WEAR TRANSITION DIAGRAM FOR SILICON-NITRIDE

Utilization of silicon nitride ceramics in applications involving cont act between two sliding surfaces requires information on the effect of contact conditions and materials microstructure on tribological perfo rmance. In the present study, unlubricated wear tests were conducted o n a hot isostatically-pressed silicon nitride under various test condi tions in self-mated sliding tests in air. Following the tests, scannin g electron microscopy (SEM) was used to elucidate the wear mechanisms and particularly to delineate the effects of load and temperature on w ear. The results of the tests and observations were used to construct a wear transition diagram, with load and temperature as the two axes. This diagram is divided into five regions plus one transition zone. Th e controlling mechanism and tribological data, i.e. friction coefficie nt and wear coefficient, in each region are unique. At low loads and r elatively low temperatures, the tribological behavior is controlled by tribochemical reactions between silicon nitride surface and water vap or in the environment. In the temperature range 400-700-degrees-C at l ow loads, selective oxidation of WC inclusions controls the wear behav ior. Formation of crystalline precipitates from the amorphous magnesiu m silicate grain boundary phase controls the wear process from 700 to 900-degrees-C at low loads. At higher temperatures, oxidation of silic on nitride dominates the wear process. A transition to severe wear by micro-fracture is observed as the contact load is increased above a pa rticular value. Detailed understanding of the fundamental mechanisms c an provide guidelines for microstructural modifications to avoid sever e wear under operating conditions.