Availableonlineatwww.sciencedirect.com SCIENCE IRECTI SOLID STATE CHEMISTRY ELSEVIER Journal of Solid State Chemistry 177(2004)449-456 http://elsevier.com/locate/jssc Boron-bearing species in ceramic matrix composites for long-term aerosp app. R. Naslain, A. Guette, F. Rebillat, R. Pailler, F. Langlais, and X. bourrat Laboratory for Thermostructural Composites UMR-5801(CNRS-SNECMA-CEA-UB1) Unirersity Bordeaux 1,3 Allee de la boetie, Received 24 September 2002: accepted 21 March 2003 Abstract Boron-bearing refractory species are introduced in non-oxide ceramic matrix fibrous composites(such as SiC/Sic composites )to improve their oxidation resistance under load at high temperatures with a view to applications in the aerospace field. B-doped pyrocarbon and hex-bn have been successfully used as interphase (instead of pure pyrocarbon) either as homogeneous or multilayered fiber coatings, to arrest and deflect matrix cracks formed under load(mechanical fuse function) and to give toughness to the materials. a self-healing multilayered matrix is designed and used in a model composite which combines B-doped pyrocarbon mechanical fuse layers and B- and Si-bearing compound (namely B. C and Sic) layers forming B2O3 based fluid healing phases when exposed to an oxidizing atmosphere. All the materials are deposited by chemical vapor infiltration. Lifetimes under tensile loading of several hundreds hours at high temperatures are reported C) 2003 Published by elsevier Inc Keywords: Ceramic matrix composites(CMC); C(B)and BN interphases; B,C/SiC multilayered matrices; B2O]healing phase; CVD-processing 1. ntroduction which is to say that the matrix failure strain is lower than that of the fibers, hence, under increasing load, it is Ceramic matrix composites(CMCs) consist of cera- the matrix which fails first. All the constituents being mic fibers embedded in a ceramic matrix. The most brittle, one has to avoid the early failure of the fibers by ommonly used CMCs are those comprising carbon or notch effect when matrix undergoes microcracking(at low SiC-based fibers(arranged in a multidirectional archi- strain). To achieve this key requirement, the FM-interface tecture)in a carbon or SiC matrix deposited according (or the interphase)should arrest or/and deflect the matrix to a gas or liquid phase route. C/C, C/SiC and Sic/Sic microcracks thus acting as a mechanical fuse(to protect composites(the fibers being quoted first)are potential the fibers). Under such conditions, CMCs display a non- candidates for a variety of applications in the aerospace linear stress/strain behavior related to various damaging field including rocket nozzles, aeronautic jet engines, phenomena at the microscopic scale (matrix cracking, heat shields and aircraft braking systems. CMCs are FM-debonding and FM-friction) and a high fracture on-brittle materials when the fiber/matrix(FM)bond- energy. In terms of interphase design, it has been ing has been properly optimized during processing, i.e. postulated that the best interphase materials might be when the FM-bonding is not too strong nor too weak those with a layered crystal structure(such as anisotropic [1-3]. The FM-bonding is controlled through the use of pyrocarbon) or microstructure (such as(PyC-SiC)m n interphase, which is a thin film of a material with a multilayers), the layers being relatively weakly bonded low shear strength, deposited on the fiber surface prior to one another, deposited parallel to the fiber surface, and to the infiltration of the matrix in the fiber architecture strongly bonded to the fiber[4-7]. Consequently and for (or fiber preform). In terms of mechanical behavior the applications mentioned previously, C/PyC/SiC or SiC/ under load, CMCs are referred to as inverse composites, PyC/Sic(the pyrocarbon interphase being quoted be- tween the fiber and the matrix)might be the best materials Corresponding author. Fax: + 33-5-56841225 in terms of mechanical functions however both carbon E-mail address: nslain @Icts. u-bordeaux fr(R. Naslain) and Sic are intrinsically oxidation-prone 0022-4596/Ssee front matter o 2003 Published by Elsevier Inc. doi:10.1016/jc.2003.03005
Journal of Solid State Chemistry 177 (2004) 449–456 Boron-bearing species in ceramic matrix composites for long-term aerospace applications R. Naslain, A. Guette, F. Rebillat, R. Pailler, F. Langlais, and X. Bourrat Laboratory for Thermostructural Composites UMR-5801 (CNRS-SNECMA-CEA-UB1) University Bordeaux 1, 3 Alle´e de La Boe´tie, 33600 Pessac, France Received 24 September 2002; accepted 21 March 2003 Abstract Boron-bearing refractory species are introduced in non-oxide ceramic matrix fibrous composites (such as SiC/SiC composites) to improve their oxidation resistance under load at high temperatures with a view to applications in the aerospace field. B-doped pyrocarbon and hex-BN have been successfully used as interphase (instead of pure pyrocarbon) either as homogeneous or multilayered fiber coatings, to arrest and deflect matrix cracks formed under load (mechanical fuse function) and to give toughness to the materials. A self-healing multilayered matrix is designed and used in a model composite, which combines B-doped pyrocarbon mechanical fuse layers and B- and Si-bearing compound (namely B4C and SiC) layers forming B2O3-based fluid healing phases when exposed to an oxidizing atmosphere. All the materials are deposited by chemical vapor infiltration. Lifetimes under tensile loading of several hundreds hours at high temperatures are reported. r 2003 Published by Elsevier Inc. Keywords: Ceramic matrix composites (CMC); C(B) and BN interphases; B4C/SiC multilayered matrices; B2O3-healing phase; CVD-processing 1. Introduction Ceramic matrix composites (CMCs) consist of ceramic fibers embedded in a ceramic matrix. The most commonly used CMCs are those comprising carbon or SiC-based fibers (arranged in a multidirectional architecture) in a carbon or SiC matrix deposited according to a gas or liquid phase route. C/C, C/SiC and SiC/SiC composites (the fibers being quoted first) are potential candidates for a variety of applications in the aerospace field including rocket nozzles, aeronautic jet engines, heat shields and aircraft braking systems. CMCs are non-brittle materials when the fiber/matrix (FM) bonding has been properly optimized during processing, i.e. when the FM-bonding is not too strong nor too weak [1–3]. The FM-bonding is controlled through the use of an interphase, which is a thin film of a material with a low shear strength, deposited on the fiber surface prior to the infiltration of the matrix in the fiber architecture (or fiber preform). In terms of mechanical behavior under load, CMCs are referred to as inverse composites, which is to say that the matrix failure strain is lower than that of the fibers, hence, under increasing load, it is the matrix which fails first. All the constituents being brittle, one has to avoid the early failure of the fibers by notch effect when matrix undergoes microcracking (at low strain). To achieve this key requirement, the FM-interface (or the interphase) should arrest or/and deflect the matrix microcracks thus acting as a mechanical fuse (to protect the fibers). Under such conditions, CMCs display a nonlinear stress/strain behavior related to various damaging phenomena at the microscopic scale (matrix cracking, FM-debonding and FM-friction) and a high fracture energy. In terms of interphase design, it has been postulated that the best interphase materials might be those with a layered crystal structure (such as anisotropic pyrocarbon) or microstructure (such as (PyC–SiC)n multilayers), the layers being relatively weakly bonded to one another, deposited parallel to the fiber surface, and strongly bonded to the fiber [4–7]. Consequently and for the applications mentioned previously, C/PyC/SiC or SiC/ PyC/SiC (the pyrocarbon interphase being quoted between the fiber and the matrix) might be the best materials in terms of mechanical functions; however, both carbon and SiC are intrinsically oxidation-prone. ARTICLE IN PRESS Corresponding author. Fax: +33-5-56841225. E-mail address: nslain@lcts.u-bordeaux.fr (R. Naslain). 0022-4596/$ - see front matter r 2003 Published by Elsevier Inc. doi:10.1016/j.jssc.2003.03.005
R Naslain et al. Journal of Solid State Chemistry 177(2004 )449-456 For long-term applications in engines, non-oxide (CVI). In this process, a fiber architecture(which ca CMCs have to be highly engineered in order to improve be either a simple fiber tow or a multidirectional fiber their oxidation resistance, particularly at the level of the preform)is chemically coated with a ceramic interphase interphase and of the matrix. In this field, boron-bearing (such as C(B) or BN) and then densified with a ceramic species appear to be promising. Firstly, anisotropic matrix(such as SiC or a multilayered Sic-based matrix pyrocarbon interphase could be replaced either by containing boron-bearing species), in a hot wall deposi- boron-doped pyrocarbon or hexagonal boron nitride, tion chamber, from appropriate gaseous precursors all of them displaying layered crystal structures with [16, 17]. Few experiments were also performed with flat weak interlayer bonding as well as an enhanced substrates(such as small disks of graphite, SiC or silica) oxidation resistance [ 8-13]. Secondly, an important in order to assess deposition kinetic laws and mechan property of all B-bearing species is to form fluid oxide isms. As an example, Fig. I shows the apparatus used to phases(B2O3 or B-M-0 ternary phase)over a broad study the kinetics of deposition of BN from BF3-NH3 temperature range(600-1200oC for B2O3) when heated gas mixtures [10] in an oxidizing atmosphere. These B-bearing species, if a variety of gaseous precursors can be used to deposit introduced in the interphase or matrix of non-oxide the ceramics which are of interest here (pyrocarbon; CMCs, could be used to design self-healing materials C(B), Sic, bn, BC and Si-B-C). Some of the most ( the fluid oxide phase formed in situ by oxidation, filling classical precursors and related overall deposition matrix cracks, slowing down the in-depth diffusion of reactions in the field of CMC-CVi processing are oxygen)with improved lifetimes [14, 15 The aim of the present contribution is to give an 2CxH,→2xC+yH2, overview of the research work, performed at LCTS and elsewhere, on the improvement of the oxidation CH3SiCl3→SiC+3HC resistance and lifetime under load in oxidizing atmo- spheres of non-oxide CMCs, through the use of BX3+ NH3 -BN +3HX with X=F,CI B-bearing species (such as B-doped carbon, hex-BN and boron carbides) in the interphase or/and matrix of 4BC13+ CH4+4H2- B4C+ 12HCI non-oxide cmcs As a matter of fact the actual deposition chemical mechanisms are much more complex and depend on deposition conditions (temperature T, pressure P,gas 2. Experimental flow rate @, precursor composition). They usually involve chemical reactions occurring in the gas phase 2.1. Materials and processing (homogeneous reactions) and leading to the formation of intermediate gaseous species as well as chemical The starting materials used had the following purity: reactions taking place at the substrate surface(hetero- CH3SiCI3(MTS): 97-99%: CH4(N35): 99.95%: C3Hs geneous reactions). Deposition is rate-controlled either N25):99.5%; BCl3(N25):99.5% and NH3(N36): by the mass transfers of gaseous species through the 99.96%with H,O 200 ppm. All the CMCs considered bo here were produced by chemical vapor infiltration control moules 3-mass flowmeters ≡ Fig. 1. Apparatus used to study the kinetics of deposition of Bn on a flat substrate (6) from BFr-NHy-Ar gaseous mixtures [10]
For long-term applications in engines, non-oxide CMCs have to be highly engineered in order to improve their oxidation resistance, particularly at the level of the interphase and of the matrix. In this field, boron-bearing species appear to be promising. Firstly, anisotropic pyrocarbon interphase could be replaced either by boron-doped pyrocarbon or hexagonal boron nitride, all of them displaying layered crystal structures with weak interlayer bonding as well as an enhanced oxidation resistance [8–13]. Secondly, an important property of all B-bearing species is to form fluid oxide phases (B2O3 or B–M–O ternary phase) over a broad temperature range (600–1200C for B2O3) when heated in an oxidizing atmosphere. These B-bearing species, if introduced in the interphase or matrix of non-oxide CMCs, could be used to design self-healing materials (the fluid oxide phase formed in situ by oxidation, filling matrix cracks, slowing down the in-depth diffusion of oxygen) with improved lifetimes [14, 15]. The aim of the present contribution is to give an overview of the research work, performed at LCTS and elsewhere, on the improvement of the oxidation resistance and lifetime under load in oxidizing atmospheres of non-oxide CMCs, through the use of B-bearing species (such as B-doped carbon, hex-BN and boron carbides) in the interphase or/and matrix of non-oxide CMCs. 2. Experimental 2.1. Materials and processing The starting materials used had the following purity: CH3SiCl3 (MTS): 97–99%; CH4 (N35): 99.95%; C3H8 (N25): 99.5%; BCl3 (N25): 99.5% and NH3 (N36): 99.96% with H2O p 200 ppm. All the CMCs considered here were produced by chemical vapor infiltration (CVI). In this process, a fiber architecture (which can be either a simple fiber tow or a multidirectional fiber preform) is chemically coated with a ceramic interphase (such as C(B) or BN) and then densified with a ceramic matrix (such as SiC or a multilayered SiC-based matrix containing boron-bearing species), in a hot wall deposition chamber, from appropriate gaseous precursors [16, 17]. Few experiments were also performed with flat substrates (such as small disks of graphite, SiC or silica) in order to assess deposition kinetic laws and mechanisms. As an example, Fig. 1 shows the apparatus used to study the kinetics of deposition of BN from BF3–NH3 gas mixtures [10]. A variety of gaseous precursors can be used to deposit the ceramics which are of interest here (pyrocarbon; C(B), SiC, BN, B4C and Si–B–C). Some of the most classical precursors and related overall deposition reactions in the field of CMC-CVI processing are: 2CxHy-2xC þ yH2; ð1Þ CH3SiCl3 ! H2 SiC þ 3HCl; ð2Þ BX3 þ NH3-BN þ 3HX with X ¼ F; Cl; ð3Þ 4BCl3 þ CH4 þ 4H2-B4C þ 12HCl: ð4Þ As a matter of fact the actual deposition chemical mechanisms are much more complex and depend on deposition conditions (temperature T, pressure P, gas flow rate Q, precursor composition). They usually involve chemical reactions occurring in the gas phase (homogeneous reactions) and leading to the formation of intermediate gaseous species as well as chemical reactions taking place at the substrate surface (heterogeneous reactions). Deposition is rate-controlled either by the mass transfers of gaseous species through the boundary layer (MTR: mass transfer regime) or by the kinetics of the chemical reactions, particularly those ARTICLE IN PRESS Fig. 1. Apparatus used to study the kinetics of deposition of BN on a flat substrate (6) from BF3–NH3–Ar gaseous mixtures [10]. 450 R. Naslain et al. / Journal of Solid State Chemistry 177 (2004) 449–456
R Naslain et al. Journal of Solid State Chemistry 177(2004 )449-456 some residual open porosity(typically 10-15%), which favors the in-depth diffusion of oxygen towards the Q.=110-120sccm interphase and the fibers when the materials are used 阝=06 under load in oxidizing atmospheres T=1250K 2.2.C The structure and microstructure of the fm-inter- facial zone(whose thickness is lower than I um)and of the matrix (which is complex wl multilayered ceramic) have been characterized by electron microscopy. The mechanical behavior was assessed through tensile tests performed on model (one-dimensional (ID) fiber architecture) or real (nD fiber architecture)composites. The oxidation of the 10 composites was studied through thermogravimetric PBFa(kPa) analysis (TGA)experiments performed in dry air. Fig. 2. Kinetic domains for the deposition of BN from BFy-NHr-Ar Finally, the durability of the composites was character- Subdomains B and C correspond to kinetic laws (5) and (6) Composition of the precursor is defined with x= PNH /PNH, +PBF, in air at different temperatures. and B= PAr/(PBF,+ PNH, )[11]. 3. Interphase design with boron-bearing species occurring at the substrate surface (CRR: chemical The objective here is to replace the pyrocarbon in which the deposition of given ceramics is rate- interphase, by a boron-containing material displaying controlled by mass transfers or reaction kinetics is mechanical fuse(matrix crack deflection), as well as important since the densification of a porous fiber better oxidation resistance(related to the formation of a preform is more homogeneous when it is performed under CRR-conditions, i.e. usually at low T, P where B2O3-condensed phase when considering an oxidizing the reaction kinetics are slow with respect to the atmosphere). Three different interphases have been diffusion of the gaseous species in the pore network considered, namely. boron-doped pyrocarbon, C(B), [16, 17]. As an example, Fig. 2 shows the kinetic hexagonal boron nitride, BN, and(BN-SiC),, multi diagram, established experimentally with the apparatus layers. For all these materials, the oxidation resistance shown in Fig. 1, for the deposition of BN from BF3 improvement is related to the formation of a B,O3or NH3-Ar, at a relatively low temperature(T=1250 K) B203-SiO2 fluid phase resulting from the oxidation of the boron and silicon-bearing species, according to the compatible with the infiltration of bn in a porous body following overall equations [Il]. At this temperature, a transition zone is observed between a mass transfer regime and chemical reaction 2B(s)+3/202(g)+B2030 regimes. Within the CRR-domain, deposition of BN occurs according to two different kinetic laws and mechanisms corresponding to different apparent activa 2BN(s)+3/2O2(g)→B2O3(u)+N2(g) tion energies and reactions orders, namely Subdomain b: SiC(s)+ 202(g)+SiO2(s, 1)+Co R=ko exp(-Ea/RT)PBE, PNH,, PHE with Ea= 100 kJ/mol SiC(s)+3/202(g)+SiO2(s 1)+CO(g) Subdomain c B2O3 has a low“ melting point”(≈450°C) and its viscosity decreases as temperature is raised. It remains in R=ko exp(-Ea/RT)PBE, PNH, PHE the liquid state up to a temperature of about 1100C in a with Ea= 185 kJ/mol (6) dry atmosphere but it is readily gasified at low The densification of a fiber preform by CVI yields Finally, the viscosity and thermal stability of the fluid composites with a high quality matrix, in terms of oxide phase can be tailored if the oxide phase contains microstructure and mechanical properties, but with both B,O3 and Sio
occurring at the substrate surface (CRR: chemical reaction regime). The knowledge of the P–T–Q domains in which the deposition of given ceramics is ratecontrolled by mass transfers or reaction kinetics is important since the densification of a porous fiber preform is more homogeneous when it is performed under CRR-conditions, i.e. usually at low T, P where the reaction kinetics are slow with respect to the diffusion of the gaseous species in the pore network [16, 17]. As an example, Fig. 2 shows the kinetic diagram, established experimentally with the apparatus shown in Fig. 1, for the deposition of BN from BF3 NH3 Ar; at a relatively low temperature (T = 1250 K) compatible with the infiltration of BN in a porous body [11]. At this temperature, a transition zone is observed between a mass transfer regime and chemical reaction regimes. Within the CRR-domain, deposition of BN occurs according to two different kinetic laws and mechanisms corresponding to different apparent activation energies and reactions orders, namely: Subdomain B: R ¼ ko exp ðEa=RTÞP0 BF3 P0 NH3 ; P1 HF with Ea ¼ 100 kJ=mol: ð5Þ Subdomain C: R ¼ ko exp ðEa=RTÞP1:5 BF3 P0 NH3 ; P2 HF with Ea ¼ 185 kJ=mol: ð6Þ The densification of a fiber preform by CVI yields composites with a high quality matrix, in terms of microstructure and mechanical properties, but with some residual open porosity (typically 10–15%), which favors the in-depth diffusion of oxygen towards the interphase and the fibers when the materials are used under load in oxidizing atmospheres. 2.2. Characterization The structure and microstructure of the FM-interfacial zone (whose thickness is lower than 1 mm) and of the matrix (which is complex when consisting of a multilayered ceramic) have been characterized by electron microscopy. The mechanical behavior was assessed through tensile tests performed on model (one-dimensional (1D) fiber architecture) or real (nD fiber architecture) composites. The oxidation of the composites was studied through thermogravimetric analysis (TGA) experiments performed in dry air. Finally, the durability of the composites was characterized through static fatigue tests (constant applied load) in air at different temperatures. 3. Interphase design with boron-bearing species The objective here is to replace the pyrocarbon interphase, by a boron-containing material displaying a layered crystal structure or microstructure, to act as a mechanical fuse (matrix crack deflection), as well as a better oxidation resistance (related to the formation of a B2O3-condensed phase when considering an oxidizing atmosphere). Three different interphases have been considered, namely boron-doped pyrocarbon, C(B), hexagonal boron nitride, BN, and (BN–SiC)n multilayers. For all these materials, the oxidation resistance improvement is related to the formation of a B2O3 or B2O3–SiO2 fluid phase resulting from the oxidation of the boron and silicon-bearing species, according to the following overall equations: 2BðsÞ þ 3=2O2ðgÞ-B2O3ð1Þ; ð7Þ 2BNðsÞ þ 3=2O2ðgÞ-B2O3ð1Þ þ N2ðgÞ; ð8Þ SiCðsÞ þ 2O2ðgÞ-SiO2ðs;1Þ þ COðgÞ; ð9Þ SiCðsÞ þ 3=2O2ðgÞ-SiO2ðs;1Þ þ COðgÞ: ð9’Þ B2O3 has a low ‘‘melting point’’ (E450C) and its viscosity decreases as temperature is raised. It remains in the liquid state up to a temperature of about 1100C in a dry atmosphere but it is readily gasified at low temperatures if the atmosphere contains moisture. Finally, the viscosity and thermal stability of the fluid oxide phase can be tailored if the oxide phase contains both B2O3 and SiO2. ARTICLE IN PRESS Fig. 2. Kinetic domains for the deposition of BN from BF3–NH3–Ar. Subdomains B and C correspond to kinetic laws (5) and (6). Composition of the precursor is defined with a = PNH3 / PNH3+PBF3 and b = PAr/(PBF3+PNH3 ) [11]. R. Naslain et al. / Journal of Solid State Chemistry 177 (2004) 449–456 451
R Naslain et al. Journal of Solid State Chemistry 177(2004 )449-456 3.1. Boron-doped pyrocarbon C(B) interphases Oxidation TGA experiments performed in dry flowing air at 700C, show that the overall weight loss rate C(B)CVD-deposits of various compositions have (resulting from the competition between the oxidation of been deposited on fat substrates or Si-C-O(Nicalon) carbon producing gaseous carbon oxides with a weight fibers from a gaseous precursor containing simulta- loss and that of boron giving condensed B,O3 with a carbon-bearing species (i.e. a hydrocarbon) and a gas the deposit increases, to become very low for specimen v. carrier [18]. The boron content ranges from 0(pure Hence, C(B) deposits display two contradictory properties pyrocarbon) to 33 at%. The addition of boron first in terms of interphase requirements: the highest aniso- increases the anisotropy of the pyrocarbon deposit tropy(fuse function) is observed for the lowest B-content (OM), X-ray diffraction(XRD) and high-resolution achieved with the highest one(specimen v) ance is (specimen ID), as supported by optical microscopy (specimen If) whereas the best oxidation resistance is transmission electron microscopy (HR-TEM) data To take into account these two contradictory features Table 1). tropy decreases as the multilayered C(B) deposits were designed and deposited B-content of the deposit increases, deposits IV(20 at% on single Si-C-O(Nicalon) fiber to produce after the B)and V (33 at% B) being almost isotropic. Hence, the deposition of Sic-matrix, So-called model microcompo- C(B) deposit with the lowest B-content(specimen ID) sites, as shown in Fig. 3a. In this complex interphase, displays a layered structure similar to(and even better each layer has a thickness of 100 nm. The first layer D) the mechanical fuse function of an interphase [5-71 bon(to achieve a good bonding with the fiber) whereas the following layers(II-IV in interphase A and II-V in interphase A) display increasing B-content. As Table I expected from the layered interphase concept [4-7 Structural parameters of C(B)deposits with increasing boron content cracks formed in the brittle Sic-matrix under tensile loading were deflected near the layers with the highest anisotropy (layer 2)(Fig. 3b). Further, static fatigue tests performed in air at 600oC under constant tensile B- content(at%)08.315.120.033 loading(the stress level being close to the proportional nd limit to create open cracks in the matrix), showed that 339 s367 the lifetime of these model microcomposites is increased 10.7 when compared to that of their counterpart with a single pyrocarbon (type D) interphase (Fig. 4). The lifetime 4 0.60.3 L,(nm) 2.8 increase is related to a self-healing behavior of the composites when they are exposed to an oxidizing Electron probe microanalysis(EPMA). b Extinction angle measured by OM in polarized light atmosphere at a sufficient temperature. Oxygen diffus Orientation angle measured on 002 arc( TEM-SAD pattern) ing along the matrix cracks reacts first with the boron rich C(B)layers (layers V to Ill) giving a fluid oxide From HR-TEM images(N: number of fringes in a stack, LI and L2 phase(mainly B2O3) which fills the cracks, slows down e of the fringes)[ 18] the in-depth diffusion of oxygen towards the B-poor I%吐日 I0%t日 FIBER ) Fig 3. Multilayered C(B)interphase(type A,)in a ID-Si-C-O(Nicalon)/SiC microcomposite: (a) composition of the interphase (b)matrix crack deflection in the interphase near layer I [18
3.1. Boron-doped pyrocarbon C(B) interphases C(B) CVD-deposits of various compositions have been deposited on flat substrates or Si–C–O (Nicalon) fibers from a gaseous precursor containing simultaneously a boron-bearing species (i.e. a boron halide), a carbon-bearing species (i.e. a hydrocarbon) and a gas carrier [18]. The boron content ranges from 0 (pure pyrocarbon) to 33 at%. The addition of boron first increases the anisotropy of the pyrocarbon deposit (specimen II), as supported by optical microscopy (OM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) data (Table 1). Then, the anisotropy decreases as the B-content of the deposit increases, deposits IV (20 at% B) and V (33 at% B) being almost isotropic. Hence, the C(B) deposit with the lowest B-content (specimen II) displays a layered structure similar to (and even better than) that of the undoped pyrocarbon, compatible with the mechanical fuse function of an interphase [5–7]. Oxidation TGA experiments performed in dry flowing air at 700C, show that the overall weight loss rate (resulting from the competition between the oxidation of carbon producing gaseous carbon oxides with a weight loss and that of boron giving condensed B2O3 with a weight gain) continuously decreases as the B-content of the deposit increases, to become very low for specimen V. Hence, C(B) deposits display two contradictory properties in terms of interphase requirements: the highest anisotropy (fuse function) is observed for the lowest B-content (specimen II) whereas the best oxidation resistance is achieved with the highest one (specimen V) [18]. To take into account these two contradictory features, multilayered C(B) deposits were designed and deposited on single Si–C–O (Nicalon) fiber to produce after the deposition of SiC-matrix, so-called model microcomposites, as shown in Fig. 3a. In this complex interphase, each layer has a thickness of 100 nm. The first layer (I) deposited on the fiber surface consists of pure pyrocarbon (to achieve a good bonding with the fiber) whereas the following layers (II—IV in interphase A and II—V in interphase A0 ) display increasing B-content. As expected from the layered interphase concept [4–7], cracks formed in the brittle SiC-matrix under tensile loading were deflected near the layers with the highest anisotropy (layer 2) (Fig. 3b). Further, static fatigue tests performed in air at 600C under constant tensile loading (the stress level being close to the proportional limit to create open cracks in the matrix), showed that the lifetime of these model microcomposites is increased when compared to that of their counterpart with a single pyrocarbon (type I) interphase (Fig. 4). The lifetime increase is related to a self-healing behavior of the composites when they are exposed to an oxidizing atmosphere at a sufficient temperature. Oxygen diffusing along the matrix cracks reacts first with the boronrich C(B) layers (layers V to III) giving a fluid oxide phase (mainly B2O3) which fills the cracks, slows down the in-depth diffusion of oxygen towards the B-poor ARTICLE IN PRESS Table 1 Structural parameters of C(B) deposits with increasing boron content Specimen I II III IV V B-content (at%)a 0 8.3 15.1 20.0 33 Ae( ) b 10 19 11 ND ND OA( ) c 94 72 107 ND ND d002 (pm)d 346 339 337 341 E367 Lc(nm)d 5.3 10.7 8.5 E1.7 E1 Ne 6 11 9 4 3 L1 (nm)e 0.4 1 0.6 0.3 0.2 L2 (nm)e 2.8 5 2.3 1.5 0.9 a Electron probe microanalysis (EPMA). bExtinction angle measured by OM in polarized light. c Orientation angle measured on 002 arc (TEM-SAD pattern). dFrom XRD-pattern. e From HR-TEM images (N: number of fringes in a stack, L1 and L2 size of the fringes) [18]. Fig. 3. Multilayered C(B) interphase (type A0 ) in a 1D-Si–C–O (Nicalon)/SiC microcomposite: (a) composition of the interphase, (b) matrix crack deflection in the interphase near layer II [18]. 452 R. Naslain et al. / Journal of Solid State Chemistry 177 (2004) 449–456
R Naslain et al. Journal of Solid State Chemistry 177(2004 )449-456 453 inner layers (layers II and D)and prevents an early fiber BN can be deposited from a variety of gaseous ure precursors including B2H6-NH3 or B3 N3H6 and BX3 NH3 (with X= F, Cl)[19. The main advantage 3.2. Hexagonal boron nitride interphases B-N-H precursors lies in the fact that they yield BN deposits at low temperatures(250-650.C)and display a Hexagonal BN is another B-containing material good compatibility with Sic-fibers. Conversely, the BN (50 at%)of potential interest in the field of interphase deposits are amorphous or poorly crystallized. Finally for non-oxide CMCs. It has a layered structure very these precursors should be handled with care(B2H6)or similar to that of pyrocarbon and a better oxidation are not readily available(B3N3H6). Hence, the most resistance, as discussed previously. However, it is often commonly used BN-precursors are BX3-NH3(with X poorly crystallized when deposited at low temperatures F, CD)[8-13, 19-21]. The main advantage of BF3-NH (CVI-conditions) and consequently it reacts with oxygen lies in the fact that Eq ( 3)can yield well-crystallized BN nd moisture. Despite this drawback, BN-interphase deposits (with the BN-layers parallel to the substrate has been used since a long time in SiC/SiC composites surface)even under the low T-P conditions required by he CVl-process(Fig urther both the BN-yield corresponding to Eq (3)and the deposition rate are low which favor infiltration. Conversely, both BF3 and HF are corrosive species for SiC fibers and the bonding between these well-crystallized bn deposits and Sic fibers is weak. One way to take into account these contradictory features is to design bi-layer interphases with: (i)a thin layer of bn deposited under low T-P - conditions, poorly crystallized but adherent to the fiber and (ii a second layer deposited under conditions yielding 3D-ordered BN(conditions 5 in Fig. 5)acting I(PyC) as a mechanical fuse(crack deflection) and exhibiting a good oxid BCl3 and the HCI reaction product(Eq (3)are less corrosive with respect to SiC fibers than their fluorine Time(h) Fig. 4. Lifetime of SiC-O(Nicalon)/siC microcomposites, under and the BN-yield higher. Conversely, boron nitride static fatigue (constant tensile 800 MPa stress) in air at 600C, with deposited from BClg-NH3 is often poorly crystallized different FM interphases. I is a single layer pyrocarbon interphase, A is the five layers nterphase shown in Fig 3 and Aa four layers and hence oxidation-prone and sensitive to moisture interphase (layer V being missing)[18] The state of crystallization of bn deposits can be 2500 9110 (100) (1o1)(102)004 Angle of diffraction: 2 0 Fig. 5. Crystallinity of BN films deposited from BF], under different T-P- conditions:(a) XRD patterns of films deposited on SHC-O Nicalon fibers. (b) SEM-image of film deposited under conditions 5[22]
inner layers (layers II and I) and prevents an early fiber failure. 3.2. Hexagonal boron nitride interphases Hexagonal BN is another B-containing material (50 at%) of potential interest in the field of interphase for non-oxide CMCs. It has a layered structure very similar to that of pyrocarbon and a better oxidation resistance, as discussed previously. However, it is often poorly crystallized when deposited at low temperatures (CVI-conditions) and consequently it reacts with oxygen and moisture. Despite this drawback, BN-interphase has been used since a long time in SiC/SiC composites [4–13]. BN can be deposited from a variety of gaseous precursors including B2H6–NH3 or B3N3H6 and BX3— NH3 (with X = F, Cl) [19]. The main advantage of B–N–H precursors lies in the fact that they yield BN deposits at low temperatures (250–650C) and display a good compatibility with SiC-fibers. Conversely, the BNdeposits are amorphous or poorly crystallized. Finally, these precursors should be handled with care (B2H6) or are not readily available (B3N3H6). Hence, the most commonly used BN-precursors are BX3–NH3 (with X = F, Cl) [8–13, 19–21]. The main advantage of BF3–NH3 lies in the fact that Eq. (3) can yield well-crystallized BN deposits (with the BN-layers parallel to the substrate surface) even under the low T–P conditions required by the CVI-process (Fig. 5). Further both the BN-yield corresponding to Eq. (3) and the deposition rate are low, which favor infiltration. Conversely, both BF3 and HF are corrosive species for SiC fibers and the bonding between these well-crystallized BN deposits and SiC fibers is weak. One way to take into account these contradictory features is to design bi-layer interphases with: (i) a thin layer of BN deposited under low T–P conditions, poorly crystallized but adherent to the fiber and (ii) a second layer deposited under conditions yielding 3D-ordered BN (conditions 5 in Fig. 5) acting as a mechanical fuse (crack deflection) and exhibiting a good oxidation resistance [22]. BCl3 and the HCl reaction product (Eq. (3)) are less corrosive with respect to SiC fibers than their fluorinebearing counterparts. Further, the BN-deposition kinetics, under commonly used CVI-conditions, is faster and the BN-yield higher. Conversely, boron nitride deposited from BCl3–NH3 is often poorly crystallized and hence oxidation-prone and sensitive to moisture. The state of crystallization of BN deposits can be ARTICLE IN PRESS Fig. 4. Lifetime of Si–C–O (Nicalon)/SiC microcomposites, under static fatigue (constant tensile 800MPa stress) in air at 600C, with different FM interphases. I is a single layer pyrocarbon interphase, A0 is the five layers interphase shown in Fig. 3 and A a four layers interphase (layer V being missing) [18]. Fig. 5. Crystallinity of BN films deposited from BF3–NH3 under different T–P–a conditions: (a) XRD patterns of films deposited on Si–C–O Nicalon fibers, (b) SEM-image of film deposited under conditions 5 [22]. R. Naslain et al. / Journal of Solid State Chemistry 177 (2004) 449–456 453