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Boron nitride (BN) has been deposited in situ on carbon fibers of 3D fibrous preforms by isopressure/isothermal chemical vapor infiltration (ICVI) from BCI,-NH,-H, mixtures, the aim being to produce a coating of uniform thickness in the whole preforms. The deposition temperature has been lowered to 773 K. Even at this temperature, the deposition rate is not totally controlled by the heterogeneous chemical kinetics; the optimization of the thickness uniformity necessitates an accurate control of both the convective plus diffusive mass transfer outside the preform and the diffusive mass transfer inside the porous medium coupled to an adjustment of the surface kinetics via the deposition temperature and the reactant composition. Under the best conditions, both the longitudinal and infiltration uniformities reach nearly 90% with a LLrelatively" high deposition rate (92 nmh), and nearly similar deposition rates on bulk and porous substrates. On the other hand, still better infiltrations have been obtained by the use of protection screens (like honeycomb) to create a partial separation of the convective and diffusive mass transport influences. We propose that the basic insights deduced here be extended to the ICVI study of other chemical systems, and they should permit an easier extension to large-scale reactors.

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