Perspective of Ionic Liquids as Lubricants

Abstract

Ionic liquids are expected to be used as novel lubricants because of their unique characteristics such as low vapor pressure, low volatility, and high thermal stabilities. Their physical properties compares favorably with conventional lubricants. However, it is well known that the halogen-containing ionic liquids cause corrosion of worn surface, although exhibit high lubricating performance. In order to preclude the corrosion, the application of halogen-free ionic liquids is proposed. The halogen-free ionic liquids such as phosphorus, sulfur, boron and cyano-based ionic liquids are expected as a substitute for the halogen ionic liquids. The phosphorus and sulfur-based ionic liquids show the inferior lubricity to that of halogen-containing ionic liquids. The boron-based ionic liquids exhibit ultralow friction, and these lubricating performance surpass the halogen-containing ionic liquids. The lubricity of cyano-based ionic liquids is improved by changing the cations and anions. In addition, the ultralow friction phenomena is confirmed in the combination of a cyano-based ionic liquid and diamond like carbon (DLC) film as sliding surface. In applying ionic liquids as lubricants, combination with friction materials is also an important subject to be studied.