Influence of hydrogen on growth and microstructure of boron nitride coatings obtained from BCl3–NH3–H2 system by chemical vapor infiltration

Abstract Hexagonal boron nitride (h-BN) interfacial coatings were prepared by chemical vapor infiltration (CVI) process from BCl3–NH3–H2 system with different hydrogen contents for improving the toughness of ceramic matrix composites. In this study, the yield of BN was found to be 94.90% without hydrogen present in the reactant system as calculated via FactSage, while it reached 99.95% at the [H2]/[BCl3] ratio of 10 and the [NH3]/[BCl3] ratio of 1, when chemical equilibrium was reached. BN interfacial coatings containing mixture of hexagonal and turbostratic phases were obtained. The deposition rate of coating increased from 18.2 ± 0.4 nm min−1 (β = 0) to 23.0 ± 0.4 nm min−1 (β = 5) with the increase of hydrogen content in reactants, then it significantly decreased when β was 10. Owing to different nucleation amounts on the surface of fibers, samples S2 (β = 2) and S3 (β = 5) exhibited particles with circular shapes and smooth surfaces, while the other coatings presented particles with polygonal shapes and rough surfaces. Moreover, the onset temperature of weight gain of sample S2 was 102 °C higher than that of sample S4, thus indicating the enhancement of the high-temperature oxidation resistance of BN coating.