Efficient one-pot synthesis of molybdenum dialkyl dithiocarbamate and investigation on evolution mechanism of tribofilms in ester-based lubricants

Abstract

Abstract Base oil polarity critically governs additive solubility, interfacial adsorption, and tribochemical reactions, which collectively determine friction, wear, and component service life. In this study, molybdenum dialkyldithiocarbamate (MoDTC) was synthesized via a facile one-pot route. Its dispersion stability and tribological properties were compared in two base oils with distinct polarities: polar diisooctyl sebacate (DIOS) and nonpolar polyalphaolefin (PAO6). The tribological behavior of the MoDTC/DIOS blend was further evaluated under variable applied loads and temperatures. The results reveal that MoDTC delivers superior friction-reducing and anti-wear performance in nonpolar PAO6, attributed to stable adsorption on friction surfaces and tribochemical decomposition into protective tribofilms. In contrast, polar DIOS enables the MoDTC/DIOS blend to reach a steady friction coefficient (COF) much faster, with a drastically shortened running-in stage. Cross-sectional characterizations demonstrate that the polar DIOS environment accelerates MoDTC decomposition and rapid formation of an amorphous Mo- and S-containing tribofilm, accounting for the sharp COF decline. Nevertheless, this amorphous tribofilm exhibits low load-bearing capacity and cannot effectively suppress interfacial wear, resulting in a larger wear scar diameter (WSD) than the MoDTC/PAO6 blend. At 25 °C, MoDTC achieves optimal lubrication, reducing the COF and WSD of neat DIOS by 50.0% and 23.3%, respectively. At 75 °C, COF reduction reaches 61.5%; at 125 °C, despite severe fluctuations in the friction curve, the WSD remains smaller than that of neat DIOS, indicating sustained anti‑wear capability. Combined with multiple modern characterizations, the underlying lubrication mechanisms is clarified: at low‑to‑medium temperatures, MoDTC preferentially generates a dense tribofilm dominated by layered MoS2; at elevated temperatures, the tribofilm transforms into a composite of MoO3 and MoSxOy. Despite increasing the COF, this composite tribofilms retain excellent load‑bearing and anti‑adhesion properties to mitigate severe wear. This study provides guidance for optimizing MoDTC synthesis, polarity matching, and tribofilm regulation, facilitating its wide application in lubrication systems.