Tribological investigations of ionic liquids in ultra-high vacuum environment
A tribological investigation of two ionic liquids (ILs) that could be used as potential lubricants for vacuum or space applications is presented.
Their tribological behaviour was evaluated using a ball-on-flat oscillating reciprocating motion CaTri ultra-high vacuum (UHV) tribometer and the results were compared against those of a conventional perfluoropolyether (PFPE) vacuum lubricant. The thermal stability of the lubricants was investigated by thermogravimetric analysis before exposing them to UHV environment. The worn steel surfaces were studied by X-ray photoelectron spectroscopy, optical and confocal microscope as well as scanning electron microscope. The tribological evaluations showed that ILs exhibited better friction reduction and anti-wear properties than the PFPE lubricant. The excellent tribological properties were attributed to the formation of a surface protective film composed of iron fluoride and iron phosphate, respectively, on the lubricated metal surfaces by a tribochemical reaction with the ILs.