A Review of Ionic Liquids for Green Molecular Lubrication in Nanotechnology

Common industrial lubricants include natural and synthetic hydrocarbons and perfluoropolyethers (PFPEs), where the latter is widely used in commercial applications requiring extreme operating conditions due to their high temperature stability and extremely low vapor pressure. However, PFPEs exhibit low electrical conductivity, making them undesirable in some nanotechnology applications. Ionic liquids (ILs) have been explored as lubricants for various device applications due to their excellent electrical conductivity as well as good thermal conductivity, where the latter allows frictional heating dissipation. Since they do not emit volatile organic compounds, they are regarded as green lubricants. In this article, we review the different types of ILs and their physical properties responsible for lubrication. We also discuss their suitability as lubricants, since the long-term performance of ILs as lubricants may be affected by issues such as corrosion, oxidation, tribochemical reactions, and toxicity. We present nanotribological, electrical, and spectroscopic studies of IL films along with conventional tribological investigations, recognizing that understanding the tribological performance at various length scales is a crucial step in selecting and designing effective lubricants.