Super-low friction of ta-C coating in presence of oleic acid

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.