Tribological study of diesel piston skirt coatings in CJ-4 and PC-11 engine oils

Abstract Piston skirt coatings are used in heavy-duty diesel engines for reduction of friction, noise, and scuffing. This study explored the friction and wear behavior of two piston skirt coatings, a polymer-graphite composite coating with a manganese phosphate (Mn-P) interlayer and a standalone Mn-P coating, in API CJ-4 and candidate PC-11 (CK-4 and FA-4) diesel engine oils. Two test configurations were used: conformal contact similar to that in an actual engine to examine the friction behavior under normal operating conditions, and point contact to study the wear performance under extreme pressure conditions. Results suggest that CK-4 could replace CJ-4 without wear penalty but FA-4 may post a potential wear challenge. The standalone Mn-P coating showed little friction benefit in the conformal contact, but made a detrimental impact on wear performance of the piston skirt in the point contact. It is hypothesized that the hard Mn-P grains worn off the coating acted as abrasive at the sliding interface to promote third-body abrasion accelerating the material removal. The polymer-graphite composite coating reduced the friction by 15–30% in the conformal contact tests in the three engine oils. However, in the point contact sliding, the polymer-graphite coating caused a higher wear rate, which is attributed to the Mn-P interlayer releasing hard particles after the top coat was worn through.