Densification of Porous Materials by Chemical Vapor Infiltration

Abstract

A mathematical model that describes the combined diffusion and reaction of trichloromethylsilane in a porous preform under typical chemical vapor infiltration (CVI) conditions is presented. The model utilizes a single pore to demonstrate the importance of the pore geometry, totalgaseous pressure, temperature, and imposed temperature gradient on the degree of achieved densification. By focusing attention on a single pore, it is possible to account for changes in the pore shape that occur during deposition of silicon carbide on the pore boundaries. Process conditions for operation in the reaction-controlled regime in order to attain complete densification are quantitatively identified even for the longer pores and the relatively higher temperatures used.Composite materials are becoming increasingly important in the aerospace, automobile, nuclear, solar energy, and manufacturing industries. Carbon fibers imbedded in a carbon matrix exhibit significant advantages, including low density to strength ratio, and excellent behavior towards friction and ablation at even high temperatures. Moreover, the use of multidirectional reinforcement leads to a quasi-isotropic behavior. However, carbon-carbon composites exhibit a few drawbacks, most important of which is their poor resistance towards oxidation at relatively low temperatures (500~176 On the other hand, monolithic ceramics (silicon carbide, silicon nitride, alumina) combine good strength and resistance to oxidation. Their main disadvantage is their sensitivity to crack propagation and poor resistance to mechanical and thermal shocks. In order to combine the advantages of both classes of materials and overcome their weaknesses, it has been suggested to even partially replace the carbon matrix, by a refractory material. More specifically, silicon carbide (SIC) and titanium carbide (TIC) are compatible with carbon and have improved mechanical properties and good oxidation resistance at high temperatures due to a protective oxide formed on their surfaces (1-3).Typical fabrication stages of fiber-reinforced ceramics include, among others, extrusion, hot-pressing, and sintering. However, during these processes the fibers may be chemically or mechanically damaged and the final product is often not of high purity but of high porosity. A most promising alternative is to subject a fibrous preform of the desired shape to the comparatively low-stress and lowtemperature process of chemical vapor infiltration (CVI) of appropriate chemical precursors and subsequent deposition of the desired product on the fibers until complete densification is achieved. Extreme care must be taken so that the deposition process is uniform and complete filling of the voids is achieved, rather than simple overcoating. To this end, the combined diffusion and reaction process must be limited by the chemical reaction allowing for indepth diffusion. Due to this requirement, the overall process is extremely slow and 300-600h of operation have been required for the full densification of laboratory-scale sampl...

Modeling of SiC-matrix composite formation by isothermal chemical vapor infiltration

“…And the fourth computation was performed using the complete model Eqs. (1)(2)(3)(4)(5)(6). The computations were carried out for the following operating conditions: p a ¼ 10; 000 Pa; T ¼ 1250 K; C a ðMTSÞ ¼ 0:9; C a ðHClÞ ¼ 0: Table 1 shows relative values of infiltration times for process within the bundles (t f ) and in the inter-bundle space (t b ) and final average porosity within the bundles ðj Sf Þ; in the inter-bundle space ðj Sb Þ and total porosity ðj S Þ: The latter values are averaged over the preform thickness: j Sn ¼ ð1=LÞ R L 0 j 3n dx: All the quantities are presented in Table 1 in the form of ratios to the respective values obtained in computation (1).…”

“…Chemical vapor infiltration (CVI) is an advanced technique for production of CMC [1,2]. This method is based on deposition of the ceramic matrix on the heated fibers by chemical decomposition of gaseous precursors infiltrating through a fibrous reinforcing substrate or ''preform''.…”

Kinetic Processes in Materials

“…The alternating SiC/carbon layers were produced by CVI of two different reactions: the decomposition of methyltrichlorosilane (MTS) in the presence of hydrogen; and the decomposition of propylene in hydrogen, respectively. There is also an excellent opportunity to model these processes with the intent at gaining better control of the CVI process [18].…”

Forced‐Flow Chemical Vapor Infiltration of Porous Ceramic Materials

“…A constant inlet mass flow rate of gaseous precursors and outlet pressure of 1 atm are maintained in the ORNL process (9). This is appropriate since mass and energy balances reach equilibrium much faster than deposition of solid on the pore surface (6). Consequently, the pressure drop across the preform, the so-called back pressure, increases.…”