The friction reducing properties of organic friction modifier additives

Abstract

Organic Friction Modifiers (OFMs) have been used as lubricant additives for over 100 years but the exact mechanism of their friction reducing properties is still uncertain. OFMs are surfactant molecules, with a polar head group and hydrocarbon tail group, which adsorb onto the surface to form self-assembled monolayers. These adsorbed layers reduce friction in mixed and boundary lubrication regimes. This thesis studies the friction reducing properties of OFM additives, which will enable the effective implementation of OFMs within a system and provide potential insight on the design of future OFM additives. The friction reducing properties of OFM additives are impacted by contaminants within an idealised system. Surface contaminants and contaminants within the base oil were studied to understand the impact these contaminants had on the friction reduction of OFMs. Effective methods were found to remove this contamination to further study the friction reduction of OFM additives. OFMs are known to adsorb onto a surface, but the link between the adsorption of these additives and their friction reduction is unclear. This thesis studied the adsorption of different OFM additives and has determined a link between the adsorption thickness and the inital friction of these additives. The mechanism of friction reduction of OFMs with an ester head group was still debated within the literature. This research has found that well adsorbed esters hydrolyse within the contact to form carboxylates on the surface. These carboxylate species dictate the steady state friction of these ester additives.