Influence of isothermal chemical vapor deposition and chemical vapor infiltration conditions on the deposition kinetics and structure of boron nitride

An experimental study has been performed to gain some insight into the corr elations between the deposition conditions and the structure of boron nitri de (BN) coatings that are used in ceramic-matrix composites. BN has been de posited at 700 degrees C from BCl3-NH3-H-2 mixtures on various substrates, by using chemical vapor deposition (CVD) and isothermal-isobaric chemical v apor infiltration (ICVI) processes, simultaneously in the same reactor. A k inetic study has shown that the CVD process is governed either by a combina tion of mass transfer with chemical kinetics at low flow rates or by the he terogeneous kinetics only at high flow velocities. In contrast, the limitin g contribution of mass transfer always is observed for the ICVI process. Th e influence of diffusion cages that are positioned around the fibrous prefo rms is reported. The structure of BN deposits has been studied as a functio n of the various deposition conditions via transmission electron microscopy . The chosen CVD conditions lead to a poor organization of the BN deposits. Fairly well-organized BN coatings are deposited on all fibers of a fibrous preform via ICVI. The results are discussed in terms of supersaturation an d deposition yields. The use of diffusion cages and the adjustment of the i nlet composition and mass flow rate seem to be very important to obtain the best BN organization and thickness uniformity.