Interactions of phosphonium-based ionic liquids and ZDDP: Both synergy and antagonism

Ionic liquids (ILs) have emerged as promising candidates for use as lubricants and lubricant additives. However, due to their high cost, it is practical to explore their application in combination with conventional lubricant formulations. This approach, although promising, necessitates further investigation into the interactions between ILs and traditional additives. In this study, two phosphonium-based ILs, tributylethylphosphonium diethylphosphate ([P₄₄₄₂][DEP]) and trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl) phosphinate ([P₆₆₆₁₄][(iC₈)₂PO₂]), were blended with a polyalphaolefin (PAO4) base oil containing zinc dialkyldithiophosphate (ZDDP), one of the most widely used antiwear additives. Tribological performance was evaluated using both ball-on-disc tribotests and in lubro atomic force microscopy (AFM)-based nanoscale experiments. Tests were conducted under boundary lubrication (BL) conditions under a constant normal load of 15 N, at room temperature and at 60 °C. The coefficient of friction (COF), wear volume, and surface morphology were recorded, followed by postmortem analyses using scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) to examine the chemical composition of the wear tracks. AFM-based imaging revealed the morphological evolution of the contact area as sliding progressed. The results demonstrated that [P₄₄₄₂][DEP] provided superior tribological performance compared to [P₆₆₆₁₄][(iC8)₂PO₂], attributed to the structural characteristics of its phosphate anion. Notably, the combination of ILs with ZDDP exhibited both synergistic and antagonistic effects on antiwear behavior, depending on the specific IL used. These findings contribute to a deeper understanding of interfacial additive interactions and shed light on the complex lubrication mechanisms of ILs.