EVALUATION OF STRENGTH, TOUGHNESS AND SUBCRITICAL CRACK EXTENSION OF A COMMERCIAL HP-SI3N4 EXPOSED TO A SULFIDIZING ENVIRONMENT

Structural ceramic materials, in particular silicon nitride, have attr acted interest for coal gasification technology at temperatures in exc ess of 1200 degrees C. In this study, a commercially available hot pre ssed silicon nitride is exposed for 200 h in a 0.4% H2S + 0.75% H2O H-2 (balance) atmosphere, at 1200 and 1300 degrees C, simulating coal gasification environments. The crack growth behaviour of the as-receiv ed material is compared to that of the exposed condition. Measurements are carried out on specimens with small natural flaws (dynamic streng th tests) and on specimens containing long cracks (chevron notch), bot h tested in four point bending. Results Of Strength tests (Modulus of rupture (MOR) and Weibull modulus) as well as toughness in both condit ions are also presented. In view of the significant changes in the hig h temperature mechanical properties caused by crystallization of the i ntergranular glassy phase at triple points, further investigations of the effect of controlled devitrification seem surely worthwhile.