Tribological Robustness of Oleylammonium Oleate-Reinforced Palm-Derived Ester Lubricants under Severe Water Contamination

Abstract Vegetable oil-derived ester lubricants are promising sustainable alternatives to conventional mineral oils. However, their susceptibility to water contamination limits their tribological robustness. This study investigates the tribological behaviour and water tolerance of a palm-derived trimethylolpropane ester (TMPE) reinforced with the protic ionic liquid (PIL) oleylammonium oleate (OAO). A two-stage evaluation framework was employed to distinguish intrinsic lubrication performance from water-induced effects. TMPE was synthesised and blended with 0.5–5 wt% OAO. Under severe water contamination (0–40 wt%), the formulation containing 1 wt% OAO exhibited the most balanced tribological response, achieving approximately a 1.5-fold improvement in the mean PCA-based performance metric and 26% lower variability relative to neat TMPE. These results show that water-tolerant lubricant design should consider not only optimal water-free friction and wear performance but also variability in performance under contamination. The combined use of friction, wear, viscosity response, short-term dispersion, corrosion resistance, and PCA-based variability analysis provides a practical framework for identifying sustainable ester lubricant formulations with improved robustness in moisture-prone lubrication environments.