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

Reciprocating ball-on-flat tests were conducted on prepared sections cut fr om cast iron, silicon nitride, and silicon nitride/12.5 vol% graphite compo site valve guides over a range of temperatures, normal loads, speeds, and l ubrication conditions. The purpose of this work was to ascertain whether th e latter ceramic composite would produce a beneficial lubricating film on t he opposing surface and serve as a self-lubricating material. Type 440C sta inless steel was used as the counterface material. Machining practices were selected to provide the surface roughness and lay (direction of grinding m arks) similar to that of actual valve guide bores. For comparison with the ceramic composite material, both cast iron and silicon nitride matrix mater ials were also tested. Tests were also performed using graphite powder on t he silicon nitride matrix material to ascertain what frictional behavior mi ght be observed in the most favorable case. Friction and wear data, combine d with surface chemical analysis confirmed that the current composite, whil e wear resistant, did not provide any lubrication advantages over silicon n itride itself. No evidence for the sliding-induced formation of a beneficia l graphite film was obtained by optical examination, scanning electron micr oscopy, or surface chemical analysis. While the type of graphite used in th e present composite fractured into fine particles and did not form a lubric ating film in our experiments, the results do not preclude the possibility of developing other ceramic composites with self-lubricating properties. (C ) 1999 Published by Elsevier Science S.A. All rights reserved.