Physicochemical and tribological comparison of bio- and halogen-based ionic liquid lubricants

Abstract

The friction and wear behavior of bio-based trihexyltetradecylphosphonium saccharinate [P6,6,6,14][Sacc] and halogen-based trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl) amide [P6,6,6,14][NTF2] ionic liquids (ILs) were studied to understand their lubrication mechanisms at steel sliding interfaces. The physicochemical and tribological properties of the ILs were characterized over a wide temperature range (10–120 ℃) to reflect the conditions present in wind turbine applications. Friction increased with increasing temperature for both ILs. At any temperature, [P6,6,6,14][Sacc] had significantly higher viscosity that provided thicker lubricant films and, in turn, better friction and wear protection than the halogen-based [P6,6,6,14][NTF2]. [P6,6,6,14][Sacc] also had lower density, comparable thermal stability, more favorable wettability, and better corrosion performance than [P6,6,6,14][NTF2]. Simulations showed that the cohesion interaction energy was stronger for [P-Sacc] due to its smaller anion-cation distance. The higher viscosity and stronger cohesion of [P6,6,6,14][Sacc] than [P6,6,6,14][NTF2] contributed to the ability of the bio-based IL to form an effective adsorption film that reduced friction and wear across a range of temperatures.