In the past large synergistic effects have been reported for combinations of overbased sulfonate and sulfurized olefin in four-ball EP testing. In this study, we examined the effect of crystallinity on the formation of the tribofilms using X-ray absorption near edge structure (XANES) spectroscopy. 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. While the overbased calcium sulfonates do not react strongly with the substrate there is evidence to suggest that the overbased sulfonate promotes decomposition of the sulfurized olefin on the surface to form both CaSO 4 and FeS, which improves the EP performance of the lubricant. A putative intermediate is H 2 S from the decomposition of the sulfurized olefin which can react with O 2 and the Fe-rich surface to form FeSO 4 and FeS, respectively. A new 'FeS' species has been detected in the tribofilms that is believed to be a non-stoichiometric Fe-enriched FeS. Depending on the presence or absence of the overbased calcium sulfonate either FeSO 4 or FeS formation is favored and changing the concentration of the crystalline, C300C, or amorphous, C400A, overbased calcium sulfonate from 5% to 10% does not appear to have any effects on FeS formation, although the C300C produces more Fe-enriched FeS than the C400A in the bulk.

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