Study of tribological mechanisms of carbon-based coatings in antiwear additive containing lubricants under high temperature

Abstract

With the development of low-emission and low-consumption engines, diamond-like carbon (DLC) coatings have been considered as promising surface coatings of engine parts in terms of friction and wear performance. However, the performance of DLC-coated parts depends on the compatibility and interaction between the coating and lubricant additives. To obtain high-performance coatings in the engine environment, the tribological behaviors of non-doped and metal-doped carbon-based coatings (a-C and a-C/WC, respectively) were studied systematically in poly-alpha-olefin (PAO) oil at 180 °C (operating temperature of engine oil) with and without antiwear additive zinc dialkyldithiophosphate (ZDDP) under various applied loads. Energy dispersive X-ray spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were performed on a friction surface to understand tribological mechanisms. Results showed that ZDDP-derived tribofilms were able to suppress the surface graphitization of carbon-based coatings. The combination of ZDDP-derived tribofilms and tribochemical products WS₂/WO₂, which formed under the joint actions of heat and contract pressure, led to improved tribological behavior of a-C/WC coating in PAO with ZDDP additive.