Forced‐Flow Chemical Vapor Infiltration of Porous Ceramic Materials

Abstract

A mathematical model for forced-flow chemical vapor infiltration (CVI) for densification of porous ceramic composites is presented. The process consists of mass transport of reactants by both convection and diffusion into a porous preform, decomposition of reactants, and subsequent deposition of solid product on pore surfaces. The preform is represented by a Bethe lattice, and percolation theory is used to account for utilized, unutilized, and blocked pore space. The effective medium approach used earlier for modeling conventional CVI is extended here to include forced flow of reactants into the preform. The effect of reactant flow rates, hot face temperature, and pore size distribution on densification is studied and compared with the conventional process. Optimum conditions for uniformity and low residual porosity in the final product are suggested.Composite materials are increasingly being regarded as the materials of the future. Their high strength-to-weight ratio and excellent friction and ablation resistance make them ideal choices for replacement of metals in several applications in the aerospace, automobile, nuclear, and other manufacturing industries. An important category of such materials is ceramic matrix composites. These composite materials, which consist of fibers or whiskers imbedded in a brittle ceramic matrix, combine the excellent mechanical properties of the fibers with increased chemical inertness of the ceramic matrix in corrosive environments (1-3). Sintering, hot-pressing, and chemical vapor infiltration (CVI) are among the more commonly used techniques for manufacturing ceramic composites. However, it is difficult to sinter a composite material containing desirable volume fractions of fibers or whiskers (>20%) (4). On the other hand, hot pressing is applicable only to relatively simple shapes and may cause damage to the fiber structure. As a ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-06-13 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-06-13 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-06-13 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-06-13 to IP

Modeling of Pore Structure Evolution Between Bundles of Plain Woven Fabrics During Chemical Vapor Infiltration Process: The Influence of Preform Geometry

“…The first one is simplifying the porous media as straight cylindrical pores, so the inhibition of pore wall to the gas diffusion is not considered. The second one is approximating the media as the capillary Bethe lattices. The third one is estimating the effective diffusivity by empirical tortuosity factors, which is a lumped one accounting for the connectivity of the pore space, an extension of the path that the gas should cross, and the size and shape nonuniformity in the pore cross section.…”

Calculation of the Evolution of Surface Area and Free Volume During the Infiltration of Fiber Felts

“…The pore structure can, for example, be represented by a cylindrical pore with characteristic dimensions or non-overlapping capillary tubes. [14][15][16] Instead of this idealized cylindrical pores model, the entire porous structure can be modeled by a Bethe lattice, [17,18] by a node-bond model, [19] or by a random pore model. [14,20,21] A different approach is the approximation of the growing solid phase instead of the pore structures.…”

Microwave Heated Chemical Vapor Infiltration: Densification Mechanism of SiCf/SiC Composites

“…6,18,19 The process is based on the decomposition or chemical reaction of a gas phase to yield a solid matrix within a heated porous preform, the latter typically formed from ceramic or carbon fibers. 21,22 However, although uniformity and efficiency are enhanced compared with ICVI, 23 disadvantages include the need for complex temperature and gas flow control, something that is magnified when processing multiple parts and geometries. It involves heating the preform very slowly to minimize the temperature gradients that tend to develop between the hot surfaces and cooler interior, 1,2 the latter resulting in preferential deposition or ''crusting'' at the exterior of the preform.…”

“…4,15,20 In an attempt to overcome this problem, an approach based on the forced flow of gases, combined with the deliberate creation of a temperature gradient across the preform (FCVI), has been developed. 21,22 However, although uniformity and efficiency are enhanced compared with ICVI, 23 disadvantages include the need for complex temperature and gas flow control, something that is magnified when processing multiple parts and geometries. The use of a pulsed-pressure CVI process (PCVI) in which the pressure is modulated 24,25 has also yielded faster infiltration rates, improved levels of densification, and greater infiltration uniformity as a result of increased mass transport compared with ICVI.…”