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

Boron nitride is isoelectronic with graphite, has a similar bonding structu re, but a stronger localization of the pi-states, and a better oxidation re sistance. 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 b oron-organic precursor and to improve the thermomechanical and hydrolytic s tability of the BN fibre coatings, we focussed our investigations on the qu estion: "In what way do carbon and oxygen become incorporated into the BN l ayer?" HREM studies showed the turbostratic structure of the BN, forming ce lls of 5 10 nm in diameter. Energy filtered electron microscopy (EFTEM) and the nanometre resolved analysis of the electron energy-loss near edge stru cture (ELNES) of the individual elements revealed a correlation between mic rostructure and chemical composition. There is a general deficiency of 10-1 5% of nitrogen with respect to boron, which is partly compensated for by co ntents 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. (C) 2000 Elsevier Science Ltd. All rights reserved.