Molecular Modeling of High-Temperature Oxidation of Refractory Borides
Final technical rept.
OHIO STATE UNIV RESEARCH FOUNDATION COLUMBUS
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Refractory diboride with silicon carbide additive has a unique oxide scale structure with two condensed oxide phases solid liquid, and demonstrates oxidation resistance superior to either monolithic diboride or silicon carbide. We rationalize that this is because the silica-rich liquid phase can retreat outward to remove the high SiO gas volatility region, while still holding onto the zirconia skeleton mechanically by capillary forces, to form a solid pillars, liquid roof scale architecture and maintain barrier function. Basic assessment of the oxygen carriers in the borosilicate liquid in oxygen-rich condition is performed based on first-principles calculations. It is estimated from entropy and mobility arguments that above a critical temperature Tc 1500C the dominant oxygen carriers should be network defects, such as peroxyl linkage or oxygen deficient centers, instead of molecular O2 as in the Deal-Grove model. These network defects will lead to sub-linear dependence of the oxidation rate with external oxygen partial pressure.
- Inorganic Chemistry