Thermochemical and Mechanical Stabilities of the Oxide Scale of ZrB2+SiC and Oxygen Transport Mechanisms

Authors

  • Ju Li,

    1. Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104
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  • Thomas J. Lenosky,

    Corresponding author
    1. Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104
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  • Clemens J. Först,

    1. Department of Nuclear Science and Engineering and Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
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  • Sidney Yip

    1. Department of Nuclear Science and Engineering and Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
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  • Y. Blum—contributing editor

  • This work was performed mostly at the Ohio State University and supported by the Ceramics and Non-Metallic Materials Program in the Air Force Office of Scientific Research (FA9550-05-1-0026).

  • Presented at the AFOSR Workshop on Ultra-High-Temperature Ceramic Materials hosted by SRI International, July 23–25, 2007.

†Author to whom correspondence should be addressed. e-mail liju99@alum.mit.edu

Abstract

Refractory diboride with silicon carbide additive has a unique oxide scale microstructure 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 using first-principles calculations. It is estimated from entropy and mobility arguments that above a critical temperature TC∼1500°C, 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 sublinear dependence of the oxidation rate with external oxygen partial pressure. The present work suggests that there could be significant room in improving the high-temperature oxidation resistance by refining the oxide scale microstructure as well as controlling the glass chemistry.

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