Tribological characteristics of phosphonium-based ionic liquids: the role of adsorption layer and tribofilms

Phosphonium-based ionic liquids (ILs) are increasingly recognized as promising lubricant additives, yet their performance under realistic automotive conditions remains poorly understood. This study evaluates three ILs [P 4442 ][DEP], [P 4444 ][C 2 C 2 PO 2 S 2 ], and [P 66614 ][NTf 2 ], as 1 wt% additives in polyalphaolefin (PAO4), benchmarked against the widely used anti-wear agent zinc dialkyldithiophosphates (ZDDP). Tribological performance was assessed via macroscopic ball-on-disc tests across λ ratios under boundary lubrication at 18 °C, 60 °C, and 80 °C, alongside nanoscale in-lubro AFM experiments to visualize tribofilm formation. SEM/EDX and Raman spectroscopy characterized the wear tracks. Results show that [P 4442 ][DEP] provides the lowest friction and wear at 18 °C, forming a thin, adsorbed, phosphate-rich tribofilm, whereas all ILs lose anti-wear efficacy at elevated temperatures, sometimes performing worse than neat PAO. AFM analyses reveal that mild sliding is insufficient to trigger tribofilm growth, suggesting that surface wear generates active sites critical for IL effectiveness. High adsorption kinetics of [P 4442 ][DEP] appear fundamental in forming a nanoscale thick, solid-like film, explaining its superior performance. This work highlights the limitations and potential of phosphonium-based ILs, emphasizing the need for tailored formulations to withstand automotive operating conditions. By linking macroscopic tribology with nanoscale film formation, it provides actionable insights for the design of next-generation additives for automotive and electric vehicle lubricants, bridging a critical gap in the field. • Tribological performance of phosphonium-based ionic liquids (ILs) were well interpreted by adjusting lambda ratios (λ). • Anti-friction and -wear effect of ILs was associated with test conditions of sliding speeds, temperature, and intrinsic properties of viscosity and molecular structures. • In lubro AFM experiments suggested the existence of a boundary adsorption layer while ILs-induced tribofilms saw rare formation as compared to ZDDP.