Unravelling the lubrication mechanism behind the enhanced tribological performance of di-imidazolium ionic liquids

Developing high-performance lubricants capable of withstanding severe operating conditions is crucial for modern engineering applications. In this work, a high-purity di-imidazolium-based ionic liquid (DImIL), 1-hexyl-3-dimethylimidazolium di[bis(trifluoromethyl)imide] (C₆di[MIM].di[TFSI]), was synthesized and systematically evaluated against a traditional monoimidazolium-based ionic liquid (MImIL), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM.TFSI) and a commercial base oil, polyalphaolefin 10 (PAO10). Tribological tests demonstrate that C₆di[MIM].di[TFSI] exhibits superior lubrication performance. Under additive-free conditions, C₆di[MIM].di[TFSI] achieves a stable friction coefficient (COF) of 0.100 and reduces the wear volume by 69% compared with EMIM.TFSI (COF of 0.125). Notably, C₆di[MIM].di[TFSI] demonstrates exceptional load-carrying capacity, maintaining a COF of 0.092 even under an extreme load of 1100 N, whereas the EMIM. TFSI fails at 450 N. The mechanism behind this enhancement was further elucidated through surface chemical analysis and molecular dynamics (MD) simulations. Chemically, C₆di[MIM].di[TFSI] facilitates the formation of a robust composite tribofilm consisting of “soft” FeS and “hard” FeS₂, providing synergistic protection and solid lubrication to the contact interface. Physically, the unique dicationic structure of C₆di[MIM].di[TFSI] enables strong anchoring to the metal substrate, forming a highly dense adsorption layer. This anchored layer minimizes internal viscous dissipation by forcing the shear motion to localize at the topmost interface. This study highlights the potential of DImILs as advanced lubricants through a unique “dual-protection” mechanism combining chemical film formation and physical shear shielding.