No abstract

The structural and morphological characteristics of flat BN coatings processed by chemical vapour deposition, using BCl3−NH3−H2 gas mixtures at low pressure (P < 1 kPa), have been investigated as a function of the deposition temperature (ranging from 900 °C to 1400 °C) and the total gas flow rate. The resulting BN coatings are mainly turbostratic but with heterogeneous microstructures, i.e. mixtures of poorly and highly organized domains. The structural homogeneity and the degree of crystallization depend notably on the nature of the dilution gas (either H2 or Ar) and the depletion of gas species. The decrease of the apparent activation energy from 80 kJ.mol -1 below 1200 °C to 40 kJ.mol -1 above 1200 °C reflects a change in the deposition regime. Ex situ Fourier transform infrared analysis of the residual gas mixture allowed intermediate species leading to poorly or highly organized BN to be tracked and then connected with the main reaction paths leading to the different BN coatings.

The ceramic matrix composites envisaged as structural parts in the hot section of aircraft engines consist of SiC-based fibres and matrices with a BN interphase coating between them. To control their service life, the oxidation/corrosion behaviour of these materials must be determined. For this purpose, samples of different geometries were prepared by chemical vapour deposition and thermally subjected to wet air. The behaviour of BN alone depends on its crystallization degree, structural homogeneity and the considered direction in the coating. In the case of a thin layer of pure BN of interphase type, recession is significantly reduced at 800°C by the formation of a sealing borosilicate phase in the confined space surrounded by SiC. At 1000°C, the protection is no longer sufficient and a steady recession remains. The presence of aluminium added to BN improves protection at 1000°C due to the formation of alumina combined with borosilicate.