Research Progress of Ionic Liquids as Lubricants

Emerging ionic liquid lubricants were discovered only 20 years ago. The superior performance of ionic liquids over traditional lubricating fluids have been reviewed, including low-temperature fluidity, viscosity–temperature properties, thermal oxidation stability, ultralow volatility, incompressibility, electrical conductivity, friction coefficient under elastohydrodynamic lubrication conditions, friction reduction and antiwear performance under boundary lubrication conditions, environmental friendliness, etc. The applications where ionic liquids are superior to traditional lubricant fluids are presented, including hydrogen compressor lubricating fluids and liquid pistons, oxygen compressor lubricating fluids, hydraulic fluids, space lubricants, vehicle engine oils, industrial gear oils, metalworking fluids, industrial coolant, micro/nano electromechanical system applications, electrical conductive lubricants, etc. The ability of ionic liquids to replace ZDDP, a key additive of lubricating oil, is introduced. The future development prospects of ionic liquid lubricants are analyzed.