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

Abstract

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 a 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 properties of the coatings. 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 CH 3  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.

Decomposition reaction rate of BCl3–CH4–H2 in the gas phase

“…6a is for the concentration of BC and (b) is for B atom. It is shown that the concentration of the products increases with increasing temperature and is consistent with the conclusion of the experiments that the higher temperature is more favorable for the reaction [10,41]. The concentration of the products may easily reach equilibrium in a very short time (e.g., about one second), or the reaction of producing BC and B is a fast process if temperature is higher than 1300 K. As shown in Fig.…”

“…The forward and reverse rate constants of the all elementary reactions (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16) (2) and (12) reported in Ref. [3] were also plotted.…”

“…The experimental kinetics researches of the reaction of BCl 3 +CH 4 +H 2 in the CVD were carried out for different conditions [10,41]. The present work examined a similar pressure 12 kPa with a large temperature range of 1200-1600 K. The initial gas-phase composition was chosen with the ratio BCl 3 :CH 4 Figure 5a is for the concentration of boron trichloride and Fig.…”

“…The fluid oxide phase would then fill the matrix cracks in situ and slow down the in-depth diffusion of oxygen [5]. Boron carbide is often produced via gasphase decomposition of boron trichloride combined with a light hydrocarbon, such as the precursors of BCl 3 -C 6 H 6 [6], BCl 3 -C 2 H 4 -H 2 [7], BCl 3 -C 3 H 6 (propene)-H 2 [8,9] or BCl 3 -CH 4 -H 2 [3,[10][11][12][13][14][15][16], with the chemical vapor deposition (CVD) technique. This process was widely employed because it allows to control easily the thickness and the texture of the materials.…”

“…A theoretical investigation is thus an important choice. For example, in a simple reaction system of BCl 3 -CH 4 -H 2 precursors, only two intermediates (IMs) HBCl 2 and HCl were observed with in situ FTIR [10], whereas 150 molecular species (including radicals) were found theoretically. Also in the theoretical study of reaction thermodynamics, 325 possible gaseous species were identified in the homologous reaction system of BCl 3 -C 3 H 6 (propene)-H 2 precursors [17].…”