Thin Film Lubrication of Ionic Liquids

Recently, room temperature ionic liquids (RT-ILs) have attracted much attention for their excellent properties, e.g., wide temperature range of liquid phase, ultra-low vapor pressure, chemical stability, potential as green solvents, and high heat capacities [64,65]. These properties make them good candidates for the use in many fields, such as thermal storage [66], electrochemical applications, homogeneous catalysis [67], dye sensitized solar cells [68], and lubricants [69,70]. 

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RT-ILs are also selected as lubricant additives. Usage of ionic liquids as boundary lubricant additives for water has resulted in dramatically reduced running-in periods for silicon nitride materials [70]. When ILs were mixed into a neat mineral oil, the mixture has proven to produce lower wear on aluminum flat than either the oil or the ionic liquid alone [72], which indicates that a small amount of ILs in the mineral oil may function as an anti-wear additive. 

However, investigations up to now have mainly concentrated themselves on ambient environments even though it is known that ionic liquids have a very low vapor pressure, making them suitable for vacuum applications such as in space mechanisms, the disk drive industry, and microelec-tromechanical systems (MEMS). Due to the ultra-low vapor pressure of most ionic liquids, they have been expected to be good lubricants in vacuum. Further experimental works are required to evaluate lubrication behavior of ionic liquids under ultra-high vacuum conditions and in inert atmospheres. 

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