Inhomogeneous oxidation of ZrB2-SiC ultra-high-temperature ceramic particulate composites and its mitigation

The ubiquitous inhomogeneous-oxidation behavior of a prototypical ZrB2-20 vol% SiC ultra-high-temperature ceramic (UHTC) particulate composite is investigated in detail. An innovative approach for enhancing the oxidation resistance of the composite and delaying the onset of its inhomogeneous oxidation is demonstrated. In this approach, a borosilicate glass coating that is highly compatible with the composite is applied to the composite surface a priori. The effect of the borosilicate-glass coating thickness on the homogeneous-oxidation kinetics of the composite is investigated quantitatively and modeled. A new phenomenon is discovered where ZrO2 precipitates of graded size are found to nucleate and grow within the borosilicate-glass layer, both in natively grown and in applied coating, during the oxidation of the composites. This is attributed to the ZrO2-solubility gradient across the borosilicate-glass layer as a result of preferential B2O3 volatilization at the surface.

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