Lubrication Mechanism of Halogen-Free Ionic Liquids

Ionic liquids have potential for use as novel high-performance lubricants because of their attractive characteristics such as low volatility, high-thermal stability, and oxidation stability. It is known that ionic liquids exhibit excellent lubricity for metals because of halogen constituents in their molecular structure. However, occurrence of corrosive damage on the contacting surfaces lubricated with the ionic liquids has also been reported. To prevent damage due to corrosion, it is necessary to use halogen-free ionic liquids whose lubricity may be inferior compared with that of halogen-containing ionic liquids. In this study, the lubricity of halogen-free ionic liquids 1-butyl-3-methylimidazolium dicyanamide ([BMIM][DCN]) and 1-butyl-3-methylimidazolium tricyanomethane ([BMIM][TCC]) was evaluated by using a reciprocating sliding friction and wear tester (SRV Optimol)) using an oscillating steel cylinder on H-free DLC disk test configuration under boundary lubrication conditions. The SRV test results showed that H-free DLC with [BMIM][TCC] at 50N exhibited superior lubricity than that with [BMIM][DCN] at the same load. In order to understand the observed, the worn surfaces of test specimens were analyzed by using Raman spectroscopy, the friction coefficient of the film by atomic force microscopy (AFM), and the chemical composition by time of flight secondary ion mass spectrometry (TOF-SIMS). From Raman spectroscopic analysis results, graphitization of the transfer films of both [BMIM][DCN] and [BMIM][TCC] did not occur. From AFM results, the topography of [BMIM][TCC] was smooth as compared with that of [BMIM][DCN]. Thus, it was considered that the surface roughness affected the lubricity. In addition, the tribofilm derived from [BMIM][TCC] also affected the lubricity because it showed low friction coefficient on the flat position. From ToF-SIMS results, it was considered that [BMIM][DCN] itself adsorbed on H-free DLC surface. On the other hand, [BMIM][TCC] is thermally decomposed by friction and generated isolated CN and covering the H-free DLC surface. The tribofilm formed by [BMIM][TCC] exhibited lower friction coefficient than that by [BMIM][DCN]