Low-friction properties and structural analysis of Ni-deposited tetrahedral amorphous carbon films in lubricants containing molybdenum dithiocarbamate

This study aimed to enhance the tribological properties of diamond-like carbon (DLC) in lubrication by depositing nickel (Ni) onto the surface layer of tetrahedral amorphous carbon (ta-C) film. Ni/ta-C of ~0.1 μm Ni exhibited significantly lower friction than ta-C on molybdenum dithiocarbamate (MoDTC) lubrication. The ta-C served as a hard, wear-resistant base, and Ni aided tribochemistry. X-ray photoelectron spectroscopy (XPS) revealed many Mo⁴⁺ peaks attributed to MoS₂ formation in Ni/ta-C surfaces, and Auger electron spectroscopy (AES) confirmed Mo and S enrichment near the surface. Detailed analysis suggested Ni did not catalyze decomposition of MoDTC to MoS₂, but promoted oxidation of transferred Fe debris, leading to formation of an Fe oxide layer. This layer likely acted as a supporting platform for subsequent formation and stabilization of the MoS₂ layer, consistent with mechanisms previously reported for transition-metal nitride coatings. This finding indicates Ni accelerates MoS₂ formation indirectly through Fe-mediated processes, resulting in substantial friction reduction and maintaining the wear resistance of DLC.