Synergistic Effects Between the Two Choline-Based Ionic Liquids as Lubricant Additives in Glycerol Aqueous Solution

Two choline-based ionic liquids (ILs) were used as lubricant additives in the aqueous glycerol solution, and the tribological properties of their different combinations were investigated. They exhibit remarkable synergistic effects on the antiwear and friction-reducing abilities. The combinations of both ILs more significantly improve the lubricity of the glycerol solution compared with the independent additives. In addition, the tribological behavior of the optimal combination was explored under different temperature and load conditions. The lubricating property of the optimal additive formulation is weakened with the increase of temperature, but enhanced with the increase of load. The worn surfaces were analyzed using SEM-EDAX and XPS. Meanwhile, Monte Carlo simulations were performed to study the adsorption behavior of the additives on iron surface in the presence of water. Remarkable synergistic lubricating interactions between the ILs are ascribed to the formation of compact and stable boundary films composed of mixed adsorbed layers and complex tribofilms.