Tribochemistry of unsaturated fatty acids as friction modifiers in (bio)diesel fuel

Abstract

The impact of fatty acids on the lubricity of diesel and biodiesel fuels was investigated in steel/steel contacts at two different temperatures of 40 °C and 100 °C. The addition of 7% v/v Fatty Acid Methyl Esters (FAMEs) to fuels, to form biodiesel, lubricates the steel–steel contact and the friction coefficient is reduced by about 30%. Further addition of fatty acids leads to a significant reduction in friction by a factor of 2 at the highest temperature of 100 °C. Some unsaturated fatty acid additives boost the friction reduction by a synergistic effect with the biofuel. The analysis of worn surfaces by XPS revealed the formation of an insulating film mainly located at the center of the tracks and the disappearance of carboxylic groups. PM-IRRAS analysis demonstrated the lack of carbon double bonds in the tribofilm and the presence of some esters (iron and/or organic soaps). By combining high-resolution photoemission soft X-ray absorption and SIMS studies, we found that the extreme surface of the tribofilm is formed by planar arrangements of sp²-rich molecules such as quinones and/or graphene oxides. It is thought that under the effects of shearing, temperature and possibly oxygen from air or fuel, complex chemical reactions between FAME and unsaturated additives occur in the diesel matrix generating a new tribofilm material on the steel surface. As double bonds are involved in the process, the basic tribochemistry mechanism is thought to be related to a reticulation process catalyzed by the formation of oxide particles in the sliding interfacial contact zone accompanied by an aromatization of the fatty acids at the extreme surface.