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

Oxidation resistance of a liquid-phase-sintered (LPS) silicon carbide (SiC) with Lu2O3–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, Qox≈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 SiO2) and the second phase (Lu2O3) produced a low-melting eutectic, resulting in accelerated oxidation. The major oxidation products consisted of SiO2 (α-cristobalite) and lutetium-disilicate (Lu2Si2O7). 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 Lu2O3.