Competitive molecular interaction between zinc dialkydithiophosphate (ZDDP) and methyl oleate along boundary lubrication regime

Abstract

Fatty acid methyl ester (FAME) possesses good lubricity properties, prompting numerous research work to explore their potential as lubricant additives. However, fully formulated lubricants consist of additive packages, which includes essential anti-wear agents, such as zinc dialkyldithiophosphate (ZDDP). Adding FAME into a lubricant could lead to adverse tribological effects if the competitive molecular interaction between different additives are not well understood. Therefore, the present study determines the tribological impact of molecular interaction between ZDDP and methyl oleate along boundary lubrication regime. Lateral force microscopy, coupled with fluid imaging approach, is adopted, allowing for the analysis to focus on asperity level interaction. A silicon nitride tip is used to slide against a mirror-polished steel substrate while being fully submerged in the tested fluids. The tested fluids are mixtures of ZDDP and methyl oleate, blended at different volumetric percentages. It is observed that the lowest friction force is measured for ZDDP-methyl oleate mixture containing 70-vol% of methyl oleate. Interestingly, the friction force measured for such mixture is found to be lower than that of neat methyl oleate. This finding indicates that an optimum blending ratio between FAME and ZDDP is essential in achieving better boundary lubrication performance for tribological conjunctions.