Unique Microstructural Development in SiC Materials with High Fracture Toughness
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structure with cubic symmetry, which is identified as 3C or the /3-phase. At high temperature, the jS-phase transforms to aphases with hexagonal or rhombohedral symmetry, with 4H, 15R, and 6H (Ramsdell notation7) being the major polytypes observed in SiC materials. Preference of the polytype selection during the /3- to aphase transformation is dependent on the chemistry of the sintering aids and metallic impurities in the grain boundaries.8 High toughness of the SiC materials was achieved when the /3-SiC fine powder was sintered with alumina as a sintering aid.9 The high toughness of the material has excellent correlation with the high aspect ratio of the rapidly grown a-SiC grains in the sintered body, which are uniformly distributed among fine, equiaxed SiC grains. Similar microstructures were obtained in the sintered bodies using the aluminum-doped /3-SiC powder10 (Ibiden Company, Japan). The powder was sintered with boron and carbon as sintering aids. Since the starting powders and sintering processes are considerably different, a comparison study of these cases is helpful in understanding the relationship of the starting powders to the final microstructures, in order to improve the toughness of the SiC materials. Sintering mechanisms of SiC materials with an alumina addition for hot pressing have been reported separately by Alliegro et al.11 and by Lange,12 wherein it was
stated that the alumina forms a type of aluminum silicate that promotes liquidphase sintering in the materials by reacting with the SiC. Lange proposed a mechanism in which the SiC particles dissolve in a liquid phase at high temperature, thus promoting densification by a solutionreprecipitation mechanism. Suzuki,13 as well as Mulla and Krstic,14 have studied the mechanical properties of SiC materials with alumina additions (5-10 wt% A12O3). In this study, analytical electron microscopy (AEM) and nuclear magnetic resonance (NMR) were used to investigate the distribution of the alumina sintering aid and all polytype distributions in the SiC grains in various morphologies, in order to understand further how some of the unique microstructures enhance flexural strength and toughness of SiC. Experimental Procedure Sample Preparation Sample 1 (SiC-Al2O3). This sample was prepared by Suzuki and Sasaki of the Asahi Glass Company.9 The SiC starting powder was composed primarily of )3phase SiC with a minor amount of 6H aphase grains (approximately 14 wt%) and a chemical purity higher than 98%. The alumina powder also had a high chemical purity (99.99%) and contained very few metallic impurities. The specific surface areas of the silicon carbide and alumina powder were 15 and 14 m2/g, respectively. The silicon carbide powder was mixed with the alumina in ethanol for 24 h by ball-milling with alumina media in a nylon container. The amount of the alumina additive was confirmed to be 5 wt% after mixing. The suspension was dried in vacuum, and the resultant powder mixture was mold-pressed into compacts with a size of 100 X 50 X 13 mm, followed by cold isos
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