Understanding the CVD process of (Si)-B-C ceramics through FTIR spectroscopy gas phase analysis

B-C and Si-B-C ceramics are used as self-healing matrices in ceramic matrix composites. They can be processed by CVD respectively from BCl3-CH4-H2 and BCl3-CH3SiCl3-H2 precursors under reduced pressure and relatively low temperature. An investigation of the CVD process is presented for the two systems on the basis of an FTIR in-situ analysis of the gas phase. By adding a porous substrate with a high internal surface in the hot zone of the reactor, the consumption of specific species is enhanced, revealing the effective precursors of the solid (e.g., HBCl2 giving rise to boron). In order to better understand the mechanisms of the solid formation, correlations are pointed out between the gas phase analysis, the deposition kinetics and the deposit physicochemical characteristics. In the B-C system, an amorphous carbon-rich boron carbide (comprising mostly B-C bonds) is obtained according to a heterogeneous reaction between HBCl2 and a carbon effective precursor (e.g. the CH3 radical), the C content of the coating increasing with the maturation of the hydrocarbon (i.e. with increasing temperature and PCH4). In the Si-B-C system, two chemical processes compete against each other. The first one, occurring at low temperature (800-900 °C), is similar to that involved for the B-C system. The second one, gradually prevailing at higher temperature (900-1000 °C), is governed by the formation of Si-C bonds according to a heterogeneous reaction between hydrocarbons and chlorosilanes. This process competes with the formation of B-C bonds and gives rise to SiC nanocrystals for the highest temperatures (T > 1000 °C), in a kinetic regime controlled by the mass transfers.