Formation of MoS₂-Rich MoDTC Tribofilm via Cr-Doped DLC Film under Ester Lubrication

A hybrid deposition technique combining bipolar plasma-based ion implantation and deposition (PBII&D) with magnetron sputtering was used to fabricate Cr-doped diamond-like carbon (Cr-DLC) coatings. An ester-based synthetic oil was selected because of its low viscosity, and its tribological properties were evaluated in the presence of molybdenum dithiocarbamate (MoDTC) and zinc dialkyl dithiophosphate (ZDDP). Under MoDTC lubrication, the coefficient of friction (COF) increased due to tribofilm wear but was significantly reduced when ZDDP was added. Raman spectroscopy confirmed the formation of a MoS₂-rich tribofilm, which was enhanced by ZDDP adsorption. Cr doping further decreased the COF by modifying the tribofilm composition. X-ray photoelectron spectroscopy (XPS) revealed the reduced MoO₃ contribution and the formation of Cr oxides. Transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) identified uniform Cr distribution within the tribofilm, suggesting that Cr captured oxygen from the lubricant. This process inhibits MoO₂ oxidation and promotes MoS₂ formation, contributing to friction reduction.