High temperature tribological studies of oil-soluble ionic liquids as extreme pressure and anti-wear additives for ester oils

In this study, four types of ionic liquids (ILs) were synthesized and their high-temperature tribological and extreme pressure properties in pentaerythritol ester (PE) oil were systematically evaluated. Compared with commercial additive zinc dialkyl dithiophosphates (ZDDP), ILs markedly improve the load-bearing capacity and high-temperature lubrication performance of PE oil. In particular, 2-n-undecylimidazolium O,O-dioctyl dithiophosphate ([C 11 -IM][C 8 -DDP]) showed the best lubricating effect at high temperature. The tribo-film of worn surface was analyzed by SEM, EDS, XPS and FIB-TEM, and the lubrication mechanism of ILs was systematically studied. Results proved that the tribochemical reactions occurred between ILs and the worn surface, resulting in the formation of a tribo-film consisting of Fe 2 O 3 , FePO 4 , FeS, Fe x (SO 4 ) y and nitrogenous organic compounds.