Tribological behaviour of silicon nitride cermaic combined with steels was investigated in fretting tests in controleed humid air. Ten types of commonly used steels were selectged, including carbon steels, stainless stelles, high speed steels, nitrided steels and bearing steels. Results indicate that the wear of friction pairs is strongly dependent on the humidity and coupling materials. Among the tested speciems, Si_3N_4/nitrided steel pair was found to have the best wear resistnat performance. Humidity affects the wear mechnaism of silicon nitride, which is precominantly through microfracture in dry air and shifts to a trobochemical reaction in humid air, manifested by a transparent layer formed on the rubbing surface of the ceramic. However, there is no evidence that this layer provides any wear protection during fetting.
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