PHASE EFFECTS ON OXIDATION RESISTANCE IN SI3N4-AL2O3-Y2O3

The oxidation resistance of Si3N4-Al2O3-Y2O 3 materials is strongly dependent on the secondary phases and processing conditions, as evidenced from the behaviour of two materials of the same starting composition but sintered in different conditions: I) when only Y-silicates are present as secondary phases (hot pressed material, 1710 C, 30 MPa, 1 hour) no modifications are detected at T < ~1100° C and the oxidation resistance is good up to~1400°C; parabolic oxidation kinetics and structure and composition of the oxide suggest the diffusion of Y and Al cations from the bulk to the reaction interface as the rate-governing step; II) when also Y-based oxynitrides form as a consequence of high temperature processing (gas-pressure sintered material, T = 1950°C, 1.5 MPa, 1.5 hours), the thermal instability at low temperatures (800-1000°C) of these phases results in linear kinetics, and at T > 1100°C parabolic kinetics are observed, but the oxidation rate constants are higher in comparison with (I). © 1990.