Reduced Phosphorus Concentration Effects on Tribological Performance of Passenger Car Engine Oils

Abstract

Phosphorus is present in engine oils in the form of the antiwear and antioxidation additive zinc dialkyldithiophosphate (ZDDP). Its effects on wear and friction were studied at different temperatures using a high-frequency reciprocating rig (HFRR). The electrically insulating tribofilm formation was measured using an electrical contact resistance (ECR) technique. The wear and friction performance of a fully formulated fresh oil containing 0.05 wt% phosphorus was compared with the corresponding used oil drained from a vehicle. The results show that the wear performance of fresh oils having phosphorus concentration from 0.02 to 0.1 wt% is very similar. Further reduction of phosphorus concentration below 0.02 wt% leads to high wear. The coefficient of friction increases with increased phosphorus concentration at temperatures above 80°C but decreases with increased phosphorus concentration at temperatures below 80°C. The used oil and the fresh 0 wt% P oil running on the original fresh steel surface exhibit higher wear than when both oils were evaluated on a previously formed film from a fresh oil containing 0.05 wt% phosphorus.

Keywords:

• Phosphorus
• Zinc Dialkyldithiophosphate
• Wear
• Friction
• Film Formation
• Oxidized Oil
• Electrical Contact Resistance

ACKNOWLEDGMENTS

The authors are grateful for the help and discussions of Milton D. Johnson at the Ford Motor Company and Kay K. Bjornen at ConocoPhillips. We also thank Kay K. Bjornen for her helpful editing work on this article.

Presented at the STLE/ASME Tribology Conference in Ponte Verda Beach, Florida October 26-29, 2003

Final manuscript approved November 18, 2003

Review led by Elaine Yamaguchi