Reactive infiltration of silicon melt through microporous amorphous carbon preforms
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I. INTRODUCTION
EXOTHERMIC reactions between a porous matrix and an infiltrating melt provide a more economic alternative for synthesizing many ceramics, intermetallics, and composites. The manufacture of reaction-bonded silicon carbide can be attained by capillary infiltration of molten silicon[1] through a carbon-containing body. These carbon preforms typically contain a large amount of SiC grains as inert fillers. The exothermic reaction between silicon and carbon results in the formation of silicon-carbide particles that bond the previously existing SiC grains. In another approach, carbonaceous materials, such as carbon fiber tow, carbon fiber cloth, or felt, are infiltrated by molten silicon in vacuum to form Si/SiC (silicon-carbide reinforced silicon composites).[1] It has recently been demonstrated that infiltration of cast microporous carbon preforms by silicon melt can be used to fabricate high-density near net-shaped silicon-carbide components at significantly reduced cost.[2,3] Components fabricated by this technique are expected to find applications as gas turbine engine components and commercial combustion nozzles, where their refractoriness (high-temperature strength), good oxidation resistance, high thermal conductivity, low density, and adequate toughness can be fully exploited. However, successful commercial exploitation of this technique requires a quantitative understanding of the various steps involved in the process. The siliconization mechanism has been extensively studied via experiments on isolated carbon fibers or plates.[4–9] Since molten silicon wets carbon, it wicks up into the microporous carbon preform when it is brought in contact with the preform, thus converting carbon into silicon carbide. It P. SANGSUWAN, formerly Graduate Student, Department of Chemical Engineering, Cleveland State University, is a Process Engineer, with 3M Thailand Limited, Bangkok, Thailand 10520. S.N. TEWARI, Professor, and J.E. GATICA, Associate Professor, are with the Department of Chemical Engineering, Cleveland State University, Cleveland, OH 44115-2426. M. SINGH, Senior Research Engineer, is with NYMA, Inc., Lewis Research Center Group, Cleveland, OH 44135. R. DICKERSON, formerly Senior Research Engineer, NYMA Inc., is Senior Scientist, Los Alamos National Laboratories, Los Alamos, NM 87545. Manuscript submitted April 9, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
was initially believed that after a thin film of silicon carbide formed on the carbon surface, diffusion of silicon or carbon through the solid was responsible for further conversion.[4,5] Grain-boundary diffusion was initially invoked to explain the fast conversion kinetics that were generally observed. It was later proposed that the initial silicon-carbide layer spalls off because of the volume increase due to the reaction, thereby exposing a fresh carbon surface to the liquid silicon. Further conversion occurred by dissolution of carbon into the molten silicon at higher temperatures. Silicon carbide then precipitated in regions of lower
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