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SOL-GEL DERIVED CERAMIC-CERAMIC COMPOSITES USING SHORT FIBERS J. J. LANNUTTI* AND D. E. CLARK**

Ceramics Division, Materials Science and Engineering, University of Florida Gainesville, Florida 32611 USA ABSTRACT Short ceramic fibers or whiskers may be ideally suited for the fabrication of ceramic-ceramic composites using the sol-gel process. The fibers can be uniformly dispersed in a low viscosity sol and then frozen into the matrix through gelation. Several ceramic composites were prepared by synthesizing an A120 3 precursor from aluminum sec-butoxide and then dispersing fibers of either zirconia, graphite or SiC. The composites were dried at room temperature and fired up to 12000C. The fibers reduce the volume shrinkages in comparison to that obtained with pure AI2 0 3 dur)ng processing. Structures with dimensions of several cm can be rapidly produced with this method without cracking. INTRODUCTION The value of composites for structural applications has been recognized for many years. A variety of composites including glass/polymer, carbon/carbon and metal/ceramic are already commercially available. Although these composites offer improved toughness over the matrix material alone, their useful temperature ranges in air are limited to below about 1200°F due to oxidation. Ceramics are more resistant to high temperature degradation and thus provide a unique advantage over other materials. Unfortunately, the ceramic composites have not received the attention that they deserve based on their potential advantages [1]. One of the reasons for this lack of attention may be due to the problems encountered in their fabrication. High temperatures and/or pressures are thought to be required in order to produce acceptable ceramic composites. Short ceramic fibers, or whiskers, are ideally suited for the fabrication of ceramic-ceramic composites using the sol-gel process. The fibers, or whiskers, can be uniformly dispersed in the sols and then "frozen" into the matrix through gelation. Subsequent densification can potentially be achieved at lower temperatures and pressures than required with conventional powder processing. In this study composites were fabricated by casting the sol containing the second phase into plastic trays and then gelling at 90°C. The presence of the second phase reduces both the shrinkage and extent of macro-cracking during drying of the gel compared to pure gel. Although several materials were studied including zirconia and graphite short fibers, the primary focus here is on SiC whiskers. Use of glycerol as a plasticizer permitted the fabrication of tensile specimens and

other intricate shapes prior to firing. These composites were characterized with scanning electron microscopy, thermal analysis, and Fourier transform infrared spectroscopy. Limited physical property measurements such as shrinkages and densities are reported. Mat. Res.

Soc.

Symp.

Proc.

Vol.

32 (1984)

Published by Elsevier Science Publishing Co.,

Inc.

370

EXPERIMENTAL Alumina sol was prepared using the Yoldas [2,3] method desc