With the increasing demand for environmentally-friendly industrial materials, the advantages of ionic liquids have constantly garnered interest from researchers and developers. Ionic liquids were first introduced as lubricants in 2001. The distinct advantages of ionic liquids over conventional lubricants lie in their better ability to form tribofilms, environmental friendliness, higher thermal stability, and adaptability to a variety of applications. Due to the efforts of many researchers, all kinds of new ionic liquids have been synthesized. The favorable physical properties of them suggest that ionic liquids are promising candidates for the next generation of lubricants .
Ionic liquids have great potential in lubricants research because of their unique properties. The most representative ionic liquid systems used for lubricants can be classified into imidazole ionic liquids, phosphonium ionic liquids, guanidinium ionic liquids and amino acid ionic liquids.
Kondo and co-workers  reported the lubricating performance of two types of imidazole ionic liquids—1-butyl-3-methylimidazolium tricyanomethanide ([BMIM] [TCC]) and 1-ethyl-3-methylimidazolium dicyanamide ([EMIM] [DCN])—for ceramics and hard coatings. Due to the chemical interaction between ceramics and ionic liquids, these ionic liquids exhibited good lubricity for sintered ceramics. However, they showed different lubricating performance with different nitride coating. [EMIM] [DCN] showed a better anti-wear than [BMIM] [TCC] on CrN coating, and its friction coefficient was higher than that of [BMIM] [TCC]. Diamond-like carbon coatings with [EMIM] [DCN] showed better lubricating performance than with [BMIM] [TCC].
Phosphonium plays an important role in friction chemistry. Therefore, phosphonium ionic liquids may also be a high-performance lubricant, which overcomes the instability of imidazole ionic liquids. A group of researchers designed and synthesized a new type of double quaternary phosphonium ionic liquid, namely 1,2-bis(trioctyl) phosphonium bis(butyl) phosphate (D-P1888P4) and 1,2-bis(trioctyl) phosphonium bis(2-ethylhexyl) phosphate (D-P1888P8), as a new type of halogen-free ionic liquid lubricant for steel/steel friction pair. These as-prepared ionic liquids are able to impart enhanced anti-wear and friction reduction property to the friction pair in contrast to the commercially available lubricants, i.e., traditional ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate and synthetic poly-α-olefin .
The guanidinium ionic liquid is also a common ionic liquid system with lubricating characteristics. A group of researchers synthesized the guanidinium ionic liquids with excellent properties. These ionic liquids have better thermal stability than 1-hexyl-3-methylimidazolium bis(trifluoromethyl sulfony)imide (LF106) and perfluoropolyether (PFPE) due to their unique molecular structure. Guanidinium ionic liquids possess better antiwear and antifriction property in a wide range of temperatures (from RT to 300 °C) than LF106 and PFPE. Meanwhile, guanidinium ionic liquids still perform excellent lubricating performance after high temperature treatment .
Amino acids can be used as both anions and cations to form amino acid ionic liquids. This kind of ionic liquid has been widely concerned by researchers due to its better environmental friendliness, biocompatibility and biodegradability. Some researchers synthesized a series of quaternary ammonium amino acid ionic liquids. They found that these ionic liquids have the advantages of simple synthesis method, cheap raw materials and stable performance. As a lubricant for steel/steel friction pairs, quaternary ammonium amino acid ionic liquids have excellent anti-wear and friction reduction property and are a new type of high-performance, green and environmentally friendly lubricant with great prospects for development.
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