Reaction synthesis of mullite-zirconia from mixtures of alumina, silica, and 3 mol% yttria-zirconia powders
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Reaction synthesis of mullite-zirconia from mixtures of alumina, silica, and 3 mol% yttria-zirconia powders Yung-Jen Lin Department of Materials Engineering, Tatung Institute of Technology, Taipei, Taiwan, Republic of China (Received 18 May 1998; accepted 10 July 1998)
Mullite-zirconia composites were synthesized by reaction-sintering powder mixtures of a-alumina, amorphous silica, and 3 mol% yttria-stabilized zirconia. It is found that the addition of 3 mol% yttria-zirconia improves sintering and lowers the mullitization temperature. It also suppresses the cristobalite formation and enhances zircon formation between 1300 and 1400 ±C. Results further suggest that these effects are consequences of modification of the silicate glass by yttria. Samples containing >20 vol% of 3 mol% yttria-zirconia could be sintered at 1500 ±C for 2 h to obtain dense composites consisting of mullite and tetragonal zirconia. The microstructural investigations of the sintered samples support the viewpoint that mullite formation is via nucleation and growth from aluminosilicate glass. They also reveal that zirconia can hinder the grain growth of mullite and that the coalescence of zirconia grains is accomplished by grain boundary diffusion.
I. INTRODUCTION
Among various structural ceramics, mullite has attracted considerable attention. It has good high temperature strength, good chemical stability, and low thermal expansion coefficient.1–5 However, the low fracture toughness of mullite and the difficulties in sintering to achieve dense monoliths have posed great obstacles in its application in industry.5,6 Many experiments on mullite-based composites have been conducted to exploit the advantages and to alleviate these disadvantages.7–11 SiC particulates, SiC whiskers, and zirconia-reinforcing phases can increase the strength and/or fracture toughness of the composites because they can increase the modulus, decrease the mullite grain size, and/or offer unique toughening mechanisms (such as crack deflection, transformation toughening, and microcracking). In the processing of mullite-based composites, several novel routes such as sol-gel technique and viscous sintering of composite powders have been developed to lower the fabrication temperature.11–17 These novel processing routes have been proved to be effective, but they require careful preparation of starting constituents (e.g., sol-gel precursors and silica-coated alumina composite powders). To fabricate mullite-zirconia, one of the commonly used mullite-based composites, reaction synthesis from zircon and alumina is an alternative method. This alternative method is easy to process and the needed powders are low in cost. Yet, the ratio of mullite and zirconia in the composites is restricted because zircon dissociates into equal moles of silica and zirconia.18–20 Optimal mullite/zirconia ratio of the composites may not be obtainable. 916
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J. Mater. Res., Vol. 14, No. 3, Mar 1999
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