Correlation between microstructure and mechanical properties in silicon carbide with alumina addition
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K. Suzuki and N. Shinohara Research Center, Asahi Glass Company, Yokohama, Japan (Received 10 June 1992; accepted 26 February 1993)
The microstructure of pressureless sintered silicon carbide (SiC) materials with alumina (AI2O3) addition was investigated using analytical electron microscopy and nuclear magnetic resonance. A sintered body with a density of higher than 99% theoretical was obtained with an addition of 5 wt. % A12O3. The sintered body (SiC-Al 2 O 3 ) has high strength, high fracture toughness, and high fatigue resistance. Its fracture toughness is approximately 5 MPa-m1/2 , which is twice as high as that of pressureless sintered SiC materials with boron and carbon additions (SiC-B-C) . The correlation between the microstructure and the mechanical properties is presented here. The starting yS-SiC powder is mostly transformed to a - S i C with various poly type distributions during the sintering process. The microstructure has homogeneously distributed, fine, plate-like interlocking grains with a high aspect ratio. Well-developed basal planes form parallel and elongated boundaries, and the crystal structure is mostly the 6H polytype (56%) mixed with thin lamellar 4H.
I. INTRODUCTION Silicon carbide (SiC) materials have been used for structural applications at high temperatures extensively in recent years. For automotive applications, the brittleness and catastrophic fracture behavior of SiC materials have limited their applications for automobile exhaust valve systems and turbocharger rotors. Further improvements are needed in the mechanical properties and fabrication processes of the materials in comparison with silicon nitride. For example, a higher fracture toughness is required. The mechanical properties of ceramic materials are greatly influenced by processing procedures and the composition of sintering aids.1 Sintering aids are required to make sintered or hot-pressed bodies with high density. Their effects on sintered bodies have been investigated. For example, Suzuki and Sasaki, Asahi Glass Company, have used alumina as a sintering aid (5 wt. % A12O3) to produce fully dense materials (higher than 99% theoretical density) with a high toughness, using a pressureless sintering process. 2 The fracture toughness is approximately 5 MPa-m 1/2 , which is almost twice as high as other SiC materials reported to date (such as SiC materials with B and C additions) and nearly the same as silicon nitride.3 The high toughness of the material has been correlated with a high aspect ratio of rapidly grown SiC grains in the sintered body, which are uniformly distributed with fine equiaxed SiC grains. However, when the SiC materials with alumina (hereafter, referred to as SiC-Al 2 O 3 ) are compared with silicon carbide maJ. Mater. Res., Vol. 8, No. 7, Jul 1993
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terials with B and C additions (hereafter, referred to as S i C - B - C ) , it is noted that the high toughness may not be explained by the aspect ratio and bimodal grain distribution alone. Sintering
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