Synergistic Effects between Plasma-Nitrided AISI 52100 Steel and Zinc Dialkyldithiophosphate Additive under Boundary Lubrication

Abstract

Plasma nitriding is applied to many industrial parts running in the boundary (or mixed) regime. It is necessary to understand the synergistic effect on tribological behavior between the nitrided steel surface and lubricant additive to achieve an effective performance. However, there has been very little work concerning the effect of plasma-nitrided steel on the tribochemical interactions of zinc dialkyldithiophosphate (ZDDP) additive and the composition of ZDDP tribofilm. In this work, the interactions and synergistic tribological effects between the plasma-nitrided AISI 52100 steel surface and ZDDP additive under boundary lubrication were investigated. The tribological behavior of plasma-nitrided and untreated steels under lubrication with different ZDDP concentrations was examined on a four-ball friction and wear tester. The chemical characteristics of the tribofilms generated on the worn surfaces were analyzed by X-ray photoelectron spectroscopy (XPS). The results showed that the obvious synergistic effects of better friction reduction and wear resistance between plasma-nitrided steel and ZDDP were attributed to the hard nitrided layer and the shorter chain phosphates tribofilm with greater carbon and phosphate contents.

KEYWORDS:

• Plasma Nitriding
• Zinc Dialkyldithiophosphate (ZDDP)
• Tribofilm
• Friction Reduction
• Wear Resistance

ACKNOWLEDGEMENTS

The authors thank the National Natural Science Foundation of China (51005218) and the Fundamental Research Funds for the Central Universities (2010ZY51, 2011YXL020). Thanks also to Haipeng Huang from the R&D Center of SINOPEC Lubricant Company (Beijing) for providing the base oil and additives.

Review led by Gary Barber