Abstract: Two synthetical ashless phosphorus/sulfur-containing compouds were used as additives in biodegradable rapeseed oil (RSO). Their anti-wear and friction-reduction performances as lubricating oil additives were evaluated using four-ball tester. The chemistry of their tribofilms and thermal films was analyzed with X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES), The results showed that the two phosphorus/sulfur-containing compounds possessed rather good anti-wear properties and friction-reducing performances. According to the XPS and XANES analytic results, the tribofilms and thermal films generated from the additives were mainly composed of adsorbed-film and reaction film, in which phosphate and pyrophosphate were main existence form of P, FeSO4 was the main existence form of S. The results also showed that tribo-heat played different roles in the formation of tribofilms generated fromthe two different additives.

Key words: Biodegradable lubricant, XPS, XANES, Tribofilm, Thermal film, Lubricating mechanism