The control of gas phase kinetics to maximize densification during chemical vapor infiltration
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A serious problem during the fabrication of composite materials by isothermal chemical vapor infiltration is that the matrix forms more rapidly at the external edges of the body and traps a large amount of porosity inside. In theory, this problem can be eliminated by controlling the gas-phase kinetics to obtain densification which is more rapid in the center of a preform than at its outer surfaces. An analysis of a first-order gas-phase reaction followed by a first-order deposition reaction indicates that improved infiltration is possible under a relatively narrow range of conditions.
I. INTRODUCTION
Composite materials can be produced by infiltrating a porous preform with a gas-phase precursor which reacts to form the matrix. This method, known as chemical vapor infiltration (CVI), is most commonly used to form ceramic or carbon matrix composites, starting with a preform which is either woven from continuous fibers or pressed from chopped fibers. In the simplest isothermal CVI reactors, the preform is placed in a flowing gas stream and the vapor species diffuse into the preform and react to form the matrix. Figure l(a) shows a hypothetical flat-plate preform of thickness 2L, with infinite length and width, such that the gases diffuse in from only two faces. The monosized fibers in Fig. 1 are parallel (i.e., unidirectional); therefore, they can be described as a two-dimensional array of uniform circles. In Fig. l(a) this array is arranged in a simple ordered structure with no contact between fibers. Compared to this simple schematic, actual preforms contain a much larger number of fibers with a distribution of sizes in a less ordered arrangement. Also, the fibers are usually oriented in multiple directions (i.e., not unidirectional), and there are numerous fiber-fiber contacts. If the deposition reaction in an isothermal system is rate limiting such that all mass transport and other reaction mechanisms are relatively fast, then infiltration will occur uniformly throughout the porous body. Even if uniform deposition occurs throughout the infiltration process, there will still be some trapped porosity in the composite [Fig. l(b)]. This minimum porosity, defined by the size, shape, and arrangement of the fibers, is the minimum percolation threshold for transport through the pore structure. The maximum theoretical density that can be obtained by CVI for a given preform architecture is defined by this minimum percolation
threshold. For the simple fiber architecture shown in Fig. 1 the maximum density is 0.907 (TT/2\/3). In isothermal CVI the matrix typically forms more rapidly near the outer surfaces of the preform.1 This process typically requires more than 100 h; thus, it is impractical to obtain more uniform infiltration by operating with slower deposition rates and longer infiltration times. More advanced CVI processes have thus been developed, where a thermal gradient is applied across the preform and the reactant flux into the preform is increased by using forced convection.2 Materials that are produced by any
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