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Preliminary experiments indicated that although silicon carbide growth rates at low temperatures and with known parameters were understood, the chemical processes involved at higher temperatures did not meet expectations. The chemical vapour infiltration (CVI) of straight cylindrical pores was analysed in 2 sets of experiments that are both based on the mass balance equations arising from the competition between the mass transport (Fick and Knudsen diffusion) and the chemical reactions that occur during movement within pores.
The 2 areas of research made similar predictions for the effect of parameters such as temperature, total pressure and geometry of the pore, and investigated:
the ratio between the rate of chemical reaction and the rate of diffusion (defined as the Damkohler number) and whether it determines the uniformity of concentration and deposit profiles;
the conservation equations for the various entities in the system, modelling the diameter variations over time and with regard to the position in the pore.
Analytical techniques used for solving the mass balance equation were found to match situations where wide pores were to be found. The numerical method used was found to give satisfactory results for narrow pores.
Validation was done using scanning electron microscopy on prepared porous substrates (pore sizes of 6 um to 1000 um) created by mechanical drilling and controlled oxidation of carbon fibre in silicon carbide substrates. From these results the researchers were able to determine that the analytical technique could be applied in situations where the pore sizes are broadly distributed. In these circumstances, the conditions met with after CVI processes matched the predictions when modelling the infiltration kinetics of silicon carbide whiskers.
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