in the coating wear track, with MoSe2 basal planes aligned parallel to the sliding direction. In contrast, under oil lubrication, the absence of a transfer layer on the ball and a crystalline tribolayer in the coating wear track resulted in higher friction (0.101). This high friction was accompanied by a 27% reduction in the wear rate due to the presence of PAO4 at the sliding contact, which served as a sealant and protected the coating from oxidation.