Tribological evaluation of oil pump relief valve coatings compatible with an aluminum oil pump body

Abstract

Wear can cause automotive relief valves to jam. In order to evaluate and screen candidate coatings for oil pump relief valves for reduction of aluminum pump cylinder bore wear and wear-related sticking, a laboratory reciprocating wear test using production parts has been developed. The coatings on valves include impinged and physical vapor deposited (PVD)-coated molybdenum disulfides, electroplated nickel–phosphorous with polytetrafluoroethylene (Ni–PTFE), electroplated bronze, and electroplated nickel–phosphorous–boron nitride (Ni–P–BN).The test results showed that the electroplated bronze coating demonstrated the best wear resistance against 380 aluminum pump bores while Ni–PTFE ranked second, PVD-coated MoS2 third, and Ni–P–BN ranked last. It was observed that the electroplated bronze coating showed only mild oxidative or abrasive wear after 20h wear test. The Ni–P–BN coating gave the worst wear resistance due to severe abrasive wear, surface scoring, and coating abrasion during the wear test. The major wear mechanisms for valve bore/relief valve can be classified at different levels from mild wear (oxidative wear or surface delamination) to abrasive wear (scoring, scuffing, and ploughing). This paper also reviewed the rooted wear mechanisms of production pump relief valves against aluminum bores based on metallographic observations of worn surfaces after field tests. This tribological investigation of valve coatings has provided insights into the fundamental wear mechanisms which depend on the compatibility of two sliding materials, protective coating composition, hard particle content, and surface interaction. The information will be useful in preventing oil pump relief valves from jamming.