Chemical Vapor Infiltration Process Modeling and Optimization
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T.M. BESMANN*, W.M. MATLIN**, AND D.P. STINTON* *Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6063, [email protected] "**Departmentof Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996
ABSTRACT Chemical vapor infiltration is a unique method for preparing continuous fiber ceramic composites that spares the strong but relatively fragile fibers from damaging thermal, mechanical, and chemical degradation. The process is relatively complex and modeling requires detailed phenomenological knowledge of the chemical kinetics and mass and heat transport. An overview of some of the current understanding and modeling of CVI and examples of efforts to optimize the processes is given. Finally, recent efforts to scale-up the process to produce tubular forms are described. INTRODUCTION Chemical vapor infiltration (CVI) is simply chemical vapor deposition (CVD) on the internal surfaces of a porous preform. It has been used to produce a variety of developmental materials. The greatest commercial interest is in continuous filament fibrous preforms, in which the high strength fibers can be aligned with the high stress directions. The preforms are infiltrated with carbon or ceramic, taking advantage of the relatively low stress CVD process, and resulting in carbon/carbon or ceramic matrix composites (CMCs). Chemical vapor infiltration originated in efforts to densify porous graphite bodies by infiltration with carbon.' The technique has developed commercially such that half of the carbon/carbon composites currently produced are made by CVI. The remainder are fabricated by curing polymer impregnated fiber layups. The earliest report of CVI for ceramics was a 1964 patent for infiltrating fibrous alumina with chromium carbides.2 In CVI, reactants are introduced into a porous preform via either diffusion or forced convection, and the CVD precursors deposit the appropriate phase(s). As infiltration proceeds, the deposit on the internal surfaces becomes thicker. Thus after some length of time the growing surfaces meet, bonding the preform and filling much of the free volume with deposited matrix. The major advantage of CVI relative to competing densification processes is the low thermal and mechanical stress to which the sensitive fibers are subjected. CVD can occur at temperatures much more modest than the melting point of the deposited material, and therefore usually well below the sintering temperature. In addition, the process imparts little mechanical stress to the preform as compared to more traditional techniques such as hot-pressing. There are a variety of techniques for infiltration classified as to whether they utilize diffusion or forced flow for transport of gaseous species, and whether thermal gradients are imposed.' Yet only two methods have been found to be practical. The most widely used commercial process is isothermal/isobaric CVI (ICVI),4' 5 which depends only on diffusion for species transport. These generally operate at reduced pressure (1-10 kPa) for deposition
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