High temperature behaviour and oxidation resistance of carbon-boron-nitrogen compounds obtained by LPCVD

Abstract
Coatings of carbon-boron-nitrogen compounds with various compositions are obtained in a low pressure chemical vapour deposition process using acetylene, boron trichloride, ammonia and hydrogen as input gases. Compositions are located in the ternary system C-BN-"BC₃". The structure of the films is a graphitic turbostratic one for the whole composition range but carbonrich deposits exhibit greater coherence length L_c than boron nitride-rich ones. The high temperature behaviour is studied for three specific compositions : C_0.82B_0.14N_0.04, C_0.50B_0.31N_0.19 and C_0.78B_0.22. In an inert atmosphere no mass loss is observed up to 2250°C while graphitization and formation of boron carbide occur. Based on electron probe micro analysis, depletion of boron and nitrogen happens only in the few micrometers near the surface of the sample. Above 2500°C, B₁₃C₂ evaporates and a graphite-like material is formed. In a dry air atmosphere, oxidation occurs above 800°C with a slow kinetics. A boron oxide layer protects the sample up to 1000°C. Above this temperature, volatilization of B₂O₃ occurs and oxidation is greatly enhanced. Of special interest seems to be the C_0.78B_0.22 coating which exhibits the lowest oxidation rate. Such deposits could then present a way to protect carbon-carbon composites at moderate temperature in the form of an external coating or better of an intermediate layer between fibres and matrix.