Sic-Reinforced Cordierite Composites Formed by Viscous Sintering of Cordierite Powders
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SIC-REINFORCED CORDIERITE COMPOSITES FORMED BY VISCOUS SINTERING OF CORDIERITE POWDERS
ANNE B. HARDY AND WENDELL E. RHINE Ceramics Processing Research Laboratory, Massachusetts Institute of Technology, Cambridge MA 02139
ABSTRACT One approach for improving mechanical properties of ceramics is to add reinforcing whiskers; however, addition of whiskers often inhibits densification. Formation of SiCcordierite composites by pressureless sintering was studied. The cordierite composites densified by viscous sintering; after densification the amorphous matrix then crystallized. It was shown that high green density composites obtained by using small particle size powder gave the highest sintered density composites. Reducing the aspect ratio of the SiC phase also improved sintered composite densities. Densities greater than 96% were obtained using small particle size cordierite powder formed by a sol-gel process and either classified SiC whiskers or SiC powder at SiC loadings of 20 vol% INTRODUCTION One approach to improving the mechanical properties of ceramic materials is to add whiskers or fibers to the ceramic matrix. However it is difficult to densify the ceramic when whiskers or fibers are added because they act as nonshrinking inclusions. At a certain volume fraction the whiskers form three dimensional networks that only shrink or deform in response to an external force. One approach to improve composite densities is to hot press the composite but this limits the product to a simple design or introduces expensive machining costs. Another approach is to improve green body characteristics so that pressureless sintering is feasible. In pressureless sintering whiskers are processed with the powders using traditional ceramic processing techniques. Research has shown the difficulty of obtaining dense composites particularly at high whisker loadings; however optimization of starting materials and processing conditions has been shown to increase composite densities [1-6, 1111. In the present study, amorphous cordierite powder was combined with SiC powders or whiskers and then densified by viscous sintering. The amorphous matrix then crystallized to form a ceramic-ceramic composite. Cordierite was chosen as the matrix material because it densifies by viscous sintering and has several properties that make it commercially interesting, including a low dielectric constant and a low thermal expansion coefficient. Many of the current proposed uses for cordierite are limited by its poor mechanical properties. By using a matrix which densifies by viscous sintering, it may be possible to minimize some of the difficulties of densifying a ceramic matrix around fully dense inclusions. This paper discusses the effect of the cordierite powder and SiC inclusion properties on the densification of SiC-cordierite composites.
EXPERIMENTAL PROCEDURE Cordierite powder was synthesized using a modified sol-gel method adapted from the literature [7,8]. Tetraethylorthosilicate (TEOS, Aldrich Chemical Co., Milwaukee, WI) was added to a HCI in ethan
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