Unveiling the Effect of Electrification on Phosphonium-Based Ionic Liquid Lubrication at Tribological Interfaces for Electric Vehicle Applications

Abstract

Conventional lubricants face significant challenges in electric vehicle (EV) systems due to their low electrical conductivity and inability to mitigate tribo-electrification effects which can result in increased friction, wear, and electrical discharge damage under external electrification. Consequently, conductive lubricants like ionic liquids (ILs) have emerged as promising alternatives, offering enhanced compatibility with EV applications. This study investigated the tribological behavior of four phosphonium-based room temperature ionic liquids (PRTILs) with trihexyltetradecyl phosphonium [P6,6,6,14] or tributyltetradecyl phosphonium [P4,4,4,14] cations and saccharinate [Sacc] or benzoate [Benz] anions under electrified conditions, targeting potential EV applications. Physicochemical properties, including viscosity and ionic conductivity, were measured using a viscometer and a conductivity meter, while tribological properties were evaluated using an electrified mini-traction machine and an electrified rotary ball-on-disk setup. The results revealed that all the PRTILs exhibited superior tribological (friction and wear) performance than mineral oil with or without electrification. PRTILs with the [Sacc] anion feature a double aromatic ring structure, while those with the [Benz] anion feature a single aromatic ring structure. Under low electrification (10 mA), [P6,6,6,14][Sacc] outperformed [Benz]-based PRTILs, showing a lower coefficient of friction and wear due to their higher viscosity and lower ionic conductivity. Additionally, [P6,6,6,14][Sacc] showed a power loss lower than [P4,4,4,14][Sacc] but higher than [Benz]-based PRTILs under tribo-electrification. The addition of graphene nanoplatelets (GNPs) reduced the power loss of [P6,6,6,14][Sacc] by 24% by reducing the electric contact resistance. Overall, double-ring aromatic [P6,6,6,14][Sacc] demonstrated superior tribological performance, and GNP additives enhanced their power efficiency, offering a promising pathway for IL-based lubricant development for electrified conditions.