FRICTION AND WEAR CHARACTERISTICS OF SILICON NITRIDE-GRAPHITE AND ALUMINA-GRAPHITE COMPOSITES

To take advantage of the beneficial properties of advanced ceramics, it is necessary that their friction coefficient be reduced to an acceptable value. One method for achieving this goal is incorporation of a solid lubricant phase in the ceramic matrix. In this study, ceramic-matrix composites were fabricated by drilling a series of small holes in alumina and silicon nitride, which were subsequently filled with NiCl2 intercalated graphite under high pressure. It was found that the addition of graphite to silicon nitride considerably reduces the friction coefficient, but alumina-graphite composites exhibit only a marginal reduction in friction coefficient compared to alumina. The reduction in friction coefficient for silicon nitride-graphite composite can be explained by the formation of transfer films consisting of a mixture of materials from both contacting surfaces. However, for alumina-graphite composites the graphite regions were completely covered with steel wear particles, inhibiting the formation of graphite-containing transfer films.