Abstract
The chemical and physical properties of boron nitride are improved as soon as its crystallisation degree in the hexagonal modification is high, the structure becoming somewhat similar to that of graphite. Under such conditions, it can be used (instead of pyrocarbon) as an interphase in ceramic matrix composites. Unfortunately, under commonly used CVD/CVI conditions, BN layers display only a poor turbostratic texture. Furthermore, when BN is deposited from the BF 3-NH 3 system, the fibrous reinforcement can be chemically damaged. A wide range of BN coatings and especially a three-dimensionally ordered hex-BN have been deposited by LPCVD at relatively low temperature. Structural evolution in BN coatings was related to change in chemical rate control regimes. Finally, a compromise between conditions yielding anisotropic coatings and conditions resulting in low degradation of the fibres has been worked out.
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