Elucidation of the Action of Functional Groups in the Coexisting Ashless Compounds on the Tribofilm Formation and Friction Characteristic of Zinc Dialkyldithiophosphate-Formulated Lubricating Oils

ABSTRACT

Zinc dialkyldithiophosphate (ZnDTP) is well known to show supreme antiwear performance by forming a thick polyphosphate-based tribofilm on the friction surface. However, it has also been demonstrated that a coexisting dispersant can interfere with the performance of ZnDTP. Because many lubricating oils contain a variety of additives in addition to ZnDTP, the effect of functional groups in the coexisting polar compounds on the tribofilm formation of ZnDTP should be clarified. In this study, several kinds of functionalized polyalkylmethacrylates (PMAs), a dispersant, and friction modifiers were used with ZnDTP, and their effects on the tribofilm formation and the friction characteristic of ZnDTP were discussed. The formulation of polar compounds with different types of functional groups significantly changed the state of ZnDTP compounds in the bulk solutions. Amino groups tended to generate coordination complex compounds and reduced the neutral ZnDTP content in the sample solutions. PMA-OH and glycerol monooleate (GMO) both had hydroxyl groups, and nonfunctionalized PMA 0 did not significantly influence the neutral ZnDTP content. PMA-COOH and stearic acid (StA) containing carboxyl group(s) significantly increased the neutral ZnDTP content. The composition of neutral ZnDTP and basic ZnDTP in the lubricating oil is a key to understanding the influence of coexisting ashless polar compounds on the formation of the ZnDTP-based tribofilms and the friction characteristics of ZnDTP.

KEYWORDS:

• Additives
• antiwear additives
• friction modifiers
• boundary lubrication
• boundary lubrication chemistry
• additives
• VI improvers

Acknowledgements

We thank Sanyo Chemical Industries for providing the PMAs and the Center for Advanced Materials Analysis, Tokyo Institute of Technology, for the assistance and discussions of the surface analytical studies. We greatly appreciate Kazuhiro Yagishita of JX Nippon Oil & Energy Corporation for his helpful advice on the ³¹P-NMR study.

Funding

This study was supported in part by the Council for Science, Technology and Innovation (CSTI); the Cross-ministerial Strategic Innovation Promotion Program (SIP), “Innovative Combustion Technology” (Funding agency: JST); and JX Nippon Oil & Energy Corporation.