On the Structure–Activity Relationship of Glyceryl oleate Friction Modifiers and Its Synergistic Mechanism on Phosphate ester Antiwear Additives

The structure–activity relationship of glyceryl oleate organic friction modifiers and the difference in interaction with antiwear additives significantly affect the friction and wear characteristics of the lubrication system. This paper comprehensively considered the differences between polar functional groups and hydrocarbon tail chains, the structure–activity relationship of three glyceryl oleate friction modifiers and the synergistic antiwear mechanism with phosphate ester additives were investigated by experiments and simulations. The results indicated that the substitution of hydroxyl groups in glyceryl oleate weakened the electrostatic and hydrogen bonding interactions with the metal interface, and the increase of hydrocarbon tail chains led to the enhanced steric hindrance effect, which in turn led to the deterioration of tribological properties. In addition, the binary additive system of glyceryl monooleate and phosphate ester exhibited significant synergistic antiwear effects. This is attributed to the chemisorption of additive O(C = O/P-O/P = O) active sites and Fe2O3 metal interface, GMO hydrogen bond interaction, the deprotonation of phosphate ester, and carbonate and phosphate protective film generated by tribochemical reaction.