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

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.