1. High-Temperature Isothermal Oxidation of Ultra-High Temperature Ceramics Using Thermal Gravimetric Analysis.
- Author
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Miller‐Oana, Melia, Corral, Erica L., and Fahrenholtz, B.
- Subjects
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HIGH temperatures , *ISOTHERMAL processes , *ZIRCONIUM boride , *THERMOGRAVIMETRY , *OXIDATION , *EQUILIBRIUM , *LAMINAR flow , *DIFFUSION - Abstract
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°C-1600°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 μm after 15 min at 1600°C in air. Isothermal oxidation followed parabolic kinetics with a mass gain that is temperature dependent from 1000°C-1600°C. The mass gain increased from ~5 to 45 g/m2 and parabolic rate constants increased from 0.037 to 2.2 g2/m4·s over this temperature range. 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. [ABSTRACT FROM AUTHOR]
- Published
- 2016
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