CVD-coated boron nitride on continuous silicon carbide fibres: structure and nanocomposition

Boron nitride is isoelectronic with graphite, has a similar bonding structure, but a stronger localization of the π-states, and a better oxidation resistance. Therefore, it is a promising alternative to pyrolytic carbon as a tool for tayloring special fibre/matrix interlayers in composites for gas turbine and aircraft applications. To understand the CVD processing via a boron-organic precursor and to improve the thermomechanical and hydrolytic stability of the BN fibre coatings, we focussed our investigations on the question: “In what way do carbon and oxygen become incorporated into the BN layer?” HREM studies showed the turbostratic structure of the BN, forming cells of 5–10 nm in diameter. Energy filtered electron microscopy (EFTEM) and the nanometre resolved analysis of the electron energy-loss near edge structure (ELNES) of the individual elements revealed a correlation between microstructure and chemical composition. There is a general deficiency of 10–15% of nitrogen with respect to boron, which is partly compensated for by contents of up to 10 and 15% of oxygen and carbon, respectively, in the layer. It could be concluded that most of the carbon is precipitated in between the BN cells, and a smaller amount is incorporated within the hexagonal BN structure.