Tribological performance and lubrication mechanism of new gemini quaternary phosphonium ionic liquid lubricants

Abstract

This paper presents the synthesis and characterization of two types of halogen-free ionic liquids (ILs) that incorporate long-chain phosphate, diisooctyl phosphate (EDHPA), and dibutyl phosphate (DBHPA) as the anions, and the gemini quaternary phosphonium as the cation. Their tribological properties are subjected to evaluation by utilizing these ILs as neat lubricants for a friction pair, constituting of a steel disc and a steel ball, for two conditions, i.e., at room temperature (RT) and high temperature of 100 °C (HT). These as-prepared ILs are found to exhibit phenomenal traits such as high thermal stability and non-corrosive properties, along with being able to impart enhanced anti-wear (AW) and friction reduction (FR) performance to the friction pair in contrast to the commercially available lubricants, i.e., synthetic poly-α-olefin (PAO 10) and traditional IL 1-butyl-3-methylimidazolium hexafluorophosphate (L-P104). Various investigations are performed employing scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and focused ion beam-transmission electron microscopy (FIB-TEM) analyses of the worn surface of the steel discs. These results yield the postulation that a chemical reaction, i.e., tribochemical reaction, is triggered during the relative motion of the two interacting surfaces. This reaction induces a protective phosphorous-containing boundary film on the surface of the worn steel, thereby evading any direct contact between the rubbing steel-steel interfaces.