This work was conducted on Pinctada maxima nacre (mother of pearl) in order to understand its multiscale ordering and the role of the organic matrix in its structure. Intermittent-contact atomic force microscopy with phase detection imaging reveals a nanostructure within the tablet. A continuous organic framework divides each tablet into nanograins. Their shape is supposed to be flat with a mean extension of 45nm. TEM performed in the darkfield mode evidences that at least part of the intracrystalline matrix is crystallized and responds like a 'single crystal'. The tablet is a 'hybrid composite'. The organic matrix is continuous. The mineral phase is thus finely divided still behaving as a single crystal. It is proposed that each tablet results from the coherent aggregation of nanograins keeping strictly the same crystallographic orientation thanks to a hetero-epitaxy mechanism. Finally, high-resolution TEM performed on bridges from one tablet to the next, in the overlying row, did not permit to evidence a mineral lattice but crystallized organic bridges. The same organic bridges were evidenced by SEM in the interlaminar sequence.

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Pyrocarbon materials containing various amounts of boron have been prepared by LPCVD from BCl 3 -C 3 H 8 -H 2 precursor mixtures. The growth rate is increased with respect to pure pyrocarbon deposition. By increasing BCl 3 / (C 3 H 8 + BCl 3 ) ratio up to 85%, the incorporation of boron can reach 33 at.%. A small amount of boron (e.g. 8 at.%) highly enhances the anisotropy of pyrocarbon, as evidenced by optical microscopy, X-ray diffraction and transmission electron microscopy (selected area diffraction and lattice fringes techniques). X-ray photoelectron spectroscopy has shown that a large fraction of the boron atoms are included by substitution in the carbon layers, the remaining boron atoms belongs to a boron-rich amorphous part of the material. As boron content increases beyond 8 at.%, the anisotropy of the boron-rich pyrocarbon decreases, due to the limited growth and stacking of the carbon layers. Also, amorphous boron-rich regions are more and more abundant as the total amount of boron increases. The oxidation resistance of the C(B) materials is better than that of pure pyrocarbon. It is mainly due to the improvement of the structural organization for the low boron content materials and to the coating of the whole material with a stable boron oxide for materials with a higher boron content.

BN films deposited from a BF,-NH, precursor, under chemical vapour infiltration conditiom, on plane sintered aSiC substrates were analysed by XPS. The films are non-stoichiometric with an N/B atomic ratio of < 1. They also contain significant amounts of oxygen atoms, homogeneously distributed in the film and thought to replace jtartly the nitrogen atoms in the turbostratic hexagonal network. As a result, ternary BN,O, species are formed locally. Near the BN/SiC interface, the oxygen concentration increases owing to the occurrence of ternary SiN,O, species, thought to be the result of an oxinitriding reaction of the substrate surface with the gas phase containing residual oxygen, at the very beginning of the BN deposition process.

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A BN interphase has been deposited, by isothermal/isobaric chemical vapor infiltration (ICVI) from BF3-NH3, within a preform made from ex-polycarbosilane (ex-PCS) fibers, at about 1000°C. In a second step, the BN-treated preform was densified with Sic deposited from CH3SiCI3-H2 at about the same temperature. From a thermodynamic standpoint, ex-PCS fibers could be regarded as unreactive vs the BF3-NH3 gas phase assuming they are coated with a thin layer of carbon or/and silica. The as-deposited interphase consists of turbostratic BN (N/B < 1) containing oxygen. The Sic infiltration acts as an annealing treatment: (i) the BN interphase becomes almost stoichiometric and free of oxygen; (ii) the fibers undergo a decomposition process yielding a Si02/C layer at the BN/fiber interface. The weaker link in the interfacial sequence seems to be the BN/Si02 interface. Deflection of microcracks arising from the failure of the matrix takes place at (or nearby) that particular interface. [

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