Glycerol oleate tribofilms: Relationships between chemical composition and tribological performance

Glycerol oleates (GO) are organic friction modifiers (OFMs) with lower toxicity than traditional P- and S-containing friction modifiers. They are increasingly used in lubricant formulations but the mechanisms by which they lower friction and protect surfaces in the boundary lubrication regime are still not well understood. We have determined tribological performance and the chemical nature of tribofilms formed on steel for model formulations of glycerol monooleate, trioleate (GMO, triolein) and their mixture with glycerol dioleate (GDO) as a function of sliding-rolling ratio (% SRR) and temperature. Formulations of GMO, triolein and the mixed GOs in PAO4 base oil were tested at a low entrainment speed of 0.02 m/s and two different temperatures, 60°C and 100°C, in a mini traction machine (MTM) tribometer, with SRRs from pure sliding to pure rolling conditions. The thickness of tribofilms formed was tracked by spacer layer image mapping (SLIM), and their chemical composition was examined using ToF-SIMS. Temperature is correlated with film thickness, which subsequently influences the frictional performance of additives. Higher temperatures promote the formation of thicker films, resulting in lower friction for all additives. ToF-SIMS analysis identified a transition from physisorption at low temperatures and SRRs to chemisorption-dominated friction reduction at higher temperatures and SRRs.