High-Temperature Isothermal Oxidation of Ultra-High Temperature Ceramics Using Thermal Gravimetric Analysis

Oxidation of ZrB2 + SiC composites is investigated using isothermal measurements to study the effects of temperature, time, and gas flow on oxidation behavior and microstructural evolution. A test method called dynamic nonequilibrium thermal gravimetric analysis (DNE-TGA), which eliminates oxidation during the heating ramp, has been developed to monitor mass change from the onset of an isothermal hold period (15 min) as a function temperature (1000 degrees C-1600 degrees C) and gas flow (50 and 200 mL/min). In comparing isothermal to nonisothermal TGA measurements, the scale thicknesses from isothermal tests are up to 4 times greater, indicating that oxidation kinetics are faster for isothermal testing, where the oxide scale thickness is 110 m after 15 min at 1600 degrees C in air. Isothermal oxidation followed parabolic kinetics with a mass gain that is temperature dependent from 1000 degrees C-1600 degrees C. The mass gain increased from similar to 5 to 45 g/m(2) and parabolic rate constants increased from 0.037 to 2.2 g(2)/m(4<bold>)s over this temperature range</bold>. The effect of flow velocity on oxidation is not significant under the given laminar flow environment where the gas boundary layer is calculated to be 4 mm. These values are consistent with diffusion of oxygen through the glass-ceramic surface layer as rate limiting.