Relationship between Surface Composition and Wear: an X-ray Photoelectron Spectroscopic Study of Surfaces Tested with Organosulfur Compounds

Abstract

An examination of steel surfaces, after wear-testing in oils containing organosulfur compounds, was conducted to investigate possible reasons for the differences in the efficiency of organosulfur compounds as antiwear additives. The tested surfaces were characterized both qualitatively and quantitatively by x-ray photoelectron spectroscopy. Qualitative analysis showed that wear-testing of several different organosulfur compounds produced metallic sulfide on the external surface. This observation indicates that the differences in efficiency cannot be ascribed to chemically different surface species. From quantitative analysis, a correlation was determined between the relative surface concentration of sulfide and wear; the higher the sulfide concentration at the end of the wear-test, the lower the wear during that test. The sulfide concentration was constant throughout the three-hour test of a given compound, but differed from one compound to another. The addition of amines to the oil formulation decreased the sulfide concentration and increased the wear. These results indicate that the surface sulfide, or its direct precursor, is the species which provides antiwear protection of steel surfaces, under the present lest conditions, when organosulfur compounds are used as anti-wear additives.