Tribological Properties of Cyano-Based Ionic Liquids under Different Environments

Abstract

Ionic liquids have been used as novel high-performance lubricants. However, fluorine-based ionic liquids cause corrosion of materials by the formation of FeF₂ and FeF₃. This study focuses on cyano-based ionic liquids, as they have low corrosion tendencies owing to the absence of fluorine in their structures. However, cyano-based ionic liquids are hydrophilic, and hence, the effect of ambient conditions (e.g., atmospheric moisture) on the tribological properties needs to be investigated. This study reports the tribological properties of two types of cyano-based ionic liquids 1-ethyl-3-methylimidazolium dicyanamide and 1-ethyl-3-methylimidazolium tricyanomethane under air, vacuum, and dry air environments. Their friction coefficients were found to be dependent on the ambient condition. Both the ionic liquids exhibited low friction coefficients in vacuum. The time-of-flight secondary ion mass spectrometry results indicated that the anion was adsorbed on the worn surface under vacuum conditions, and the adsorbed anion exhibited a low friction. In dry air, the cation was adsorbed on the worn surface, and the adsorbed cation exhibited a higher friction coefficient than the adsorbed anion. In air, the cyano-based ionic liquids caused corrosion of the sliding material and exhibited a high friction coefficient, because of the absorption of moisture from air.