Study of surface films of overbased sulfonates and sulfurized olefins by X-Ray Absorption Near Edge Structure (XANES) spectroscopy

The X-Ray Absorption Near Edge Structure (XANES) spectroscopy technique has been successfully applied to complex matrices such as tribological films, coal, DLC films and, where other instrumental analytical techniques are less sensitive for molecular analysis. Four-ball tests were performed for lubricating oils containing the synergistic combination of overbased sulfonate (calcium and magnesium) and sulfurized olefin. The surface films were analyzed by XANES using C K-edge, Ca L-edge and S L- and K-edge. Using this technique we were able to differentiate the carbonate from organic carbon in C K-edge spectra, as well as, determining the difference between FeS, FeS_2, RSO ₃ ⁻ and SO ₄ ²⁻ in the sulfur spectra. It was found while the sulfurized olefin reacts very strongly with the substrate to form FeS, the overbased calcium sulfonate and overbased magnesium sulfonate do not react strongly with the substrate without sulfurized olefin present. There is evidence to suggest that the overbased sulfonate promotes decomposition of the sulfurized olefin on the surface to form both CaSO₄ and FeS, which improves the EP performance of the lubricant.