β-Mipron–Cyclic Amidine-Based Protic Ionic Liquids as Steel Lubricants: Synergistic Construction of a Dual-Layer Inorganic–Organic Tribofilm for Enhanced Tribological Performances

The tribological behaviors of three as-synthesized β-mipron-cyclic amidine-based protic ionic liquids (PILs) were studied as lubricant additives in poly-alpha-olefin (PAO) for steel contact pairs. Compared to PAO10 base oil, these additives not only improved thermal stability and wettability but also achieved a maximum 67% reduction in the coefficient of friction and an 80% decrease in the wear volume. Further studies demonstrated that the tribological performance of PIL additives exhibited a significant structural dependence: Bicyclic PIL additives outperformed their single-ring counterparts. Analysis of worn surfaces and tribofilm formation revealed a dual-layer structure consisting of organic carbonaceous species (upper layer) and inorganic iron oxides (base layer), which contributed to the exceptional tribological performance without requiring long alkyl chains or active elements (e.g., P or S). The bicyclic amidine-based PIL additives demonstrate substantial industrial potential as novel oil-soluble lubricant additives while providing a valuable reference for designing cost-effective and high-performance PIL additives.