Abnormal thermal shock behavior of Ti 3 SiC 2 and Ti 3 AlC 2
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Many layered ternary ceramics displayed unusual thermal shock behavior, i.e., the retained strengths of as-quenched samples increased with increasing quench temperature above a critical quench temperature. However, the causation was not clear even though this phenomenon has been observed for 10 years. In this study, the thermal shock behavior of Ti3SiC2 and Ti3AlC2, two representative members of layered ternary ceramics, was investigated. The results indicated that the formation of surface oxides was responsible for this abnormal phenomenon. These results might contribute to the understanding of this unusual behavior of other layered ternary ceramics.
I. INTRODUCTION
Ceramics are ideally suited for high-temperature applications because of their high melting point and excellent high-temperature mechanical properties. However, their characteristic brittle failure nature leaves them susceptible to thermal shock damage and failure. After thermal shock, most ceramics show catastrophic drops in mechanical properties such as flexural strength and elastic modulus.1–5 This poor resistance to thermal shock has limited wide application of ceramics at high temperatures. Recently, a family of layered ternary ceramics exhibited striking thermal shock resistance: they withstood a thermal shock greater than 1000 °C without a catastrophic drop in mechanical properties.6–13 Moreover, these layered ternary ceramics combine the properties of metals and ceramics. Like ceramics, they have low density, high strength and modulus, and excellent hightemperature oxidation resistance. Like metals, they have high thermal and electrical conductivity, excellent thermal shock resistance, and are damage tolerant and machinable by conventional high-speed steel tools and electrodischarge machining methods. Therefore, these materials are promising candidates in high-temperature applications. The response of layered ternary ceramics to thermal shock is unique: residual strengths of as-quenched samples gradually change with quench temperatures. Furthermore, a noteworthy phenomenon of quench strengthening is observed: the retained strengths of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0289 J. Mater. Res., Vol. 21, No. 9, Sep 2006
http://journals.cambridge.org
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water-quenched samples increase with increasing quench temperatures above a critical quench temperature. Such quench-strengthening behavior has never been observed in any other ceramics to date. In fact, as temperature rapidly decreases, transient tensile stresses are generated in the surface layer, which easily results in the damage of ceramics. The transient tensile stress can be described as =
␣E⌬T , 1−
(1)
where ␣ is the expansion coefficient, E is the elastic modulus, ⌬T is the temperature difference, and is the Poisson’s ratio. Obviously, the increase of the quench temperature enhances the temperature difference ⌬T, and then increases the transient thermal stresses. Therefore, the damage degree of thermal sh
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