Ionic liquids as lubricants: An overview of recent developments

•Biocompatible ILs indicate reduced toxicity and improved biodegradation.•Biocompatible ILs provide efficient lubrication while being environment friendly.•Alkyl chain length of the cation has significant effects on the performance of ILs.•ILs and NPs as hybrid additives in lubricants show synergistic behaviour. Minimizing energy losses and ensuring smooth motion between engine components is the critical role of lubricants. Extensive research over the past decade has explored various lubricant types and additives. Ionic liquids (ILs) have emerged as promising candidates due to their exceptional tribological performance, which is attributed to their unique physiochemical properties. This review delves into the potential of ILs as both lubricants and additives, focusing on their structure-activity relationship in the quest for identifying green lubricants. Compared to neat base oils, ILs significantly reduce friction and wear. This review explores the role of ILs in water-based lubricants (WBLs) and analyzes the impact of tribo-testing conditions based on different tribometers. A recent trend involves the use of ILs and nanoparticles (NPs) as hybrid lubricant additives. The review examines the synergistic behaviour of these hybrid additives in different base oils and proposes a lubrication mechanism for phosphonium ILs based on tribo-film formation induced by tribo-chemical reactions during the rubbing process. The lubrication mechanism of hybrid nano-lubricants is also comprehensively reviewed to explain why combining NPs and ILs results in such remarkable reductions in friction and wear. Overall, this review provides a comprehensive overview of the promising potential of ILs in lubrication, highlighting their advantages, diverse applications, and underlying mechanisms.