ournal J An. Ceran. Soc., 82 [5] 1187-95(1999) Influence of Isothermal Chemical Vapor Deposition and Chemical Vapor Infiltration Conditions on the deposition Kinetics and Structure of boron nitride Marc Leparouxt and Lionel Vandenbulcke Laboratoire de Combustion et Systemes Reactifs, Centre National de la Recherche Scientifique(CNRS) 45071 Orleans france Christian clinard Centre de Nanoscopie Electronique Analytique (CNEA), Faculte des Sciences, Universite d'Orleans, 45067 Orleans, france An experimental study has been performed to gain some fibers and the matrix during composite insight into the correlations between the deposition condi- carbon interphase that is deposited on the to matrIx tions and the structure of boron nitride(BN) coatings that manufacturing leads to this nonbrittle me behavior 5-k are used in ceramic-matrix composites BN has been depos Unfortunately, the efficiency of these int limited by ited at 700oC from BCly-NH3-H, mixtures on various sub- he oxidation phenomena.9, 10 strates, by using chemical vapor deposition(CVD)and iso. Efforts have been made to improve the oxidation resistance thermal-isobaric chemical vapor infiltration(ICvI) of CMCs with a carbon interphase. Initially, external sealant processes, simultaneously in the same reactor. a kinetic coatings that contained silicon and/or boron were pro study has shown that the Cvd process is governed either by posed 11-13 Another approach considered a multilayered C/SiC a combination of mass transfer with chemical kinetics at matrix or interphase that may permit more crack deflec low flow rates or by the heterogeneous kinetics only at high issues are examined together with SiC/SiBC/ flow velocities. In contrast, the limiting contribution of SiC sequenced matrices, 7 to delay the access of oxygen to the mass transfer always is observed for the ICvI process. The carbon interphase or the fiber/interphase interface. In other influence of diffusion cages that are positioned around the studies, the oxidation resistance of the pyrocarbon interphase fibrous preforms is reported. The structure of BN deposits itself is improved by some change of its composition, as in has been studied as a function of the various deposition boron-doped pyrocarbon or in a compositional gradient layer conditions via transmission electron microscopy. The cho- the composition of which varies continuously from the fiber sen CVD conditions lead to a poor organization of the Bn interface(carbon) to the matrix interface( Sic). deposits. fairly well-organized bn coatings are depos On the other hand, boron nitride(Bn) has been proposed as on all fibers of a fibrous preform via ICVI. The results are an alternative, because of its graphitelike structure and its bet- discussed in terms of supersaturation and deposition ter resistance to oxidation. 20 It has been successfully used in yields. The use of diffusion cages and the adjustment of the various composite systems. 21-29 Particular attention has been inlet composition and mass flow rate seem to be very im- given to the interfacial zones, in terms of chemistry and mi- portant to obtain the best BN organization and thickness crostructure 26-29 These studies revealed that many interfacial noforn carbon-and/or SiO2-rich sublayers can be present between the SiC fibers(NicalonTM(Nippon Carbon, Tokyo, Japan)or L. Introduction TyrannoTM(UBE Industries, Yamaguchi, Japan) and the BN film. To improve the mechanical properties, especially at high HE potential of fiber-reinforced ceramic-matrix composites temperature, it is fundamental either to protect the carbon-rich (CMCs)for thermomechanical applications is well estab- sublayers from the external atmosphere or, even better, to pre lished, because of their fracture toughness, damage tolerance. vent their development. This condition implies the formation of and low density. The mechanical properties of CMCs are de- ndent largely on the fiber-matrix bonding, which must be ganization to deflect the cracks inside the interphase. Such an weak enough to allow crack deflection along the interface, yet optimized structure can be examined by controlling the classi- strong enough to retain load transfer from the matrix to the al experimental deposition parameters. Together with the pro- cess type(either chemical vapor deposition(CVD)in a hot wall reactor or isothermal-isobaric chemical vapor infiltration CVi), these variables produce different deposition condi- tions at the gas-phase/substrate interface which also are a func- R. Naslain-contributing edito tion, in any case, of the total amount of fibers to be coated Moreover, adequate deposition conditions can prevent the de velopment of amorphous glassy sublayers as SiO,. In previous studies, 26-29the interfaces have been principally observed and Manuscript No. 191278. Received January 14, 1997, approved September 21, generally, the evolution of the structure in the bn deposit has Supported by the Societe Europeenne de Propulsion(SEP) and Region Centre, not been described, especially as a function of the deposition conditions In this edat700°C pResent address: Fraunhofer-Iws,WinterbergstraBe 28, 0127 Dresden, Ger- from the BCl-NHa-H2 gas mixture, which was shown to be pResent address: CNRS-CRMD, IB Rue de la Ferollerie, 45071 Orleans, france less aggressive than the recursor under the infiltra 1187Influence of Isothermal Chemical Vapor Deposition and Chemical Vapor Infiltration Conditions on the Deposition Kinetics and Structure of Boron Nitride Marc Leparoux† and Lionel Vandenbulcke* Laboratoire de Combustion et Syste`mes Re´actifs, Centre National de la Recherche Scientifique (CNRS), 45071 Orle´ans, France Christian Clinard‡ Centre de Nanoscopie Electronique Analytique (CNEA), Faculte´ des Sciences, Universite´ d’Orle´ans, 45067 Orle´ans, France An experimental study has been performed to gain some insight into the correlations between the deposition condi￾tions and the structure of boron nitride (BN) coatings that are used in ceramic-matrix composites. BN has been depos￾ited at 700°C from BCl3–NH3–H2 mixtures on various sub￾strates, by using chemical vapor deposition (CVD) and iso￾thermal–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 cho￾sen 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 im￾portant to obtain the best BN organization and thickness uniformity. I. Introduction THE potential of fiber-reinforced ceramic-matrix composites (CMCs) for thermomechanical applications is well estab￾lished, because of their fracture toughness, damage tolerance, and low density. The mechanical properties of CMCs are de￾pendent largely on the fiber–matrix bonding, which must be weak enough to allow crack deflection along the interface, yet strong enough to retain load transfer from the matrix to the fibers.1–4 A carbon-rich layer that is formed in situ between the fibers and the matrix during composite processing or a pyro￾carbon interphase that is deposited on the fibers prior to matrix manufacturing leads to this nonbrittle mechanical behavior.5–8 Unfortunately, the efficiency of these interphases is limited by the oxidation phenomena.9,10 Efforts have been made to improve the oxidation resistance of CMCs with a carbon interphase. Initially, external sealant coatings that contained silicon and/or boron were pro￾posed.11–13 Another approach considered a multilayered C/SiC matrix or interphase that may permit more crack deflec￾tions.14–16 These issues are examined, together with SiC/SiBC/ SiC sequenced matrices,17 to delay the access of oxygen to the carbon interphase or the fiber/interphase interface. In other studies, the oxidation resistance of the pyrocarbon interphase itself is improved by some change of its composition, as in boron-doped pyrocarbon18 or in a compositional gradient layer, the composition of which varies continuously from the fiber interface (carbon) to the matrix interface (SiC).19 On the other hand, boron nitride (BN) has been proposed as an alternative, because of its graphitelike structure and its bet￾ter resistance to oxidation.20 It has been successfully used in various composite systems.21–29 Particular attention has been given to the interfacial zones, in terms of chemistry and mi￾crostructure.26–29 These studies revealed that many interfacial carbon- and/or SiO2-rich sublayers can be present between the SiC fibers (Nicalon™ (Nippon Carbon, Tokyo, Japan) or Tyranno™ (UBE Industries, Yamaguchi, Japan)) and the BN film. To improve the mechanical properties, especially at high temperature, it is fundamental either to protect the carbon-rich sublayers from the external atmosphere or, even better, to pre￾vent their development. This condition implies the formation of strong interfacial bonding and the optimization of the BN or￾ganization to deflect the cracks inside the interphase. Such an optimized structure can be examined by controlling the classi￾cal experimental deposition parameters. Together with the pro￾cess type (either chemical vapor deposition (CVD) in a hot￾wall reactor or isothermal–isobaric chemical vapor infiltration (ICVI)), these variables produce different deposition condi￾tions at the gas-phase/substrate interface, which also are a func￾tion, in any case, of the total amount of fibers to be coated. Moreover, adequate deposition conditions can prevent the de￾velopment of amorphous glassy sublayers as SiO2. In previous studies,26–29 the interfaces have been principally observed and, generally, the evolution of the structure in the BN deposit has not been described, especially as a function of the deposition conditions. In this paper, the BN deposition was performed at 700°C from the BCl3–NH3–H2 gas mixture, which was shown to be less aggressive than the BF3–NH3 precursor under the infiltra￾R. Naslain—contributing editor Manuscript No. 191278. Received January 14, 1997; approved September 21, 1998. Supported by the Socie´te´ Europe´enne de Propulsion (SEP) and Re´gion Centre, via a grant given to author ML. *Member, American Ceramic Society. † Present address: Fraunhofer–IWS, Winterbergstraße 28, 01277 Dresden, Ger￾many. ‡ Present address: CNRS–CRMD, 1B Rue de la Fe´rollerie, 45071 Orle´ans, France. J. Am. Ceram. Soc., 82 [5] 1187–95 (1999) Journal 1187