The preparation of carbon reinforced silicon carbide composites using the isothermal forced flow chemical vapour infiltration technique

This paper describes the densification of 3D carbon (continuous) fibrous preforms with a SiC matrix using methyltrichlorosilane (MTS) as the precursor in a gas mixt. of hydrogen and argon. The reproducibility in infiltration time, matrix gain and relative composite d. as estd. from three forced-flow CVD infiltration (FCVI) expts. infiltrated at the same process conditions was found to be less than 10%. At a preform temp. of 1273 K and a total outlet pressure of 6.65 kPa, the effect of both the total gas flow rate of the gas mixt. and the initial input MTS concn. on the infiltration duration and relative composite d. is presented. Characteristic nos. are presented which describe the SiC deposition profile in a cylindrical pore for both isothermal isobaric CVI (ICVI) and isothermal FCVI. Low Damkohler nos. will increase the uniformity of the deposition profile within the pore. Increase of the MTS input concn. will enhance both uniform and rapid infiltration. For ICVI, both rapid and uniform deposition cannot be achieved by changing any other process variable. In the FCVI case, rapid and uniform deposition can be achieved at high gas velocity.