The forced—flow thermal gradient chemical vapor infiltration (FCVI) process for fabricating ceramic matrix composites (CMCs) was modeled and analyzed based on the finite element method (FEM).

The modeling study was focused on the fabrication of silicon carbide (SiC) matrix composites from methyltrichlorosilane (MTS) precursors because of their high strength, high modulus and excellent oxidation resistance properties at high temperatures.

Unlike other available FCVI models, which use lumped reaction schemes, both gas phase and surface reactions of the FCVI process were explicitly considered for the present FEM FCVI model.

The kinetics of SiC deposition from MTS precursor were derived by analyzing our own deposition rate data as well as reported results. The SiC deposition process was modeled using the following reactions — (1) : gas phase decomposition of MTS molecules into two major intermediates, one containing silicon and the other containing carbon; (2) : adsorption of the intermediates onto the surface sites of the growing film; (3) : reaction of the adsorbed intermediates to form silicon carbide.

The equilibrium constant for the gas phase decomposition process was divided into the forward and backward reaction constants as 2.0E+25 exp[(—448.2 kJ/mol)/RT] and 1.1E+32 exp[(-416.2 kJ/mol)/RT], respectively. Equilibrium constants for the surface adsorption reactions of silicon—carrying and carbon—carrying intermediates were determined to be 0.5E+11 exp[(—21.6 kJ/mol)/RT] and 7.1E+09 exp[(—33.1 kJ/mol)/RT], while the rate constant for the surface reaction of the intermediates was 4.6E+05 exp[(—265.1 kJ/mol)/RT].

Effects of the deposition temperature and vapor pressure variations on the density profiles of the composite preform were studied based on this FEM FCVI model. It was found that the advantages of the commonly used ambient—pressure FCVI process (APFCVI) are likely to be limited by the equipment and the accumulation of gaseous components around the entrance sides, which could render the deposition process to be mass transport limited. A conceptual multi-step FCVI process was proposed to alleviate this problem and obtain products of good final density profiles within reasonable processing times.

This multi—step FCVI process involved deposition under ambient—pressure to improve the density profiles and shorten the processing times. This was followed by the sub ambient—pressure FCVI process (LPFCVI) process to overcome the mass transfer limitations caused by the entrance accumulation effect and possible limitations on the equipment.

A balance between processing time and final density profile can be achieved through the use of this multi-step FCVI process. Advantages of this process has been demonstrated by studying the densification process in large size specimens.