The Load-Carrying Mechanism of Organic Sulfur Compounds—Application of Electron Probe Microanalysis

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Abstract
A detailed literature survey has shown that considerable controversy exists on the mode of action of organic sulfur compounds as load-carrying additives. Previous work by the authors, using four ball extreme-pressure and wear tests, has suggested that under mixed lubrication (antiwear) conditions, organic disulfides are adsorbed on to the metal surface with cleavage of the sulfur-sulfur bond to form an iron mercaptide layer. However, under more severe conditions such as exist in the extreme-pressure region, cleavage of the carbon-sulfur bond occurs to give an inorganic sulfur-containing layer. Thus the different relative performances of disulfides in the mixed lubrication and extreme-pressure regions are readily explained. This paper now presents the results of a detailed examination of the wear scars obtained with certain disulfides using an electron probe microanalyser. The results illustrate the use of this new technique for studying the nature of worn surfaces but, even more important, lend strong support to the suggested mechanism of action of organic disulfides. The results of this study also offer an explanation for the apparently conflicting rig test results often reported in the literature.