The Effect of a Friction Modifier Additive on Micropitting

Abstract

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 dialkyl-dithio-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.

KEY WORDS:

• Micropitting
• Antifatigue Additives
• Antiwear Additives
• Fatigue
• Gear Lubricants
• Gears
• Mineral Base Stocks
• Rolling-Contact Fatigue

ACKNOWLEDGEMENTS

The authors are grateful to Caterpillar Inc. for financial support and for permission to publish this article. The XPS work was carried out at the University of Illinois at Champaign–Urbana, whose assistance is gratefully acknowledged.

Review led by Bob Errichello