Toward Oxidation-Resistant ZrB 2 -SiC Ultra High Temperature Ceramics
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INTRODUCTION
ULTRA high temperature ceramics (UHTCs) are designed to operate in extreme environments such as those experienced on leading edges of hypersonic vehicles or in propulsion components of missiles. In recent years, there has been a resurgence of interest in UHTCs, particularly methods of improving their hightemperature capabilities. For UHTCs to maintain their structural integrity during service, exceptional oxidation resistance is paramount. Diborides of the transition metals such as hafnium and zirconium have proven to be some of the best candidate UHTC materials to date. For this review, only zirconium diboride-based ceramics will be considered unless research into hafnium diboride provides useful insight. Several reviews have summarized the history of UHTC research, material properties, and testing methods,[1,2] and these reviews consider the candidate materials and selection procedures necessary to produce a UHTC that fulfils the stringent requirements for these extreme environment applications.[3–5] The purpose of this review is to summarize the advances in research conducted in recent years that identify the possible methods of improving the oxidation resistance of existing UHTC materials.
EMILY EAKINS, PhD Student, DONI DANIEL JAYASEELAN, Post Doctoral Research Associate and WILLIAM EDWARD LEE, Professor, are with the Centre for Advanced Structural Ceramics (CASC) and Department of Materials, Imperial College, London SW7 2AZ, U.K. Contact e-mail: w.e.lee@imperial. ac.uk Manuscript submitted June 7, 2010. Article published online November 23, 2010 878—VOLUME 42A, APRIL 2011
II. OXIDATION RESISTANCE AND METHODS FOR IMPROVEMENT A. Oxidation The oxidation performance of zirconium diboride has been investigated since the 1960s,[6,7] and the temperature range in which the ceramics have been tested has increased with the testing capabilities of the time. For monolithic ZrB2, the oxide formed on oxidation is B2O3, which is liquid above 723 K (450 C) and wets the oxide grains until it volatilizes at temperatures above 1373 K (1100 C). Below 1373 K (1100 C), the oxidation kinetics are controlled by the diffusion of oxygen through the liquid boria that surrounds the ZrO2 grains. Between 1373 K and 1473 K (1100 C and 1400 C), the oxidation kinetics display paralinear characteristics because of the mass gain from ZrO2 and B2O3 formation, and mass loss from B2O3 vaporization. It has long been known that the addition of SiC improves the properties of ZrB2 UHTCs significantly,[4,8–12] and it is concluded generally that the optimum amount of SiC is between 15 and 20 vol pct. The addition of SiC increases the sinterability of the starting powders by facilitating liquid phase sintering via formation of a borosilicate liquid. This in turn allows greater control of the diboride grain growth which has a beneficial effect on mechanical properties.[13,14] Other benefits reported include improved thermal shock resistance and oxidation resistance.[15] The impact of the presence of a connected low thermal conductivity and b
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