Influence of Base Oil Polarity on the Tribological Performance of Surface Active Engine Oil Additives

Friction, wear and tribofilm growth of organic friction modifiers (glycerol monooleate and oleamide), antiwear additive (ZDDP) and binary additive system comprising the organic friction modifiers and ZDDP were studied in polyalphaolefin (PAO) and ester oil. The mechanisms underlying base oil polaritydependent frictional performance of the OFM and AW additives at high temperature (140 oC), either singly or in combination, was investigated in the light of chemical composition analysis of the tribofilms post friction measurements using energy-dispersive X-ray spectroscopy (EDX), static and dynamic timeof-flight secondary ion mass spectrometry (ToF-SIMS). Depending on the rubbing conditions, the boundary friction coefficient of the binary additive systems were found to be either lower than that of individual additives or to lay between the values for the individual additives. Chemical composition analysis of the tribofilms indicated that the nature of base oil controlled interactions between ZDDP and OFM and consequently adsorption and reactive tribofilm formation in the boundary lubrication layer. Surface roughness and wear scar width measured post tribological tests using 3D surface profiler showed improved wear performance in both PAO and ester-based additive formulations.