Oxidation/corrosion of BN-based coatings as prospective interphases for SiC/SiC composites

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.