Influence of sintering atmosphere on the fabrication of SiC ceramics with a powder mixture of sialon composition
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Kyung-Seok Kim School of Mechanical Engineering, Chosun University, Kwang-ju 501-759, Korea (Received 2 October 2000; accepted 28 March 2001)
Silicon carbide ceramics with a powder mixture of sialon composition as a densification aid were prepared by hot pressing through a transient liquid phase, and the effects of sintering atmosphere and starting phases on the microstructural characteristics were investigated. The sintered SiC with the additive of a Y–␣-sialon composition in argon showed high sintered density and grains with large aspect ratio. But sintered specimens in nitrogen yielded grains with low aspect ratio and small grain size, because of the retardation of phase transformation and grain growth. The SiC specimen prepared from the starting –SiC powder with Y–␣-sialon composition showed the highest fracture toughness of approximately 6.0 MPa m1/2.
I. INTRODUCTION
Since silicon carbide has a high covalent bond above 87% and low diffusion coefficients, silicon carbide is usually densified with sintering aids which promote the matter transport at grain boundaries.1,2 Two kinds of sintering aids, that is, oxides and non-oxides, respectively, are usually used for high densification to attain good mechanical and thermal properties of sintered SiC. In the case of nonoxide sintering additives, i.e., B, C, and Al, etc., SiC powder sinters at about 2150 °C by a solid state diffusional process3 and those nonoxide sintering additives contributed to the obtention of highly densified SiC. It has high strength and good corrosion resistance and thermal properties but low fracture toughness of 3–4 MPa m1/2 due to the large grain size. SiC can be also sintered with oxide additives such as Al2O3 and Y2O3, which result in lower sintering temperature, smaller grain size, and higher fracture toughness compared to solid-state sintered SiC through the microstructure control during sintering or heat treatment.4,5 These oxides form a liquid phase at the sintering temperature and aid the mass transportation by solution reprecipitation through the liquid phase.6,7 However, on cooling, the liquid generally transforms into a residual intergranular amorphous phase whose quantity, distribution, and chemical composition control the mechanical properties at high or room temperature.
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J. Mater. Res., Vol. 16, No. 6, Jun 2001 Downloaded: 21 Mar 2015
Therefore, to achieve sintered SiC with excellent properties, it is necessary to choose the optimal sintering additives and methods, which require a high densification rate during sintering and strong grain boundary phase at high temperature. Of particular interest is Y–␣-sialon which is the well-known material for superior mechanical properties, i.e., high strength and creep resistance etc.8–10 In this study, we chose a Y–␣-sialon composition as sintering additive, which could enhance mass transport through oxynitride liquid and form a sialon upon cooling. Also, we investigat
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