THE LOW-TEMPERATURE OXIDATION PROBLEM IN YTTRIA-DENSIFIED SILICON NITRIDE CERAMICS.

Silicon nitride ceramics containing greater than 10 w/o Y//2O//3 show extensive cracking when oxidized at 1000 degree C. This phenomenon was originally attributed to the presence of crystalline yttrium silicon oxynitride phases (especially N-melilite, Y//2O//3 multiplied by (times) Si//3N//4) in the as-fired product which on oxidation resulted in a very large specific volume change. At higher temperatures the silica in the oxidized surface starts to soften and the problem is no longer observed. Because these materials exhibit excellent mechanical properties above 1200 degree C there is considerable interest in understanding this behaviour in more detail. The problem also occurs in silicon nitride ceramics densified with mixed magnesia/yttria additions in which the grain boundary phase is entirely glassy. Results are presented for a range of samples in which the grain boundary phase is either vitreous or crystalline and a more generalized explanation is offered in terms of the total volume change of the specimen. If this is larger than about 1% cracking is likely to occur; this is in good agreement with strength and modulus data for dense silicon nitride ceramics.