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STUDIES ON HOT--PRESSED SILICON CARBIDE

Jiang Dong-liang;Pan Zhen-su;Wang Da-qian;Huang Yu-zhen;Wang Ju-hong;Lin Qing-ling Shanghai Institute of Ceramics, Academia Sinica  
Silicon carbide is a highly feasible material for high temperature engineering applica-tions. Recently extensive work has been directed to high strength and high oxidation resi-stant SiC bodies for gas turbine and other engineering applications. This paper describes the effects of additatives (B_4C and C) and other processing para-meters on the sintering behaviour and mechanical properties of hot-pressed SiC, includingthe weight gain and the change of strength after oxidation under various temperatures andwet-oxygen conditions. The experimental results can be summarized as follows: (1) Boron carbide and carbon are jointly necessary as additives for obtaining hot pres-sed SiC of high density. The lower limit of B_4C or C addition required to give maximumdensity is about 0.5% respectively. (2) The strength of hot-pressed SiC is about 500MN/m~2 which remains almost unchang-ed from room temperature up to 1400℃ and is nearly irrelevant to carbon content up to3%. As fracture at high temperature has been assumed to be governed by grain boundaryphases, the preservation of strength at high temperature rules out the possibility of forma-tion of low melting compounds at grain boundary, more so for the fact that the strengthof SiC has been observed even to increase slightly at higher temperatures. (3) The sample of SiC containing 1% B_4C and 3% C hot pressed at 2050℃ for 45 minutesunder pressure of 40 MN/m~2 possesses the following properties: density 3.17g/cm~3, bendingstrength at room temperature 480 MN/m~2, coefficient of thermal expansion 4.6 10~(-6)℃~(-1),hard-ness HRA93.5. (4) The oxidation resistance of hot-pressed SiC is quite good under 1280℃. The rateof oxidation varies with time according to a parabolic function. After oxidation thestrength of SiC under room temperature remains almost unchanged.
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