Contribution of Gas-Phase Reactions to the Deposition of SiC by A Forced-Flow Chemical Vapor Infiltration Process
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CONTRIBUTION OF GAS-PHASE REACTIONS TO THE DEPOSITION OF SiC BY A FORCED-FLOW CHEMICAL VAPOR INFILTRATION PROCESS
CHING-YI TSAI* and SESHU B. DESU** *Department of Engineering Science and Mechanics "**Department of Materials Engineering Virginia Polytechnic Institute and State University, Blacksburg, VA, 24061 ABSTRACT A model, incorporating both gas-phase and surface reactions, for simulating thickness profile of SiC, deposited from trichloromethylsilane (TMS), along the longitudinal direction of a single pore is presented in this paper. The transport mechanisms considered include both forced-flow and diffusion. With the nonlinear nature of this model, a finite element model was developed to solve the problem Simulation results were in good agreement with the reported numerically. experimental data by Fedou et al. (1990). Effects of critical parameters, such as deposition temperature, ratio of sticking coefficients of TMS and intermediate species, and forced-flow, on the deposition thickness profile were investigated. Forced-flow effect was found to be small for the chemical vapor infiltration (CVI) processes at high deposition temperatures. INTRODUCTION Ceramic materials have long been considered as the ideal materials for high temperature applications because of their good stability, resistance to corrosion, and high strength. However, the brittle nature of ceramics has precluded their use in applications where significant levels of toughness are required. Primary goal of many ceramic materials researchers has been to alter the properties of ceramic materials through composition, design, and processing in order to minimize the effects due to the brittle nature and retain other desirable properties. To achieve this goal, researchers have developed the ceramic matrix composites (CMCs), which basically consists of a ceramic matrix reinforced with high performance fibers. Recently, chemical vapor infiltration (CVI) processes have received a considerable attention as a strong candidate for the fabrication of CMCs because of its versatility in creating all major families of ceramic matrices by a single, continuous deposition step, low processing temperature feature, and geometry-preserving properties. The development of forced-flow CVI processes makes the CVI processes more compatible because they ensure better uniformity of the deposit and reduce the processing time significantly [1]. Extensive work is being done in the area of CVI process modeling in an effort to find an optimum relationship between the processing conditions and product properties [2-11]. Geometrical considerations of these models range from single-pore [2-5], overlap pore [6], unit-cell configuration [7], to effective medium [8-9], which considers the porous medium as pore networks. Diffusion is assumed to be the main transport mechanism of the gas species [2-8]. Depending on the CVI process to be modeled, convective flow (forced-flow) was also used [9-10]. In general, chemical reactions involved in the CVI processes are quite complex and can be b
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