Sinterability of SiC powder coated uniformly with Al ions
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Sinterability of SiC powder coated uniformly with Al ions Soichiro Sameshima, Keisuke Miyano, and Yoshihiro Hirataa) Department of Applied Chemistry and Chemical Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890, Japan (Received 19 August 1997; accepted 30 December 1997)
SiC particles coated uniformly with Al ions (0.25 mass % Al2 O3 ) in an aluminum nitrate solution were consolidated by filtration through a gypsum mold. Hot-pressing in vacuum gave dense SiC (above 99% relative density) in the temperature range of 1900–1950 ±C under a pressure of 39 MPa. The microstructures of dense SiC consisted of 2–5 mm grains of low aspect ratios (below 2). The fracture toughness and flexural strength of SiC increased gradually as the hot-pressing temperature became higher and were 4.3 MPa ? m0.5 and 350 MPa, respectively, with hot-pressing at 1950 ±C. Crack propagation in SiC shifted from intergrain to intragrain with increasing hot-pressing temperature.
I. INTRODUCTION
Silicon carbide (SiC) is potentially useful as a high temperature structural material because of its high strength, high hardness, high creep resistance, and high oxidation resistance. Recently, the addition of sintering additives such as Al2 O3 plus Y2 O3 to SiC powder by chemical methods has been reported to control the liquid phase sintering and resultant microstructures of SiC.1,2 The precipitation of Al and Y hydroxides often results in inhomogeneous microstructure in sintered SiC owing to the nonuniform coating of the SiC surface with the hydroxides. Furthermore, precipitation-coating is accompanied by an increase in specific surface area and a change of surface charge after the calcination of SiC powder. Handling of the powder becomes generally difficult with increasing specific surface area. In this study, electrostatic adsorption3–5 of a small amount of Al ions [added as Al(NO3 )3 ] was applied to coat uniformly the surface of SiC particles in an Al(NO3 )3 suspension. Consolidation was effected by filtration through a gypsum mold so as to form a dense homogeneous microstructure. By heating above 200 ±C, Al(NO3 )3 in the consolidated SiC compact decomposes to Al2 O3 . This paper describes the sintering behavior of SiC powder coated uniformly with Al ions. In our previous work,6–8 the following conclusions were derived for the dissolution of the SiO2 films on a SiC surface and adsorption of Al ions onto a SiC surface. In a suspension of a –SiC coated with amorphous SiO2 films 0.67 6 0.34 nm at pH 3–7, the SiO2 films and H2 O interacted to form soluble H4 SiO4 . The amount of SiO2 dissolved in the SiC suspensions increased at 22 pH 10 due to the formation of H3 SiO2 4 and H2 SiO4 a)
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J. Mater. Res., Vol. 13, No. 4, April 1998
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ions. Addition of Al(NO3 )3 to the neutral and basic suspensions suppressed the dissolution of the SiO2 films.
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