Influence of Isothermal Chemical Vapor Deposition and Chemical Vapor Infiltration Conditions on the Deposition Kinetics and Structure of Boron Nitride

Abstract

An experimental study has been performed to gain some insight into the correlations between the deposition conditions and the structure of boron nitride (BN) coatings that are used in ceramic-matrix composites. BN has been deposited at 700°C from BCl 3 -NH 3 -H 2 mixtures on various substrates, by using chemical vapor deposition (CVD) and isothermal-isobaric chemical vapor infiltration (ICVI) processes, simultaneously in the same reactor. A kinetic study has shown that the CVD process is governed either by a combination of mass transfer with chemical kinetics at low flow rates or by the heterogeneous kinetics only at high flow velocities. In contrast, the limiting contribution of mass transfer always is observed for the ICVI process. The influence of diffusion cages that are positioned around the fibrous preforms is reported. The structure of BN deposits has been studied as a function 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 and deposition yields. The use of diffusion cages and the adjustment of the inlet composition and mass flow rate seem to be very important to obtain the best BN organization and thickness uniformity.

Chemical Vapor Infiltration of the Hexagonal Boron Nitride Interphase for SiC Fiber-Reinforced HP-Si₃N₄ Matrix Composite

“…7) As previously mentioned, the relation between the diffusion mechanism controlling the CVI of BN and a well-organized structure of BN has been observed by several authors. 8,9) Both studies noted that a diffusion screen that promotes kinetics controlled by mass transport promotes the organization of hexagonal BN. However, in the case of BN infiltration of large fibrous preforms, such a mechanism tends to increase the heterogeneity of the thickness of the deposit between the outer part and the inner part of the preform.…”

Carbothermal synthesis of boron nitride coating on PAN carbon fiber

“…[12][13][14] Up till now, boron nitride thin coatings for fibers in reinforced CMC materials have been prepared primarily by chemical vapor deposition method. [15][16][17] However, CVD techniques have some disadvantages too. For instance, CVD process may require high vapor pressure of some hazardous and expensive chemical precursors, such as BCl 3 and NH 3 , for boron nitride CVD coating.…”

Effect of intermediate heat treatment on mechanical properties of SiCf/SiC composites with BN interphase prepared by ICVI

“…Some studies have been therefore carried out on postdeposition heat treatments at high temperatures ranging from 1000 to 1700 • C 26,35 to reduce the reactivity of the boron nitride with oxygen. The stability is obtained by decomposing un-reacted species in the deposited BN interface and by increasing the degree of crystallisation of BN.…”