Influence of structural factors on the tribological performance of organic friction modifiers

The influence of structural factors on the lubrication performance of organic friction modifiers (OFMs) formulated in Group V (polyol ester oil) base oil was studied using a ball-on-disk tribometer. The results show that OFMs can mitigate friction under heavy loads, low sliding speeds, and high temperatures. These conditions are commonly encountered in internal-combustion engines between cylinder liners and piston rings. The reduction in friction is ascribed to the boundary lubrication film containing the OFM. The chemical composition analysis of the metal disk surface using energy dispersive X-ray spectroscopy (EDS) confirmed the presence of a protective film of OFM on the wear track, albeit inconsistently deposited. Although the adsorption of the OFM on the metal surface was observed to be dependent on the chemical reactivity of the functional groups, levels of unsaturation, and hydrocarbon chain length of the OFM, the frictional performance was not always directly correlated with the surface coverage and tribofilm thickness. This implies that the friction reduction mechanism can involve other localized processes at the interface between the metal surface and lubricant oil. The occasional variation in friction observed for these OFMs can be attributed to the stability and durability of the boundary film formed during the rubbing phase.