Tribochemical Reactions of Ionic Liquids Under Vacuum Conditions

Ionic liquids are expected to function as novel lubricants owing to their attractive characteristics such as high thermal stability and low vapor pressure. In order for ionic liquids to be used as lubricants, knowledge of their corrosion and lubricating properties must be obtained. However, the reaction mechanism and decomposition of ionic liquids have not yet been sufficiently clarified. In this study, we elucidate the tribological properties and tribochemical reaction mechanism by analyzing outgassing generated by the decomposition or reaction of ionic liquids [EMIM][DCN], [EMIM][TCB], and [BMPL][TCB] on a sliding surface. From our results, [BMPL][TCB] showed the lowest friction coefficient and [EMIM][DCN] had a lower friction coefficient than [EMIM][TCB]. In all cases, outgassing from the ionic liquid was confirmed, and main outgassing products were derived from the cation. [BMPL][TCB] had the largest amount of outgassing. Time-of-flight secondary ion mass spectroscopy analysis showed that tribochemical reactions involving the anion occurred. From sliding tests and experimental analyses, it is revealed that the [DCN] anion showed superior lubricating properties to the [TCB] anion.