Friction tests have been carried out on bearing steel friction pairs coated with a-C:H and ta-C under lubricated conditions with 1-hexadecene and pure oleic acid, respectively. Main results show that ta-C leads to the super-low regime with oleic acid (friction coefficient below 0.01) and that a-C:H is quite well lubricated by 1-hexadecen. Surface analyses by coupling X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) have been carried out in the case of oleic acid with ta-C and a-C:H coatings. Results show that tribochemical reactions have occurred. OH groups terminate the ta-C surface whereas a-C:H is covered by a thin tribofilm and is terminated by oleic acid monolayers. The superlow regime in the case of ta-C is mainly attributed to the presence of the OH-termination.

This paper investigates the effect of unsaturated fatty acid additives on the lubricating properties of a biodiesel, composed of a blend of fatty acid methyl esters (FAMEs) and a conventional diesel fuel. The presence of fatty acids as additives for biodiesel improves their lubricating properties at elevated temperature conditions. The formation of a protective lubricating tribochemical film on the steel contact surfaces has been evidenced. Elevated temperature conditions and possibly oxygen are needed to form this film that reduces friction and limits wear. Several analytical tools have revealed the insulating nature of the formed tribofilm and its chemical properties. Friction-induced reticulation is thought to play a major role in the growth of this insulating film.