Microwave enhanced chemical vapour infiltration of silicon carbide fibre preforms

An investigation into the fundamentals of the deposition of silicon carbide within porous silicon carbide fibre preforms using microwave-enhanced chemical vapour infiltration has been carried out. The study of the kinetics of deposition revealed an Arrhenius behaviour of the matrix growth rate against the temperature in the range 800-1000°C and a linear dependence on the pressure in the range 20 - 70 kPa. This is typical of a surface-reaction limited regime. The morphology of the SiC deposited changed with both temperature and pressure. Increases in both lead to a transition from a smooth, globular deposit morphology to something that was rougher and more angular; this corresponded to the transition from a nucleation to a growth regime. Stoichiometric SiC was predominantly found in the central region of the samples infiltrated at 1000°C, but the deposit became more silicon-rich (up to 2.6 at %) the farther from the initial deposit. Dielectric properties showed that ZMI Tyranno silicon carbide fibres readily absorbed microwave energy. In specific conditions of temperatures and pressures, 900-950°C and 50 kPa, an inside-out deposition pattern was observed indicating a temperature gradient across the preform. Deposition of silicon carbide and silicon caused the gradual flattening of the temperature gradient