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

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 oxygendeficient 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. Comments Copyright The American Ceramic Society, 2008. Reprinted from Journal of the American Ceramic Society, Volume 91, Issue 5, May 2008, pages 1475-1480. Publisher URL: http://dx.doi.org/10.1111/j.1551-2916.2008.02319.x This journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/151 Thermochemical and Mechanical Stabilities of the Oxide Scale of ZrB21SiC and Oxygen Transport Mechanisms Ju Li and Thomas J. Lenosky Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Clemens J. Först and Sidney Yip Department of Nuclear Science and Engineering and Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Refractory diboride with silicon carbide additive has a unique oxide scale microstructure with two condensed oxide phases (solid1liquid), 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 TCB15001C, 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.