Study of surface films of amorphous and crystalline overbased calcium sulfonate by XPS and AES

We have shown that the crystalline overbased sulfonate shows superior Reichert test results over the conventional amorphous overbased materials, and a subsequent X-ray photoelectron spectroscopy (XPS) analysis revealed that thicker Ca films were observed on a test specimen with tribofilms formed from crystalline overbased sulfonate compared to amorphous overbased sulfonate. From the XPS analysis of the wear scars produced in the Reichert test, it appears that amorphous overbased and crystalline overbased calcium sulfonate both form thin layers of CaCO3 on top of a thicker layer of CaO (from CaCO3 decomposition) on the surface of the test piece. Additionally, we examined the morphology of the Ca and S films using auger electron spectroscopy (AES). In the presence of sulfurized olefin, an additional AES analysis reveals that a layer of both S and Ca cover the surface. From the AES analysis of selected regions, the surface either possessed high S or high Ca concentrations, but not both together. The authors hypothesize that there is a reaction between the sulfurized olefin and the carbonate of the overbased sulfonate, which forms a more "active" film during tribotesting.