Gas Flow Simulation of Chemical Vapour Infiltration in a Vertical Hot-wall Reactor

Deposition of silicon carbide (Sic) into two types of capillaries (1.5 mm dia. x 6 mm depth, 1.0 mm dia. x 4 mm depth) has been attempted by a vertical hot-wall-type chemical vapour infiltration (CVI) process using an SiCl4-CH4-H2 system. Dependence of the deposition temperature (Tdep) and the total gas pressure (Ptot) on the thickness distribution of Sic in the two types of the capillaries (parallel and perpendicular to the gas stream) was studied. Activation energy of the deposition of S ic was 42 Wmol in the Tdep range of 1673 to 1773 K, and 167 Wmol in the Tdep range of 1573 to 1673 K. The most suitable CVI condition was a Tdep below 1673 K accompanied by the reaction-rate controlled process. The uniformity of the film thickness obtained at Ptotbelow 0.13 kPa was lower than that obtained at Ptot below 13 kPa. The lower the Ptot and the closer the deposition was to the capillary entrance, the higher the vorticity in the two types of capillaries became. The film thickness at the capillary entrance increased coincident with the increase in vorticity. Vorticity in the capillaries perpendicular to the gas stream was higher than that parallel to the gas stream. In the diffusion-rate controlled process (Tdep>1673 K), in which decreasing Ptot was accompanied by increasing vorticity, film thickness uniformity in the capillary decreased Results of the CVI experhen& and the gas flow simulation suggest that lower vorticity under reaction-rate controlled conditions is important for obtaining uniformity of S ic thickness in the capillaries.