The addition of friction reducing additives, or friction modifiers (FMs), to lubricants is an important strategy for improving efficiency of lubricated equipment. The performance of these additives is dependent on their surface activity and ability to form adsorbed layers on lubricated surfaces. However, the extent of surface coverage (mass per unit area) required for effective friction reduction is currently unknown. In this presentation we discuss the adsorption of stearic acid on iron oxide and compare our results to earlier studies.   We show that surface coverage is affected by solvent (i.e. basestock), surface packing and free energy of adsorption.   Dimerization of stearic acid is also considered, a factor which is not fully developed in earlier work.   Solvent can also impact adsorption kinetics.  For example,   molecular dynamic simulations reveal that hexadecane tends to align on the iron oxide surface, increasing the energy barrier for the adsorption of stearic acid.   Heptane does not align at the surface to create an adsorption barrier.  Our results also suggest that full coverage of the surface with friction modifier is not necessary for significant friction reduction.    While friction of systems lubricated with organic FMs decreases with increasing coverage, complete coverage of the surface is difficult to achieve.  Moreover, full surface coverage is not necessary for effective friction reduction in the systems and conditions studied here.<br/><br/>

Influence of surface coverage on friction performance of stearic acid on iron oxide

2019 Fall National Meeting

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