Effect of rare-earth cation additions on the high temperature oxidation behaviour of LPS-SiC

Abstract

Abstract Oxidation resistance of a liquid-phase-sintered (LPS) silicon carbide (SiC) with Lu 2 O 3 –AlN additive system was investigated in air between 1200 and 1500 °C, for up to 100 h. Oxidation followed parabolic kinetics in the temperature range 1200–1500 °C with an activation energy, Q ox ≈400±50 kJ/mol. The material showed high oxidation resistance at and below 1400 °C, however, the oxidation resistance degraded significantly at 1500 °C. At 1500 °C, reaction between the growing oxide layer (mainly SiO 2 ) and the second phase (Lu 2 O 3 ) produced a low-melting eutectic, resulting in accelerated oxidation. The major oxidation products consisted of SiO 2 (α-cristobalite) and lutetium-disilicate (Lu 2 Si 2 O 7 ). These oxidation products crystallised from the surface amorphous silicate phase during oxidation. The microstructure of the oxidised surfaces was shown to be dependent on oxidation temperature. The most probably rate-limiting steps were (1) the migration of additive cations along the residual intergranular phase to the interface between the oxide layer and the SiC bulk; and (2) the interfacial reactions between growing oxide layer and Lu 2 O 3 .