Structural changes of CVD Si-B-C coatings under thermal annealing

Si-B-C coatings were prepared by chemical vapor deposition (CVD) from CH₃SiCl₃/BCl₃/H₂ gas mixtures at 900-1000°C and under low total pressure (5kPa). A selection of coatings was synthesized at different deposition temperatures and initial compositions of the gas phase. They were characterized in terms of morphology, elemental composition and structure, both at the atomic scale (by nuclear magnetic resonance and X-ray absorption) and long range (by X-ray diffraction and transmission electron microscopy). The as-deposited materials consist of an amorphous boron carbide phase (a-B_xC) including icosahedron-like units and enclosing SiC nanocrystals. The SiC grain size increases with the silicon concentration in the solid, i.e. with the initial CH₃SiCl₃ concentration in the gas phase. The amorphous structure and the excess carbon are partly accommodated by unusual BC₃ environments in boron carbides. Thermal annealing in inert atmosphere at increasing temperature and time gradually modifies their structure at short and long range. A disordered polycyclic carbon phase appears first, while the SiC grain size increases progressively, and rhombohedral boron carbide (B₄C) finally crystallizes around 1300°C. The organization of the SiC₄ sites is improved and the BC₃ sites are partially transformed into more usual intra and intericosahedral environments. Whereas the temperature has a strong influence on the structural changes, the annealing time has only a limited effect, the structure being rapidly "frozen" in a metastable state.