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MATERIALS RESEARCH

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Fabrication of silicon carbide nanoceramics Mamoru Mitomo National Institute for Research in Inorganic Materials, 1-1, Namiki, Tsukuba-shi, Ibaraki, 305, Japan

Young-Wook Kim Korea Institute of Science and Technology, Cheongryang, Seoul, Korea

Hideki Hirotsuru Research Center, Denki Kagaku Kogyo Co., 3-5-1 Asahimachi, Machida, Tokyo, 194, Japan (Received 8 May 1995; accepted 28 February 1996)

Ultrafine silicon carbide powder with an average particle size of 90 nm was densified by hot-processing with the addition of Al2 O3 , Y2 O3 , and CaO at 1750 ±C. Silicon carbide nanoceramics with an average grain size of 110 nm were prepared by liquid phase sintering at low temperature. The materials showed superplastic deformation at a strain rate of 5.0 3 1024ys at 1700 ±C, which is the lowest temperature published. The microstructure and deformation behavior of materials from a submicrometer powder were also investigated as a reference.

Silicon carbide (SiC) ceramics are very hard and have been used as wear components. Silicon carbide is difficult to densify without additives because of the covalent nature of Si–C bonding and low self-diffusion coefficient.1 The ceramics have been fabricated at about 2000 ±C with additions of boron, aluminum, carbon, or their compounds as sintering aids.2 Liquid phase sintering with the addition of Y2 O3 and/or Al2 O3 has been also investigated at 1850–2000 ±C.3–6 The sintering aids accelerate the diffusion of materials at grain boundaries. The powder compacts shrink 15 –20% during sintering, which makes the fabrication of precision components difficult. Superplastic deformation is expected to solve the problem through near-net-shape fabrication. There is a great deal of data on creep deformation of silicon carbide ceramics at 1400–1600 ±C,7–10 but only one attempt has been tried for superplastic deformation of silicon carbide under compressive stresses at 1800–2060 ±C.11 The deformation temperature was about the same as the sintering temperature. Although the average grain size in the materials was about 2 mm12 and the smallest among commonly sintered silicon carbide ceramics,1,8,9 high deformation temperature suggested that the grain size was still too large. It is suspected that fine-grained ceramics with glassy phase at grain boundaries might have superplasticity.13–16 The hot-pressing of fine-grained silicon nitride was very effective to densify at low temperature. The grain growth during sintering was depressed so that the preparation of nanoceramics was possible.16 The fabrication of fine-grained silicon carbide, i.e., nanoceramics, has been investigated in the present work by liquid phase sintering of fine powder under hot-pressing. Ultrafine b-powder was prepared by gas phase reaction (Sumitomo-Ohsaka Cement Co., Tokyo, Japan, T-1 grade). The powder contains a large amount of J. Mater. Res., Vol. 11, No. 7, Jul 1996

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free carbon, i.e., 4.15 wt. %, which was detrimental to sinter