Processing of silicon carbide ceramics using chemically modified polycarbosilanes
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Processing of silicon carbide ceramics using chemically modified polycarbosilanes Sachiko Okuzaki,a) Yuji Iwamoto, and Shinji Kondoh Fine Ceramics Research Association Synergy Ceramics Laboratory, 2-4-1 Mutsuno Atsuta-ku, Nagoya 456, Japan
Koichi Kikutab) National Industrial Research Institute of Nagoya, 1 Harate-cho, Kita-ku, Nagoya 462, Japan
Shin-ichi Hirano Department of Applied Chemistry, School of Engineering, Nagoya University, 1 Furo-cho, Chikusa-ku, Nagoya 464-01, Japan (Received 5 March 1997; accepted 11 April 1998)
Chemically modified polycarbosilane (PC) which contains Si–Al –C–O component, PCOAl, was synthesized using PC and aluminum triisopropoxide. Ceramic yield was greatly improved through the modification of PC with a metal alkoxide. The phase transformation behavior and microstructure development of silicon carbide (SiC) were studied on b –SiC powders coated with chemically modified PC. The b-a phase transformation of SiC was enhanced by the coating of chemically modified PC on b –SiC powder. A unique microstructure with submicron-sized plate-like grains was developed, since the fine a phase produced at low temperature served as a nucleation site for the b-a phase transformation of SiC.
I. INTRODUCTION
Silicon carbide (SiC) has been used widely for engineering and electrical ceramics because of its excellent properties, especially strength and oxidation resistance at high temperature. Usually, the sintering of pure SiC is difficult due to its covalent nature, so the fabrication of SiC ceramics has been carried out by hot-pressing, or pressureless sintering of SiC powders with sintering additives.1,2 In the case of hot-pressing, fully dense ceramics are obtained relatively easily, because this technique supplies not only thermal energy but also pressure as a driving force for sintering. On the other hand, pressureless sintering has been recognized to have an advantage to produce various shapes of ceramics. As one of the examples of the pressureless sintering, the SiC ceramics can be sintered with boron and carbon additives by a solid state sintering mechanism.3 Another approach of a pressureless sintering at relatively lower temperatures is a liquid phase sintering. Several oxide additive systems, pure aluminum oxide (Al2 O3 ),4,5 or a mixture of rear-earth oxides with Al2 O3 ,6–8 have been investigated. In these systems, elongated SiC grains are formed during sintering and some of these ceramics show high fracture toughness.9,10 a)
Now at Hitachi Research Laboratory, Hitachi Ltd., 7-1-1 Oomikacho, Hitchi-shi, Ibaraki 319-12, Japan. b) Now at Nagoya University, 1 Furo-cho, Chikusa-ku, Nagoya 464-01, Japan. J. Mater. Res., Vol. 14, No. 1, Jan 1999
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However, these sintering additives segregated heterogeneously at grain boundaries, which caused to impair the mechanical properties of the ceramics at high temperatures.11 In order to suppress the segregation of Al2 O3 , it is useful that the minimum
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