Influence of BCl₃/NH₃ flow ratio on growth and microstructure of CVI-processed boron nitride interfacial coatings

Chemical vapor infiltration (CVI) is usually utilized to deposit hexagonal boron nitride (h-BN) interfacial coatings with layered microstructure. The deposition rate of BN coating increased with the increase of ammonia flow, indicating that the nucleation occurs on the surface of fibers rather than the gaseous phase. However, when ammonia flow is increased from 3.0 L min⁻¹ to 7.5 L min⁻¹, the deposition rate exhibits an increase of only 30%, which implies that the deposition of BN coatings is controlled by the boron deposition. Furthermore, the coating samples, prepared with ammonia flow of 3.0–4.2 L min⁻¹, exhibits a relatively smooth surface, whereas the flow rates of 5.4 and 7.5 L min⁻¹ result in strumae with a diameter of >4 μm. The smooth coatings demonstrate a near-stoichiometric ratio of boron and nitrogen, and render excellent thermal stability from room temperature to 1300 °C in argon and air. Moreover, the chemical composition of strumae-containing coatings is far from the desired stoichiometry and both coatings contain a lower amount of hexagonal phase, which can be ascribed to the lower thermal stability in the presence of oxygen and water vapors. The present study provides useful insights into the influence of CVI process parameters on the quality of BN coatings.