ABSTRACT

In certain applications involving high temperatures or vacuum environments, liquid lubricants cannot be used effectively. Self-lubricating materials containing solid lubricants may provide an alternative lubrication method for these applications. This chapter reviews recent results that demonstrate the feasibility of this approach. First, the frictional behavior of alumina and silicon nitride lubricated with nickel chloride intercalated graphite is reviewed. It is shown that the friction coefficients of these ceramics sliding against steel are reduced by graphite lubrication. The reduction in the friction coefficient is shown to be related to the formation of a transfer layer containing a mixture of materials from the surfaces in contact. Considered next are the results on a series of self-lubricating 'ceramic matrix composites that were prepared by drilling small holes in the ceramics and filling the holes with the intercalated graphite and hexagonal boron nitride solid lubricants. A frictjon coefficient as low as 0.17 is observed for the silicon nitride-graphite composites. The friction and wear behaviors of self-lubricating composites prepared by casting slurries of zirconia powder mixed with intercalated graphite are also reviewed. The zirconia-graphite composites in sliding contact with steel from room temperature to 400 °C exhibit lower friction coefficients than the cast zirconia sliding against steel. At room temperature, the

I. INTRODUCTION High wear resistance combined with high hardness, low density, the ability to retain strength and hardness at elevated temperatures, and corrosion resistance make ceramic materials attractive for applications at high temperatures and in harsh environments. Investigations of the friction and wear behavior of these materials, however, have revealed that they exhibit high friction coefficients, typically 0.5-0.8, under unlubricated sliding conditions [1-8]. The high friction coefficients of these materials limit their use as tribological components. To take advantage of the beneficial properties of advanced ceramic materials, their friction coefficient must be reduced to 0.1 or lower.