EFFECTS OF HEAT-TREATMENTS ON THE CREEP-PROPERTIES OF A HOT-PRESSED SILICON-NITRIDE CERAMIC

Effects of heat treatment in an argon atmosphere at high temperatures for varying times on the creep properties of a Y2O3-Al2O3 (8-2 wt%) doped hot pressed silicon nitride (HPSN) ceramic were investigated. It was observed from the creep measurements that higher temperature, i.e. greater than or equal to 1360 degrees C, and longer time, i.e. greater than or equal to 8 h, heat treatment in an argon atmosphere improved the creep properties, (e.g. secondary creep rate) of this material. Heat treatment at a lower temperature of 1300 degrees C and for a shorter time of 4 h did not change the creep behaviour. Improvement of the creep properties was related to the crystallization of an amorphous grain boundary phase by heat treatment. Secondary creep rate parameters of the as-received material: stress exponent, n (2.95-3.08) and activation energy, Q (634-818 kJ mol(-1)), were in the range of values found by other investigators for various hot pressed silicon nitride ceramics.