Oxidation of yttria- and alumina-containing dense silicon nitride ceramics.

The oxidation behaviour of Y2O3 plus Al2O3-doped Si3N4 based ceramics was investigated in air up to 1500 degree under isothermal and nonisothermal conditions. The oxidation resistance of materials is strongly dependent on the content of additives, secondary phases and processing methods, as evident from the behaviour of five materials of the same starting composition prepared by hot pressing and gas pressure sintering. Gas pressure sintered ceramics containing crystalline yttrium-silicon oxynitrides in their intergranulare phase showed instability at intermediate temperatures and lower oxidation resistance than hot pressed ceramics prepared from the same powder mixtures. Hot pressed ceramics exhibited excellent resistance to oxidation by air at temperatures up to 1400./ 1450 degree, because of the formation of a protective oxide layer. A decrease of oxidation rates with the increase of additive content was found at temperatures below 1300 degree, while the reverse was the case for all materials above 1400 degree.