Relationship between Interfacial Adsorption of Additive Molecules and Reduction of Friction Coefficient in the Organic Friction Modifiers-ZDDP Combinations

Organic friction modifiers (OFMs) are often used with zinc-dialkyl-dithio-phosphate (ZDDP) in engine oil formulations to reduce boundary friction while maintaining the anti-wear property of ZDDP. This study investigated the interaction between fatty acids and ZDDP. The results showed that the adsorption of fatty acids determines the friction coefficient when fatty acids are used alone. The friction coefficient was smaller when a denser fatty acid adsorbed layer was formed. Neutron reflectivity (NR) measurement revealed that, for a fatty acid-ZDDP combination, the fatty acid also contributes to friction behavior; a synergistic effect occurred only with a reduction in the interface metal thickness. These results suggest that the fatty acid-ZDDP combination promotes metallic soap formation, which reduces friction. • Neutron reflectivity was used to measure additive molecular adsorption. • Adsorption density of OFM molecule corresponds with friction coefficient. • Saturated/unsaturated fatty acids exhibited different friction coefficients when used with ZDDP. • Further friction reduction can be achieved with additive combinations.