Interfacial Energy Tuning for Shear-Resilient Boundary Films in Organic Friction Modifier Systems

Lubricant additive optimisation, such as using an Organic Friction Modifier (OFM), often relies on empirical methods because the role of interfacial energetics in boundary lubrication remains uncertain. This study explores how interfacial energy interactions affect the tribological performance of glycerol monooleate (GMO)–polyalphaolefin-4 (PAO4) blends using ball-on-disk friction and wear tests. The 7 wt% GMO blend showed the best results, with friction reduced by about 4 times and the wear scar diameter by nearly 6 times compared to neat PAO4. Film-thickness estimates indicate that all tests operated within the boundary-to-mixed lubrication regime, suggesting that friction reduction is associated with interfacial interactions rather than hydrodynamic film formation. The Owens–Wendt–Kaelble surface energy analysis reveals that increasing GMO concentration shifts the lubricant’s dispersive–polar balance, with the 7 wt% formulation exhibiting dispersive–polar characteristics closer to those of the steel substrate. Low friction persisted as sliding velocity increased, and rupture-threshold analysis is consistent with improved tribological response under increasing load and sliding conditions. These findings suggest that the favourable tribological response observed for the investigated formulations may be associated with balanced interfacial energetic characteristics, particularly between dispersive and polar interactions. The observed friction and wear behaviour under increasing sliding conditions is interpreted in terms of friction and wear responses under boundary-dominated conditions, rather than through direct structural characterisation of boundary films. These trends are interpreted in relation to changes in dispersive and polar interactions at the interface. The results provide an interpretive framework for designing OFM systems that may be relevant to high-shear environments, including applications such as hydrogen internal combustion engines.