S. Stöckel

Influence of composition and structure on the mechanical properties of BCN coatings deposited by thermal CVD

BCN films were deposited by isothermal chemical vapour deposition from gaseous mixtures of trimethylborazine, toluene and ammonia. The films were analysed with respect to chemical state, composition, morphology and microstructure on the one side oxidation behaviour and hardness on the other side. X-ray spectroscopy (WDX), Raman and infrared spectroscopy, differential thermal analysis, X-ray diffraction and transmission electron spectroscopy were employed for film characterization. A microhardness of maximum 20 GPa was achieved, affected by carbon content, by the way of its incorporation into the hexagonal turbostratic lattice as well as by the crystallite size and its texture.

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

Abstract 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.

Preparation of boron nitride thin films by microwave PECVD and their analytical characterisation

Hexagonal boron nitride films on Si(100) substrates are formed from the system N2/B(OCH3)3 (TMOB) by microwave plasma enhanced chemical vapour deposition (PECVD). The parameters substrate temperature, deposition time and composition of the process gas mixture were varied. The stability of the deposited layers mainly depends on the substrate temperature during the deposition process. Layers formed below 650°C showed strong decomposition features by contact with air humidity. Above this temperature very stable layers could be produced. This was confirmed by hydrolysis tests. The structural growth of the boron nitride layers was investigated by means of transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Three different growth zones were observed within the layers. The composition was determined by electron probe microanalysis (EPMA) and elastic recoil detection analysis (ERDA). Infrared and Raman spectroscopy were used for qualitative investigation of the BN layers.