Reciprocating friction and wear behavior of a ceramic-matrix graphite composite for possible use in diesel engine valve guides

Abstract

Reciprocating ball-on-flat tests were conducted on prepared sections cut from cast iron, silicon nitride, and silicon nitride/12.5 vol% graphite composite valve guides over a range of temperatures, normal loads, speeds, and lubrication conditions. The purpose of this work was to ascertain whether the latter ceramic composite would produce a beneficial lubricating film on the opposing surface and serve as a self-lubricating material. Type 440C stainless steel was used as the counterface material. Machining practices were selected to provide the surface roughness and lay (direction of grinding marks) similar to that of actual valve guide bores. For comparison with the ceramic composite material, both cast iron and silicon nitride matrix materials were also tested. Tests were also performed using graphite powder on the silicon nitride matrix material to ascertain what frictional behavior might be observed in the most favorable case. Friction and wear data, combined with surface chemical analysis confirmed that the current composite, while wear resistant, did not provide any lubrication advantages over silicon nitride itself. No evidence for the sliding-induced formation of a beneficial graphite film was obtained by optical examination, scanning electron microscopy, or surface chemical analysis. While the type of graphite used in the present composite fractured into fine particles and did not form a lubricating film in our experiments, the results do not preclude the possibility of developing other ceramic composites with self-lubricating properties.