Friction experiments were conducted with a reciprocating friction test apparatus to determine the effect of humidity and oxide films on the friction characteristics of ceramics slid against pure metals. The friction coefficients of silicon carbide were not affected very much by varying humidity or metals. On the other hand, the friction coefficients of nitride were affected by both humidity and the type of metal. Some of these metals showed high friction coefficients in high humid air. Preexisting thick oxide layers, which had formed on metal surfaces before sliding, caused an increase in the friction coefficients in an environment of 50 percent relative humidity (RH) air. The friction coefficients of silicon nitride to metal contacts were strongly affected by the oxidation of the metal surfaces, and those in 90 percent RH air correlated well with the Gibbs free energy of formation of the lowest metal oxide.

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