Colloidal processing and mechanical properties of silicon carbide with alumina

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Colloidal processing and mechanical properties of silicon carbide with alumina Yoshihiro Hirata, Kouji Hidaka, Hiroaki Matsumura, Yasuo Fukushige, and Soichiro Sameshima Department of Applied Chemistry and Chemical Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890, Japan (Received 13 March 1996; accepted 24 March 1997)

Submicrometer-sized SiC coated with SiO2 of 0.4–1.8 wt. % and a –Al2 O3 powder of median size 0.2 mm were mixed in aqueous solutions in the pH range 3.0–10.0. The SiCyAl2 O3 (4.3–6.9 wt. %) powders were consolidated by filtration through gypsum molds and hot-pressed at 1600±– 2040 ±C under a pressure of 39 MPa. These compacts were densified to near the theoretical density at 1700±–1800 ±C. The sintering mechanisms are discussed based on the analysis of shrinkage curves of SiCyAl2O3 compacts during hot-pressing. The equiaxed SiC grains grew with low aspect ratios below 1800 ±C and changed to plate-like grains at 1900 ±C. The fracture toughness of SiC as a function of average grain size reached a maximum of 5 MPa ? m0.5 at 2.5 mm grains of low aspect ratios of 1–2. The flexural strengths at room temperature were 230–430 MPa in the SiC above 98% of the theoretical density and showed a similar grain size dependence.

I. INTRODUCTION

SiC sintered with Al2 O3 has a high flexural strength of 600 MPa at 25 ±C and a high fracture toughness reaching 7 MPa ? m0.5 .1–3 A high temperature strength of 450–500 MPa is also reported in this material at 1400 ±C. The fracture toughness of the SiCyAl2 O3 system is closely related to the aspect ratio of plate-like SiC grains.3–6 The purpose of this study is to apply colloidal processing to lower the sintering temperature of SiC with Al2 O3 .7–9 Colloidal processing is generally able to give a uniform microstructure and a high density green compact and resultant sintered body by adjustment of the interaction energy (summation of repulsion and attraction energy) between particles in a suspension.10–12 The interaction between particles is controlled electrostatically by charging the particles or electrosterically by adsorbing charged polyelectrolytes on the surfaces of particles.13 Due to the above reason, a low sintering temperature is expected in colloidal processing. In particular this process may be effective in lowering the high sintering temperature, generally required for the achievement of dense SiC above 2000 ±C. The second purpose is to study the sintering behavior and microstructural development of SiCyAl2O3 . It is reported that Al2 O3 additives interact with SiO2 on the surface of SiC to form an Al2 O3 -rich liquid at a high temperature that promotes the densification of SiC.3,14,15 In this experiment, the role of Al2 O3 was investigated by 3146

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J. Mater. Res., Vol. 12, No. 11, Nov 1997

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analyzing the shrinkage curves of SiCyAl2 O3 compacts as a function of sintering time. Finally, the mechanical properties of SiC were evaluated in relation t