Mechanistic insights into the effect of structural factors on film formation and tribological performance of organic friction modifiers

Abstract The influence of molecular structure of organic friction modifiers (OFMs) on their adsorption behaviour on iron oxide surface immersed in PAO oil was investigated using neutron reflectometry (NR). The effects of carbon chain length and levels of unsaturation on film formation was studied using a series of fatty acids with 18-20 carbon atoms in the temperature range between 25 and 100 °C. The results revealed that saturated OFM molecules generate thicker films compared to its unsaturated counterparts. Further, a decrease in film thickness was observed with the degree of unsaturation with the exception of linolenic acid. The film thickness and grafting density of the OFM increased with temperature. A decreased film thickness for the binary additive system comprising GMO and saturated molecules suggested that the adsorption of OFMs on the surface is hindered in the presence of glycerol mono oleate (GMO). A lower boundary friction coefficient and wear exhibited by OFMs with longer chain and linear structure suggest that their tribofilms are more effective in withstanding shear. The tribofilm thickness measured using optical interferometry showed no correlation with the adsorbed layer thickness determined from NR data; this lack of correspondence is attributed to the load and shearing forces experienced by the OFM molecules at the contact surfaces during friction measurements.