Boron carbonitride coatings synthesized by LPCVD, structure and properties

Thin films of carbon-rich boron carbonitride (h-B-C-N) were prepared by low pressure chemical vapor deposition from C2H2, BCl3, NH3 and H2 mixtures. After an overall investigation of the deposition kinetics and the composition of the solid, four deposition conditions were selected to study in details the structure, the oxidation behavior and the mechanical properties of the deposits. The influence of the gas phase composition on the atomic concentration and the microstructure of the material was first investigated. Carbon rich coatings are turbostratic and highly textured, whereas boron nitride-rich coatings are more disordered and less anisotropic. We examined their oxidation behavior in the range 450 °C-700 °C, under both dry and ambient air. The oxidation resistance of the h-B-C-N coatings strongly depends on the deposition temperature and is improved by heat treatments. It is significantly better than that of pyrolytic carbon in dry air but worse in wet air, because of the reactivity of B2O3 with H2O. The room temperature stress/strain behavior of unidirectional SiC/SiC composites with h-B-C-N interphases is elastic and damageable as in the case of reference composites having a pyrolytic carbon interphase.

282