WEAR TRANSITION DIAGRAM FOR SILICON-NITRIDE

Utilization of silicon nitride ceramics in applications involving contact between two sliding surfaces requires information on the effect of contact conditions and materials microstructure on tribological performance. In the present study, unlubricated wear tests were conducted on a hot isostatically-pressed silicon nitride under various test conditions in self-mated sliding tests in air. Following the tests, scanning electron microscopy (SEM) was used to elucidate the wear mechanisms and particularly to delineate the effects of load and temperature on wear. 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. The controlling mechanism and tribological data, i.e. friction coefficient and wear coefficient, in each region are unique. At low loads and relatively low temperatures, the tribological behavior is controlled by tribochemical reactions between silicon nitride surface and water vapor in the environment. In the temperature range 400-700-degrees-C at low loads, selective oxidation of WC inclusions controls the wear behavior. Formation of crystalline precipitates from the amorphous magnesium silicate grain boundary phase controls the wear process from 700 to 900-degrees-C at low loads. At higher temperatures, oxidation of silicon nitride dominates the wear process. A transition to severe wear by micro-fracture is observed as the contact load is increased above a particular value. Detailed understanding of the fundamental mechanisms can provide guidelines for microstructural modifications to avoid severe wear under operating conditions.