Tribological performance of environmental friendly ionic liquids for high-temperature applications

Abstract

High-temperature liquid lubricants are often composed of petroleum-based oils. Recently, the lubrication industry has begun to shift towards environmentally benign lubricants to reduce the use of toxic petroleum-based oils. As a result, there has been a resurgence in the use of bio-based lubricants. These “biolubricants” consist of vegetable oils, synthetic ethers, and polyalkyleneglycols (PAGs) for use as high-temperature lubricants. The largest drawbacks to biolubricants are neither their toxicity nor their lubricity, but rather their thermal instability. Room-temperature ionic liquids (RTIL) are emerging biolubricants that has the potential to replace conventional lubricants at high temperatures. In this investigation 1,3-diakylimidazolium and trihexyl(tetradecyl)phosphonium cations are combined with carboxylate anions to create eco-friendly lubricants for high-temperature applications. Here, these RTILs are compared with bio-based and petroleum-based oils to evaluate their tribological properties through pin-on-disk testing and thermal stability via thermogravimetric analysis (TGA). Additionally, the influence of differing alkyl chain imidazolium cations of the 1-decyl through 1-allyl equivalents and various shape and sized anions are studied. Results indicate that the eco-friendly RTILs exhibit the lowest friction and wear properties as well as superior thermal stability because of their lamellar-like liquid crystal structure, electrostatic potential dipolar, and molecular adsorption layer.