Tuning the Surface Reactivity and Tribological Performance of Phosphonium-Based Ionic Liquid at Steel/Steel Interfaces by Bromide/Phosphate Anion Mixtures

Abstract Phosphonium phosphate ionic liquids (ILs) are promising lubricant additives for engine oils owing to a combination of good tribological performance and attractive properties, such as high thermal stability and good miscibility with mineral and synthetic oils. Here, we evaluate the dependence of the lubricating performance of trihexyltetradecylphosphonium ([P6,6,6,14])-based ILs when used in steel/steel contacts on the relative concentration of bis(2-ethylhexyl)phosphate ([DEHP]) and bromide (Br-) anions. The results indicate no changes in friction and wear upon increasing the [P6,6,6,14]Br-to-[P6,6,6,14][DEHP] volume ratio up to 0.2:1, while a further increase in this ratio leads to a reduction of friction and wear. X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry measurements highlight that the dependence of the lubricating properties of the ILs on the [P6,6,6,14]Br-to-[P6,6,6,14][DEHP] ratio originates from the balance between corrosivity and surface reactivity of this IL: while [DEHP] acts as corrosion inhibitor in the non-contact region, the formation of species containing phosphorus-oxygen moieties in the wear track as a result of the hydrolysis of phosphonium ions increases the surface coverage of phosphorus. This is proposed to decrease friction by decreasing adhesion at steel/steel contacts and lowering the interfacial shear strength.