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%) do ped hot pressed silicon nitride (HPSN) ceramic were investigated. It w as 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. He at treatment at a lower temperature of 1300 degrees C and for a shorte r time of 4 h did not change the creep behaviour. Improvement of the c reep properties was related to the crystallization of an amorphous gra in boundary phase by heat treatment. Secondary creep rate parameters o f the as-received material: stress exponent, n (2.95-3.08) and activat ion energy, Q (634-818 kJ mol(-1)), were in the range of values found by other investigators for various hot pressed silicon nitride ceramic s.