Effect and mechanism of additives for ionic liquids as new lubricants

Ionic liquids are unique compounds, which exhibit low viscosity, non-flammability, low vapor pressure, and extremely high thermal stability. Therefore, they are expected as candidates for advanced lubricants. Several ionic liquids, derived from cations such as imidazolium, pridinium, ammonium, and anions such as BF4-, bis(trifluoromethylsulfonyl)imide (TFSI-)(TFSI-) were examined under boundary conditions. It was found that tribological properties of ionic liquids are better than those of conventional lubricants such as synthetic hydrocarbons, synthetic esters, and fluorinated ethers. Careful analysis of worn surface revealed that tribochemical reactions of ionic liquids take place under these conditions. For example, organic fluoride, iron fluoride, iron sulfate derived from anionic moiety of the ionic liquids were detected by the instrumental surface-analysis with TOF-SIMS and XPS. Tricresylphosphate (TCP) and dibenzyldisulfide (DBDS) were found to improve anti-wear properties of ionic liquids to some extent. Interestingly, both additive-derived compounds and anionic moiety derived ones were detected on the worn surface. Moreover, additive response of ionic liquids was found to be superior to those for conventional lubricants. It was speculated that the unique tribochemical reactions will lead us to design tailor-made lubricants. Lubrication mechanism of ionic liquids is discussed from the viewpoint of tribochemistry.