The Effect of a Friction Modifier Additive on Micropitting

Previous studies have shown that many anti-wear additives have a tendency to aggravate micropitting damage in hard steels subjected to rolling sliding contact at low lambda values. This is apparently because anti-wear additives suppress the gradual smoothing of the rough surfaces that takes place when a pure base stock is used under mild conditions. However, in practice, it is common to combine anti-wear and friction modifier additives in formulated oils, a combination that leads to reduced boundary friction. In the work described in this article, we added a common friction modifier agent, a molybdenum bis-diethylhexyl dithio-carbamate (MoDTC), to an oil containing an anti-wear additive, a secondary zinc dialkyldithio-phosphate (ZDDP) in a mineral base-stock. We examined micropitting behavior using a disc tester. The oil with both MoDTC and ZDDP showed initial micropitting that gradually disappeared with continued running, whereas ZDDP alone led to continuing severe damage. Analysis of the counter-discs after the test showed the presence of MoS2 deposits on the asperity crests. Reduced boundary friction was confirmed using a tribometer test. It is speculated that the improved micropitting behavior resulted from the effect of a reduction in local tensile stress due to reduced asperity friction. This may have reduced the opening of the surface cracks and inhibited their extension.