OXIDATION RESISTANCE OF ALUMINA TITANIUM NITRIDE AND ALUMINA TITANIUM CARBIDE COMPOSITES

The presence of TiC or TiN paritcles in an Al2O3 matrix affects the thermal stability of the composites in oxidizing environments. In isothermic oxidation tests at 700-degrees, 800-degrees, 900-degrees, 1000-degrees, and 1100-degrees-C for up to 20 h, two different oxidation regimes have been observed at T < 900-degrees-C and at 900-degrees-C less-than-or-equal-to T less-than-or-equal-to 1100-degrees-C. At low temperatures (T < 900-degrees-C), the oxidation follows a phase-boundary reaction; the reaction product initially consists of aggregates of submicrometer needlelike TiO2 rutile crystals that subsequently grow and coalesce. When a continuous TiO2 rutile layer is formed (T greater-than-or-equal-to 900-degrees-C), the oxidation kinetics change to parabolic, and the diffusion of O2 through a thick TiO2 layer is proposed as the governing step.