SOLID STATE ELSEVIER Solid State ionics 141-142(2001)541-548 elsevier. com/locate/ssi Synthesis of highly tailored ceramic matrix composites by pressure-pulsed CVI R.R. Naslain',R. Pailler, X. Bourrat, S Bertrand, F. Heurtevent, P. Dupel F Lamouroux Laboratory for Thermostructural Composites, UMR-5801(CNRS-SNECMA-CEA-UBD, University Bordeaux 1, 3 Allee de La boetie, 33600 Pessac. france Abstract Pressure-pulsed chemical vapor infiltration(P-CVD) is a new processing technique to produce ceramic matrix composites (CMCs)(C/C, C/SiC and SiC/SiC) from gaseous precursors with highly tailored interphases and matrices. P- sed to control the microtexture of pyrocarbon deposited from hydrocarbon in a porous body. It is also used to form (PyC-SiC) or(BN-SiC) interphases with elementary layer thickness of a few nanometers. Finally, P-CVI is an efficient way to produce self-healing matrices comprising mechanical fuse layers (PyC or BN) and glass-former layers(B13C2 or/and SiC). The resulting model or real composites display an oxidation resistance, in air at 600-700oC and under load which is improved by two orders of magnitude. C 2001 Elsevier Science B V. All rights reserved Keywords: Chemical vapor infiltration; CVI; Ceramic matrix composite; Multilayered ceramics; Carbon; Silicon carbide; Boron nitride 1. Introduction matrix. The interphase has several key functions including load transfer, matrix crack deflection Ceramic matrix composites(CMCs) are promis- (mechanical fuse function)and diffusion barrier. The ing materials for structural applications at high tem- best interphase materials might be those with a lay- peratures. They comprise a fiber architecture ered crystal structure, such as graphitic carbons (I embedded in a ceramic matrix often deposited by rocarbons or PyC) or hexagonal boron nitride, or chemical vapor infiltration(CVD) from a gaseous with a multilayered microstructure, such as(PyC precursor. The most common are the C/C,C/Sic SiC), or(BN-SiC)n, the layers being parallel to the and SiC/SiC composites(the fibers being specified fiber surface [2, 5 ] The concept of multilayered ce first)[1.CMCs tough when the fiber/matrix ramics has been extended to the matrix itself, with a (FM) bonding is neither too strong nor too weak view to increase the oxidation resistance. In CMCs That bonding is controlled through an interphase deposited on the fiber prior to the infiltration of the sists of thin layers of ceramics acting as mechanical fuses and thin layers of glass formers [6, 7] Pressure-pulsed chemical vapor infiltration (P Corresponding author. LCTS, UMR-47(CNRS-SEP-UBI) Domaine Universitaire. 3 Allee de La boetie. 33600 Pessac CVI) is a novel technique for depositing ceramics france.Tel:+33-556-84-4700;fax:+33-556-84-1225 ithin the porosity of a substrate, such as a fiber E-Inail address: naslainalcts u-bordeaux. fr(R. R. Naslain) preform In P-CVl, a given sequence of elementary 0167-2738/01/S- see front matter C2001 Elsevier Science B V. All rights reserved. P:S0167-273801)00743-3
Solid State Ionics 141–142 2001 541–548 Ž . www.elsevier.comrlocaterssi Synthesis of highly tailored ceramic matrix composites by pressure-pulsed CVI R.R. Naslain), R. Pailler, X. Bourrat, S. Bertrand, F. Heurtevent, P. Dupel, F. Lamouroux Laboratory for Thermostructural Composites, UMR-5801 CNRS-SNECMA-CEA-UB1 , Uni ( ) Õersity Bordeaux 1, 3 Allee de La Boetie, ´ ´ 33600 Pessac, France Abstract Pressure-pulsed chemical vapor infiltration P-CVI is a new processing technique to produce ceramic matrix composites Ž . Ž .Ž . CMCs CrC, CrSiC and SiCrSiC from gaseous precursors with highly tailored interphases and matrices. P-CVI can be used to control the microtexture of pyrocarbon deposited from hydrocarbon in a porous body. It is also used to form Ž .Ž . PyC–SiC or BN–SiC interphases with elementary layer thickness of a few nanometers. Finally, P-CVI is an efficient n n way to produce self-healing matrices comprising mechanical fuse layers PyC or BN and glass-former layers B C Ž. Ž 13 2 orrand SiC . The resulting model or real composites display an oxidation resistance, in air at 600–700 . 8C and under load, which is improved by two orders of magnitude. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapor infiltration; CVI; Ceramic matrix composite; Multilayered ceramics; Carbon; Silicon carbide; Boron nitride 1. Introduction Ceramic matrix composites CMCs are promis- Ž . ing materials for structural applications at high temperatures. They comprise a ceramic fiber architecture embedded in a ceramic matrix often deposited by chemical vapor infiltration CVI from a gaseous Ž . precursor. The most common are the CrC, CrSiC and SiCrSiC composites the fibers being specified Ž first 1 . CMCs are tough when the fiber . w x rmatrix Ž . FM bonding is neither too strong nor too weak. That bonding is controlled through an interphase deposited on the fiber prior to the infiltration of the ) Corresponding author. LCTS, UMR-47 CNRS-SEP-UBI , Ž . Domaine Universitaire, 3 Allee de La Boetie, 33600 Pessac, ´ ´ France. Tel.: q33-556-84-4700; fax: q33-556-84-1225. E-mail address: naslain@lcts.u-bordeaux.fr R.R. Naslain . Ž . matrix. The interphase has several key functions including load transfer, matrix crack deflection Ž . mechanical fuse function and diffusion barrier. The best interphase materials might be those with a layered crystal structure, such as graphitic carbons py- Ž rocarbons or PyC or hexagonal boron nitride, or . with a multilayered microstructure, such as PyC– Ž SiC or BN–SiC , the layers being parallel to the .Ž . n n fiber surface 2,5 . The concept of multilayered ce- w x ramics has been extended to the matrix itself, with a view to increase the oxidation resistance. In CMCs displaying a self-healing behavior, the matrix consists of thin layers of ceramics acting as mechanical fuses and thin layers of glass formers 6,7 . w x Pressure-pulsed chemical vapor infiltration P- Ž CVI is a novel technique for depositing ceramics . within the porosity of a substrate, such as a fiber preform. In P-CVI, a given sequence of elementary 0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0167-2738 01 00743-3 Ž
R.R. Naslain et al/ Solid State ionics 141-142(2001)541-548 steps is repeated vs time. Each sequence comprises: BX -NH, ( with X= F, CD), respectively. Under low i the evacuation of the deposition chamber, (i) the mperature(T)and pressure(P), the process yields injection of the gaseous reactant(s)and (iii)a hold- a deposit of high microstructural quality and the ing period during which deposition occurs. P-CVI preform porosity can be almost totally filled with the has been first introduced as a way to shorten the deposit [ densification [8-11]. Then, it has been shown that In P-CVI, the gaseous precursor is periodically P-CVI could be used to control the maturation of the injected in the deposition chamber as short pressure gas phase and, hence, the microtexture of, e.g. pyro- pulses(Fig. 1). Each pressure pulse corresponds to carbon deposits [12-14]. More recently, P-CVI has the following sequence of events. First, the deposi been used to tailor the composition and microstruc- tion chamber is evacuated by pumping. Then, the ture of multilayered interphases and matrices gaseous precursor is injected in the reactor and its [6,7,15-17] internal pressure reaches almost instantaneously a The aim of the present contribution is to show preset value. In a third step, deposition occurs during how P-CVI can be used to produce CMCs with a given time(the residence time, tg). This elemen ilored interphases at the nanometer scale, as well as tary sequence is repeated automatically vs. time, the self-healing matrices combining crack arrestor and inlet and outlet valves ng and closing being glass-former layers controlled with a computer. If T, P, tg are low enough, deposition can occur atomic layer by atomic layer. Finally, if one periodically switches from one 2. Experimental given precursor(C,H, for carbon) to a different precursor (MTS-H,, for SiC), a multilayered de In conventional CVI, a porous fiber substrate(the yC-SiC), is formed, in which the thickness preform) is set in a hot-wall deposition chamber of the sublayers can be as low as a few nanometers fed with a gaseous precursor flowing under a con- Different substrates have been used including: (i) stant pressure. Carbon, SiC and BN are deposited model pores open at both ends(length: 20 mm) with from hydrocarbons C,H,, CH; SiCI,(MTS)-H2 and rectangular cross sections(60X 2000, 120X 2000 depositi O: chamber 128 furnace Pumps MrS Fig. I. P-CVI apparatus for the deposition of carbon, silicon carbide or(PyC-SiC)n multilayers(1: sample, 2: electrical furnace; 3: precursors; 4: MTS vaporization chamber; 5: liquid nitrogen traps; 6: evacuated tank; 7: rotary vacuum pump)
542 R.R. Naslain et al.rSolid State Ionics 141–142 2001 541–548 ( ) steps is repeated vs. time. Each sequence comprises: Ž. Ž . i the evacuation of the deposition chamber, ii the injection of the gaseous reactant s and iii a hold- Ž. Ž . ing period during which deposition occurs. P-CVI has been first introduced as a way to shorten the densification 8–11 . Then, it has been shown that w x P-CVI could be used to control the maturation of the gas phase and, hence, the microtexture of, e.g. pyrocarbon deposits 12–14 . More recently, P-CVI has w x been used to tailor the composition and microstructure of multilayered interphases and matrices w x 6,7,15–17 . The aim of the present contribution is to show how P-CVI can be used to produce CMCs with tailored interphases at the nanometer scale, as well as self-healing matrices combining crack arrestor and glass-former layers. 2. Experimental In conventional CVI, a porous fiber substrate the Ž preform is set in a hot-wall deposition chamber . fed with a gaseous precursor flowing under a constant pressure. Carbon, SiC and BN are deposited from hydrocarbons C H , CH SiCl MTS –H and Ž . x y 33 2 BX –NH with X 3 3 Ž . sF, Cl , respectively. Under low temperature Ž. Ž . T and pressure P , the process yields a deposit of high microstructural quality and the preform porosity can be almost totally filled with the deposit 1 . w x In P-CVI, the gaseous precursor is periodically injected in the deposition chamber as short pressure pulses Fig. 1 . Each pressure pulse corresponds to Ž . the following sequence of events. First, the deposition chamber is evacuated by pumping. Then, the gaseous precursor is injected in the reactor and its internal pressure reaches almost instantaneously a preset value. In a third step, deposition occurs during a given time the residence time, Ž . t . This elemen- R tary sequence is repeated automatically vs. time, the inlet and outlet valves opening and closing being controlled with a computer. If T, P, tR are low enough, deposition can occur atomic layer by atomic layer. Finally, if one periodically switches from one given precursor C H for carbon to a different Ž . x y precursor MTS–H , for SiC , a multilayered de- Ž . 2 posit PyC–SiC is formed, in which the thickness Ž .n of the sublayers can be as low as a few nanometers. Different substrates have been used including: iŽ . model pores open at both ends length: 20 mm with Ž . rectangular cross sections 60 Ž =2000, 120=2000 Fig. 1. P-CVI apparatus for the deposition of carbon, silicon carbide or PyC–SiC multilayers 1: sample; 2: electrical furnace; 3: Ž. Ž n precursors; 4: MTS vaporization chamber; 5: liquid nitrogen traps; 6: evacuated tank; 7: rotary vacuum pump
R.R. Naslain et al/ Solid State ionics 141-142(2001)541-548 and 320 X 2000 um2)machined in purified graphite, conventional CVI [1]. It also depends upon an easily (ii)single SiC fibers(Nicalon NLM-202 or Hi-Nica- adjustable parameter, the residence time, fg.Further, lon fibers, 12-15 um in diameter, from Nippon tg also controls the maturation of the gas phase, i.e arbon, Tokyo), attached to rectangular SiC holder, the formation of intermediate species, and, hence for P-CVD experiments, (i single carbon (T300 the microtexture of the pyrocarbon deposited. The from Toray, Tokyo) or SiC (Nicalon or Hi-Nicalon) influence of T, P and Ig on pyrocarbon deposition fiber tows, also attached to carbon or SiC holders, was studied through experiments performed on the for the preparation of model ' minicompositesand model pores finally, (iv) real carbon or SiC fiber preforms dis- The thickness profile of pyrocarbon deposited chitecture with an initial open porosity of 602%er ar- from propane along the 60-um pore(drawn for a playing a two(or pseudo-two)-dimensional fibe time corresponding to pore entrance blocking) de- Carbon was deposited from C,H, hydrocarbons pends on fg(Fig 2a). The related pore filling ratio 8 ainly propane but methane, propene or benzene, (calculated by integrating the thickness profile along have also been used for the purpose of comparison) the pore length and dividing the result by half the at P=1-10 kPa, T=900-1100@C and Ig=0.2-60 initial pore volume) goes through a minimum for s. Silicon carbide was deposited from MTS-H, at intermediate tg values(Fig 2b). The highest pore 900-1000C and IR=1-10 S In filling is achieved for the lowest residence times, i.e order to form smooth deposits, i.e. deposits with a when the gas phase undergoes little maturation, py nanocrystalline microstructure, SiC in multilayered rocarbon being formed from small species, which ceramics, was codeposited with 3% carbon(to penetrate easily the pore. Conversely, for intermedi prevent SiC grain growth), with a=H2/MTS ate IR values, aromatic species are thought to be formed in the gas phase, which are preferentially precursor, at P=2-5 kPa, T=900-1000C, IR adsorbed on the pore wall near the pore entrance, 0. 1-5 s and B=NH3/BCl3=1. 2. For the deposi- resulting in an early pore sealing and a low filling tion of multilayers, temperature was maintained con- ratio. For high tg values corresponding to an impor stant and the thickness of the layers controlled tant gas phase maturation, the amount of aromatic through the residence times and the numbers and species is thought to decrease (gas phase reactant durations of pressure pulses depletion and hydrogen concentration increase)to he deposits were characterized according to vari- the benefit of smaller species diffusing readily along ous techniques including, electron probe microanaly the pore and yielding again a relatively high pore sis(EPMA), Auger electron spectroscopy (AES), filling. Pore filling ratio increases as P decreases,a X-ray diffraction (XRD), transmission and scanning pressure of about I kPa being necessary to com electron microscopies(TEM and SEM). The anisot- pletely fill the 60-um pore at T=950C for t=10 ropy of the pyrocarbon was assessed by optical S Increasing T favors pore entrance plugging partic- microscopy in polarized light and the so-called ex- ularly when the pore size is small. An optimum tinction angle, Ae, measured. Pyrocarbon was re- temperature seems to be about 950C. Finally, the smooth laminar (SL) for 120<A<18, dark lami- ness profile and filling ratio, when propane(or nar(DL)for 4<A< 12 and, finally, isotropic(d propylene) is replaced by methane [13] when a<4°[14 The anisotropy of pyrocarbon deposited along a pore can be controlled by playing with the T, P and tR parameters, I.e. with the maturation of the gas 3. Results and discussion 3.1. P-CV as a way to control pyrocarbon microt and P=3 kPa, pyrocarbon is deposited along the ture pore: (i) with a homogeneous microtexture, either poorly anisotropic (SL/DL; A=12%) or highly The PyC densification of a pore by P-cvi de- anisotropic (RL, Ae=20%), for IR=0.5 and 10 S, pends on P and T, as previously reported, for respectively, or (ii) with an anisotropy gradient fo
R.R. Naslain et al.rSolid State Ionics 141–142 2001 541–548 ( ) 543 2 and 320=2000 mm machined in purified graphite, . Ž. Ž ii single SiC fibers Nicalon NLM-202 or Hi-Nicalon fibers, f12–15 mm in diameter, from Nippon Carbon, Tokyo , attached to rectangular SiC holder, . for P-CVD experiments, iii single carbon T300 Ž. Ž from Toray, Tokyo or SiC Nicalon or Hi-Nicalon .Ž . fiber tows, also attached to carbon or SiC holders, for the preparation of model ‘minicomposites’ and, finally, iv real carbon or SiC fiber preforms dis- Ž . playing a two or pseudo-two -dimensional fiber ar- Ž . chitecture with an initial open porosity of 60%. Carbon was deposited from C H hydrocarbons x y Žmainly propane but methane, propene or benzene, have also been used for the purpose of comparison. at Ps1–10 kPa, Ts900–11008C and tRs0.2–60 s. Silicon carbide was deposited from MTS–H at 2 Ps1–5 kPa, Ts900–10008C and tRs1–10 s. In order to form smooth deposits, i.e. deposits with a nanocrystalline microstructure, SiC in multilayered ceramics, was codeposited with f3% carbon toŽ prevent SiC grain growth , i.e. with . asH2rMTS -1. Finally, BN was deposited from BCl –NH 3 3 precursor, at Ps2–5 kPa, Ts900–10008C, tRs 0.1–5 s and bsNH rBCl s1.2. For the deposi- 3 3 tion of multilayers, temperature was maintained constant and the thickness of the layers controlled through the residence times and the numbers and durations of pressure pulses. The deposits were characterized according to various techniques including, electron probe microanalysis EPMA , Auger electron spectroscopy AES , Ž . Ž. X-ray diffraction XRD , transmission and scanning Ž . electron microscopies TEM and SEM . The anisot- Ž . ropy of the pyrocarbon was assessed by optical microscopy in polarized light and the so-called extinction angle, A , measured. Pyrocarbon was re- e ferred to as rough laminar RL when Ž . A )188, e smooth laminar SL for 12 Ž . 8-A -188, dark lami- e nar DL for 4 Ž . Ž. 8-A -128 and, finally, isotropic I e when A -48 w x 14 . e 3. Results and discussion 3.1. P-CVI as a way to control pyrocarbon microtexture The PyC densification of a pore by P-CVI depends on P and T, as previously reported, for conventional CVI 1 . It also depends upon an easily w x adjustable parameter, the residence time, t . Further, R t also controls the maturation of the gas phase, i.e. R the formation of intermediate species, and, hence, the microtexture of the pyrocarbon deposited. The influence of T, P and tR on pyrocarbon deposition was studied through experiments performed on the model pores. The thickness profile of pyrocarbon deposited from propane along the 60-mm pore drawn for a Ž time corresponding to pore entrance blocking de- . pends on tR Ž . Fig. 2a . The related pore filling ratio d Žcalculated by integrating the thickness profile along the pore length and dividing the result by half the initial pore volume goes through a minimum for . intermediate t values Fig. 2b . The highest pore Ž . R filling is achieved for the lowest residence times, i.e. when the gas phase undergoes little maturation, pyrocarbon being formed from small species, which penetrate easily the pore. Conversely, for intermediate t values, aromatic species are thought to be R formed in the gas phase, which are preferentially adsorbed on the pore wall near the pore entrance, resulting in an early pore sealing and a low filling ratio. For high t values corresponding to an impor- R tant gas phase maturation, the amount of aromatic species is thought to decrease gas phase reactant Ž depletion and hydrogen concentration increase to . the benefit of smaller species diffusing readily along the pore and yielding again a relatively high pore filling. Pore filling ratio increases as P decreases, a pressure of about 1 kPa being necessary to completely fill the 60-mm pore at Ts9508C for tRs10 s. Increasing T favors pore entrance plugging particularly when the pore size is small. An optimum temperature seems to be about 9508C. Finally, the quality of the infiltration is better, in terms of thickness profile and filling ratio, when propane or Ž propylene is replaced by methane 13 . . w x The anisotropy of pyrocarbon deposited along a pore can be controlled by playing with the T, P and t parameters, i.e. with the maturation of the gas R phase during the holding step Fig. 3 . At Ž . Ts9508C and Ps3 kPa, pyrocarbon is deposited along the pore: i with a homogeneous microtexture, either Ž . poorly anisotropic SL Ž . rDL; Aes128 or highly anisotropic RL; Ž . A s208 , for t s0.5 and 10 s, e R respectively, or ii with an anisotropy gradient for Ž
R.R. Naslain et al/ Solid State ionics 141-142(2001)541-548 0.5s 15 0123 distance (mm) ng. 2. Densification of a model pore(60 x 2000 um2 in cross section; 20 mm in length) open at both ends by P-CVI from propane at 050%C and P=l kPa:(a)pyrocarbon thickness profiles along the pore length for tR=0.5, 5 and 60 s, (b)pore filling ratio as a function of Ig, according to Ref. [13]. fR=60 s. These features could tentatively be ex- deposit might be low molecular weight hydrogenated plained as follows. When residence time is very radicals, yielding a poorly organized pyrocarbon short, the maturation process is very limited and (SL/DL). When fg is raised, the maturation process the species in the gas phase responsible for the is more significant and the pyrocarbon highly anisotropic(RL). TEM data show that the size of the carbon layers is relatively large, i.e. 1.9 nm as a mean at pore center(with some layers as large as 4 Ingesting an echanism from gaseous species still limited in size(to diffuse far from pore entrance before reacting). Finally long residence times, the increase in hydrogen con- centration at pore center, might again favor the formation of hydrogenated species of relatively small molecular weights, yielding pyrocarbon of lower ani sotropy(Ae=16). Further, the anisotropy and oxi dation resistance of pyrocarbon can be improved by doping the gas phase with a boron-bearing gaseous 0.5s species [18] 0 1 2 34 5 6 78 9 10 3.2. P-Cl as a way to tailor multilayered inter- depth(mm) Anisotropy of pyrocarbon deposited along a 60-um model ore(with rectangular cross section) from propane at T=950/C and P=3 kPa for different residence times (Ig=0.5, 10 and 60 From a mechanical standpoint, anisotropic pyr s). The anisotropy is expressed in terms of extinction angle, A rbon is probably the best FM interphase for non- oxide CMCs [2-5]. Unfortunately, it is oxidation
544 R.R. Naslain et al.rSolid State Ionics 141–142 2001 541–548 ( ) Ž 2 Fig. 2. Densification of a model pore 60=2000 mm in cross section; 20 mm in length open at both ends by P-CVI from propane at . Ts10508C and Ps1 kPa: a pyrocarbon thickness profiles along the pore length for Ž . t s0.5, 5 and 60 s, b pore filling ratio as a Ž . R function of t , according to Ref. 13 . w x R t s60 s. These features could tentatively be ex- R plained as follows. When residence time is very short, the maturation process is very limited and the species in the gas phase responsible for the Fig. 3. Anisotropy of pyrocarbon deposited along a 60-mm model pore with rectangular cross section from propane at Ž . Ts9508C and Ps3 kPa for different residence times ŽtRs0.5, 10 and 60 s . The anisotropy is expressed in terms of extinction angle, . Ae w x 14 . deposit might be low molecular weight hydrogenated radicals, yielding a poorly organized pyrocarbon Ž . SLrDL . When t is raised, the maturation process R is more significant and the pyrocarbon highly anisotropic RL . TEM data show that the size of the Ž . carbon layers is relatively large, i.e. 1.9 nm as a mean at pore center with some layers as large as 4 Ž nm , suggesting an in situ lateral growth mechanism . from gaseous species still limited in size to diffuse Ž far from pore entrance before reacting . Finally, for . long residence times, the increase in hydrogen concentration at pore center, might again favor the formation of hydrogenated species of relatively small molecular weights, yielding pyrocarbon of lower anisotropy Ž . A s168 . Further, the anisotropy and oxi- e dation resistance of pyrocarbon can be improved by doping the gas phase with a boron-bearing gaseous species 18 . w x 3.2. P-CVI as a way to tailor multilayered interphases From a mechanical standpoint, anisotropic pyrocarbon is probably the best FM interphase for nonoxide CMCs 2–5 . Unfortunately, it is oxidation w x
R.R. Naslain et al/ Solid State Ionics 141-142(2001)541-548 prone, its oxidation starts at about 450C. As a result, the fiber is exposed to the oxidizing atmo- sphere (oxygen diffusing along the annular pore formed around the fiber) and the relatively weak FN bonding due to pyrocarbon is either totally destroyed or replaced by a strong FM coupling due to silica formed by the oxidation of the annular pore wall, depending on temperature [ 19]. In the first case, the omposite remains tough, but its failure strength is low. whereas in the second. it becomes brittle. In order to improve the oxidation resistance, it has been suggested to replace part of the pyrocarbon by a F glass former, such as SiC, to yield multilayered 500nm interphases [2-5]. In (PyC-SiC) interphases, as well as in their(BN-SiC)n counterparts, pyrocarbon Fig. 5.(PyC-SiC)multilayered se in Hi-Nicalon(F)/ (or BN) acts as mechanical fuse and Sic as glass omposite(TEM image)deposited by P-CVD from former, the silica-based liquid /glass filling cracks P roane and MTS-H, at T=900C. The deposition conditions and slowing down the in-depth diffusion of oxygen. s and a-1/4 for the Sic(c) layers of the interphase; and P=3 P-CVI is a very suitable technique to form multilay Pa, IR=2 s and a=3 for the SiC matri ered ceramics in a porous body, i.e. a fiber preform ts effectiveness has been assessed both for micro- composites'(deposition on a single fiber)[15, 16] and minicomposites'(infiltration in a single fiber tow). SiC), interphase, a given number of C H, hydrocar- The nature of the gaseous precursor is periodi- bon pressure pulses followed by a given number of cally changed vS. time to produce a multilayered MTS-H, pressure pulses, are repeatedly injected in deposit by P-CVI (or P-CVD)(Fig 4). For a(PyC- the deposition chamber, vS time. For low T, P and R, the deposit thickness per pulse can be lower than I nm and, hence, multilayered ceramics comprising extremely thin layers can be deposited on fibers. An SiC deposition y C deposition examI deposited on a single fiber is shown in Fig. 5. The overall interphase thickness is =420 nm and, hence, Qo闷 that of the elementary PyC-SiC sequence about 42 nm. The thickness of the PyC sublayer is about 10 nm(which has required five pressure pulses of propane)and that of its SiC counterpart about 30 nm (5 MTS-H, pressure pulses). The smooth interphase morphology has been achieved by selecting 7-P-a conditions yielding nanocrystalline SiC +C sublay- ers. The TEM picture has been recorded from a microcomposite containing a few SiC matrix cracks Such cracks are nicely deflected by each pyrocarbon sublayer and ultimately by that deposited first on the fiber surface, the fiber remaining unbroken [15, 16] Fig. 4. Deposition of (PyC-SiC), interphase P-CVD/CVI Multilayered(Py C-SiC), and(BN-SiC)inter from propane(PyC deposition)and MTS-H,(SiC(C)deposition) (gas introduction(0.3 s);() deposition step(SiC(C). ig=1-20 phases were also infiltrated in Hi-Nicalon tows(each PyC: Ig-5-30 s), (d gas evacuation (2.5 s) and (v) vacuum tow comprising =500 fibers) and the lifetime at holding (30 s) high temperature in air of the SiC/SiC minicompos-
R.R. Naslain et al.rSolid State Ionics 141–142 2001 541–548 ( ) 545 prone, its oxidation starts at about 4508C. As a result, the fiber is exposed to the oxidizing atmosphere oxygen diffusing along the annular pore Ž formed around the fiber and the relatively weak FM . bonding due to pyrocarbon is either totally destroyed or replaced by a strong FM coupling due to silica formed by the oxidation of the annular pore wall, depending on temperature 19 . In the first case, the w x composite remains tough, but its failure strength is low, whereas in the second, it becomes brittle. In order to improve the oxidation resistance, it has been suggested to replace part of the pyrocarbon by a glass former, such as SiC, to yield multilayered interphases 2–5 . In PyC–SiC interphases, as w x Ž .n well as in their BN–SiC counterparts, pyrocarbon Ž .n Ž . or BN acts as mechanical fuse and SiC as glass former, the silica-based liquidrglass filling cracks and slowing down the in-depth diffusion of oxygen. P-CVI is a very suitable technique to form multilayered ceramics in a porous body, i.e. a fiber preform. Its effectiveness has been assessed both for ‘microcomposites’ deposition on a single fiber 15,16 and Ž . w x ‘minicomposites’ infiltration in a single fiber tow . Ž . The nature of the gaseous precursor is periodically changed vs. time to produce a multilayered deposit by P-CVI or P-CVD Fig. 4 . For a PyC– Ž .Ž . Ž Fig. 4. Deposition of PyC–SiC interphase by P-CVD Ž . rCVI n from propane PyC deposition and MTS–H SiC C deposition : Ž . Ž Ž. . 2 Ž. Ž . Ž . Ž Ž . I gas introduction 0.3 s ; II deposition step SiC C : tRs1–20 s; PyC: t s5–30 s ; III gas evacuation 2.5 s and IV vacuum .Ž . Ž . Ž . R holding 30 s . Ž . Fig. 5. PyC–SiC multilayered interphase in Hi-Nicalon F Ž . Ž. 10 r SiC M microcomposite TEM image deposited by P-CVD from Ž. Ž . propane and MTS–H at Ts9008C. The deposition conditions 2 were: Ps3 kPa and tR R s5 s for pyrocarbon; Ps5 kPa, t s10 s and a s1r4 for the SiC C layers of the interphase; and Ž . Ps3 kPa, t s2 s and a s3 for the SiC matrix. R SiC interphase, a given number of C H hydrocar- .n xy bon pressure pulses followed by a given number of MTS–H pressure pulses, are repeatedly injected in 2 the deposition chamber, vs. time. For low T, P and tR, the deposit thickness per pulse can be lower than 1 nm and, hence, multilayered ceramics comprising extremely thin layers can be deposited on fibers. An example of a PyC–SiC multilayered interphase Ž .10 deposited on a single fiber is shown in Fig. 5. The overall interphase thickness is f420 nm and, hence, that of the elementary PyC–SiC sequence about 42 nm. The thickness of the PyC sublayer is about 10 nm which has required five pressure pulses of Ž propane and that of its SiC counterpart about 30 nm . Ž . 5 MTS–H pressure pulses . The smooth interphase 2 morphology has been achieved by selecting T–P–a conditions yielding nanocrystalline SiCqC sublayers. The TEM picture has been recorded from a microcomposite containing a few SiC matrix cracks. Such cracks are nicely deflected by each pyrocarbon sublayer and ultimately by that deposited first on the fiber surface, the fiber remaining unbroken 15,16 . w x Multilayered PyC–SiC and BN–SiC inter- Ž .Ž . n n phases were also infiltrated in Hi-Nicalon tows each Ž tow comprising f500 fibers and the lifetime at . high temperature in air of the SiCrSiC minicompos-
R.R. Naslain et al/ Solid State ionics 141-142(2001)541-548 ites compared to that of similar minicomposites pre- multilayered interphases can actually be deposited pared with one single 100-nm pyrocarbon interphase. within a porous body by P-CVI. Further, the concept A TEM picture of such a multilayered interphase is of an interphase strongly bonded to the fiber surface shown in Fig. 6. The interphase was designed to and that of a multilayered interphase combining me- have an overall thickness of 700 nm (its thickness chanical fuses and glass formers, effectively increase being actually close to 600 nm), n=10(PyC-Sic) the oxidation resistance of SiC/SiC minicomposites sequences and elementary sublayer thicknesses of 20 nm for pyrocarbon and 50 nm for SiC(C). The fibers ere pretreated (proprietary treatment of Snecma) before depositing the interphase in order to achieve a 3.3. P-CV as a way to engineer self-healing matri- strong bonding between the fiber surface and the ces first pyrocarbon sublayer. Under such a condition cracks are again nicely deflected by each pyrocarbon The multilayering concept has been extended to sublayer [20]. The lifetimes of the minicomposites, the matrix itself through the infiltration of boron- as assessed through tensile static fatigue tests in air doped pyrocarbon mechanical fuses and various glass at 700@C under constant load (at a stress level higher formers, with a view to entrap oxygen 'far'from the than the proportional limit to microcrack the Sic fibers as it diffuses along the microcrack network matrix)were as follows: (i2 h for the minicompos- formed under load and, hence, to increase the oxida- ite with a 100-nm single Pyc interphase produced tion resistance from as-received fibers, (ii) 100 h for their counter- An example of such highly engineered composite part fabricated from pretreated fibers and (ii) 150 h is shown in Fig. 7. This minicomposite was fabri for the minicomposite prepared with the multilayered cated from a carbon fiber tow, from BX3-C H -H (PyC-SiC)o interphase from pretreated fibers. Simi- precursor for the boron-doped pyrocarbon(C(B)) lar results were obtained for minicomposites in which mechanical fuse and boron carbide (referred to as pyrocarbon was replaced by BN. Hence, complex Bu3 C2) glass former, on the one hand, and from MTS-H, precursor for the Sic glass former, on the other hand. The two first(C(B)/B13C2/C(B)/SiC) sequences(S, and S,)have a thickness of about 1 Matrix um, whereas the two others, S, and S4, display a ( Fiber surface thickness of 4 and 6 um, respectively(Fig. 7a, b) Each carbon fiber is actually coated by the S, and S, multilayers(single arrow) only(Fig. 7c). The third sequence coats a group of fibers( double arrow)and, finally, the last sequence(S4) acts as a multilayered seal-coating for the whole fiber tow(triple arrow) The lifetime of the minicomposite(tensile tests in air at 700%C under a constant 250-MPa stress). i.e. >70 h, was considerably improved with respect to that(1 h) of a conventional C/PyC/SiC minicomposite pyrocarbon interphase. Similar re- sults were obtained for real composites fabricated with a complex 2.5D fiber architecture [6]. These data show again how flexible and effective the p-cvi Fig. 6. Multiple matrix crack deflection in a Hi-Nicalon/(Pyc process is to engineer non-oxide ceramic matrices in SiC),o/Sic minicomposite prepared by P-CVI from propane and CMCs. In these matrices, oxygen diffusing along MIS-H ursors at T=950C under the following condi- extremely tortuous paths(tortuosi from and a-1/4 for the sic(C)), and P=3 kPa, Ig-2 s and a-6 the occurrence of multiple crack deflections)is en- for the SiC matrix(TEM picture from a longitudinal thin foil) trapped by forming liquid/ glass oxide phases with
546 R.R. Naslain et al.rSolid State Ionics 141–142 2001 541–548 ( ) ites compared to that of similar minicomposites prepared with one single 100-nm pyrocarbon interphase. A TEM picture of such a multilayered interphase is shown in Fig. 6. The interphase was designed to have an overall thickness of 700 nm its thickness Ž being actually close to 600 nm ,.Ž. ns10 PyC–SiC sequences and elementary sublayer thicknesses of 20 nm for pyrocarbon and 50 nm for SiC C . The fibers Ž . were pretreated proprietary treatment of Snecma Ž . before depositing the interphase in order to achieve a strong bonding between the fiber surface and the first pyrocarbon sublayer. Under such a condition, cracks are again nicely deflected by each pyrocarbon sublayer 20 . The lifetimes of the minicomposites, w x as assessed through tensile static fatigue tests in air at 7008C under constant load at a stress level higher Ž than the proportional limit to microcrack the SiC matrix were as follows: i 2 h for the minicompos- . Ž. ite with a 100-nm single PyC interphase produced from as-received fibers, ii 100 h for their counter- Ž . part fabricated from pretreated fibers and iii 150 h Ž . for the minicomposite prepared with the multilayered Ž . PyC–SiC interphase from pretreated fibers. Simi- 10 lar results were obtained for minicomposites in which pyrocarbon was replaced by BN. Hence, complex Fig. 6. Multiple matrix crack deflection in a Hi-NicalonrŽPyC– SiC. 10 rSiC minicomposite prepared by P-CVI from propane and MTS–H precursors at Ts9508C under the following condi- 2 tions: Ps3 kPa, t s5 s for pyrocarbon; Ps5 kPa, t s1 s R R and a s1r4 for the SiC C ; and Ž . Ps3 kPa, tRs2 s and a s6 for the SiC matrix TEM picture from a longitudinal thin foil . Ž . multilayered interphases can actually be deposited within a porous body by P-CVI. Further, the concept of an interphase strongly bonded to the fiber surface and that of a multilayered interphase combining mechanical fuses and glass formers, effectively increase the oxidation resistance of SiCrSiC minicomposites w x 7 . 3.3. P-CVI as a way to engineer self-healing matrices The multilayering concept has been extended to the matrix itself through the infiltration of borondoped pyrocarbon mechanical fuses and various glass formers, with a view to entrap oxygen ‘far’ from the fibers as it diffuses along the microcrack network formed under load and, hence, to increase the oxidation resistance. An example of such highly engineered composite is shown in Fig. 7. This minicomposite was fabricated from a carbon fiber tow, from BX –C H –H 3 x y 2 precursor for the boron-doped pyrocarbon C B Ž Ž .. mechanical fuse and boron carbide referred to as Ž B C glass former, on the one hand, and from . 13 2 MTS–H precursor for the SiC glass former, on the 2 other hand. The two first C B ŽŽ. Ž. . rB C13 2rC B rSiC sequences S and S have a thickness of about 1 Ž . 1 2 mm, whereas the two others, S and S , display a 3 4 thickness of 4 and 6 mm, respectively Fig. 7a,b . Ž . Each carbon fiber is actually coated by the S and S 1 2 multilayers single arrow only Fig. 7c . The third Ž .Ž . sequence coats a group of fibers double arrow and, Ž . finally, the last sequence S acts as a multilayered Ž . 4 seal-coating for the whole fiber tow triple arrow . Ž . The lifetime of the minicomposite tensile tests in air Ž at 7008C under a constant 250-MPa stress , i.e. . )70 h, was considerably improved with respect to that 1Ž h of a conventional C . rPyCrSiC minicomposite with one single pyrocarbon interphase. Similar results were obtained for real composites fabricated with a complex 2.5D fiber architecture 6 . These w x data show again how flexible and effective the P-CVI process is to engineer non-oxide ceramic matrices in CMCs. In these matrices, oxygen diffusing along extremely tortuous paths tortuosity resulting from Ž the occurrence of multiple crack deflections is en- . trapped by forming liquidrglass oxide phases with
R.R. Naslain et al/ Solid State ionics 141-142(2001)541-548 7 um S1S2 fiber Ms3$4 (b) Fig. 7. SEM micrographs of a model minicomposite fabricated by P-CVi from carbon fiber tow with a multilayered self-healing matrix: (a) detail of the S, and S, multilayers deposited on a single carbon fiber, b) the full S, to s, multilayer sequence and (c) cross section of the composite(partial view)[6] the Bu3C2(at low temperature)and SiC (at higher Acknowledgements temperature)glass former layers. These fluid oxide phases fill the microcracks, thus, increasing the life This work has been supported by CNRS through time of the composites under load PhD or post-doctoral grants given to S.B., F.H., P.D. 4. Conclusion References From the data presented in Section 3, the follow- ing conclusions can be drawn: (i) P-CVI is a novel [I] R. Naslain, F. Langlais, Mater. Sci. Res. 20(1986)145 processing technique suitable for infiltrating single 2) R, Naslain, Ceram. Trans. 58(1995)23 or multilayered interphases and matrices, in fiber [4]R.R. Naslain, Compos. Part A 29A(1998)1157. preforms, (i) multilayered interphases and matrices [5]R.R. Naslain, Ceram. Tr combining mechanical fuse layers(PyC or BN) and [6] F Lamouroux, S Bertrand, R. Pailler, R. Naslain, M Cataldi, glass-former layers(boron or/and silicon carbides) Comp. Sci. Technol. 59(1999)1073 improve the oxidation resistance in air of non-oxide nd, P. Forio, R. Pailler, J. Lamon, J. Am. Ceram CMCS. It is anticipated that P-CVI could be ex Soc.82(9)(1992465 [8]R L. Beattly, J. Nucl. Appl. Technol. 8(1970)45 tended to other multilayered ceramics and porous [9] W.A. Bryant, J Cryst. Growth 35(1976)257 bodie [10] K. Sugiyama, T. Nakamura, J. Mater. Sci. Lett. 6(1987)331
R.R. Naslain et al.rSolid State Ionics 141–142 2001 541–548 ( ) 547 Fig. 7. SEM micrographs of a model minicomposite fabricated by P-CVI from carbon fiber tow with a multilayered self-healing matrix: aŽ . detail of the S and S multilayers deposited on a single carbon fiber, b the full S to S multilayer sequence and c cross section of the Ž. Ž. 1 2 1 4 composite partial view 6 . Ž . w x the B C at low temperature and SiC at higher Ž .Ž 13 2 temperature glass former layers. These fluid oxide . phases fill the microcracks, thus, increasing the lifetime of the composites under load. 4. Conclusion From the data presented in Section 3, the following conclusions can be drawn: i P-CVI is a novel Ž . processing technique suitable for infiltrating single or multilayered interphases and matrices, in fiber preforms, ii multilayered interphases and matrices Ž . combining mechanical fuse layers PyC or BN and Ž . glass-former layers boron or Ž . rand silicon carbides improve the oxidation resistance in air of non-oxide CMCs. It is anticipated that P-CVI could be extended to other multilayered ceramics and porous bodies. Acknowledgements This work has been supported by CNRS through PhD or post-doctoral grants given to S.B., F.H., P.D. and F.L. References w x 1 R. Naslain, F. Langlais, Mater. Sci. Res. 20 1986 145. Ž . w x 2 R. Naslain, Ceram. Trans. 58 1995 23. Ž . w x 3 R. Naslain, Ceram. Trans. 79 1996 37. Ž . w x 4 R.R. Naslain, Compos. Part A 29A 1998 1157. Ž . w x 5 R.R. Naslain, Ceram. Trans. 99 1998 167. Ž . w x 6 F. Lamouroux, S. Bertrand, R. Pailler, R. Naslain, M. Cataldi, Comp. Sci. Technol. 59 1999 1073. Ž . w x 7 S. Bertrand, P. Forio, R. Pailler, J. Lamon, J. Am. Ceram. Soc. 82 9 1999 2465. Ž .Ž . w x 8 R.L. Beattly, J. Nucl. Appl. Technol. 8 1970 488. Ž . w x 9 W.A. Bryant, J. Cryst. Growth 35 1976 257. Ž . w x 10 K. Sugiyama, T. Nakamura, J. Mater. Sci. Lett. 6 1987 331. Ž
548 R.R. Naslain et al/ Solid State ionics 141-142(2001)541-548 [11]SV. Sotirchos, AlChE J. 37(9)(1991)1365 [17] K.A. Appiah, Z.L. Wang, W.J. Lackey, J. Mater. Sci. 35 [12] P. Dupel, R. Pailler, F. Langlais, J. Mater. Sci. 29(1994) (2000)1979 [ 18]S. Jacques, A. Guette, F. Langlais, R. Naslain, J. Mater. S [13] P. Dupel, R. Pailler, X. Bourrat, R. Naslain, J Mater. Sci. 29 32(1997)983 (1994)1056 [19]L. Filipuzzi, R. Naslain, J. Am. Ceram Soc. 77(2)(1994) [14] P. Dupel, X. Bourrat, R. Pailler, Carbon 33(9)(1995)1193 [15] F. Heurtevent, R. Pailler, X. Bourrat, Carbon(in press) 20]S. Bertrand, C, Droillard, R. Pailler, X. Bourrat, R. Naslain [16] F. Heurtevent, R. Pailler, X. Bourrat, J. Mater. Sci.(in J. Eur. Ceram Soc. 20(2000)I
548 R.R. Naslain et al.rSolid State Ionics 141–142 2001 541–548 ( ) w x 11 S.V. Sotirchos, AIChE J. 37 9 1991 1365. Ž .Ž . w x 12 P. Dupel, R. Pailler, F. Langlais, J. Mater. Sci. 29 1994 Ž . 1341. w x 13 P. Dupel, R. Pailler, X. Bourrat, R. Naslain, J. Mater. Sci. 29 Ž . 1994 1056. w x 14 P. Dupel, X. Bourrat, R. Pailler, Carbon 33 9 1995 1193. Ž .Ž . w x 15 F. Heurtevent, R. Pailler, X. Bourrat, Carbon in press . Ž . w x 16 F. Heurtevent, R. Pailler, X. Bourrat, J. Mater. Sci. inŽ press .. w x 17 K.A. Appiah, Z.L. Wang, W.J. Lackey, J. Mater. Sci. 35 Ž . 2000 1979. w x 18 S. Jacques, A. Guette, F. Langlais, R. Naslain, J. Mater. Sci. 32 1997 983. Ž . w x 19 L. Filipuzzi, R. Naslain, J. Am. Ceram. Soc. 77 2 1994 Ž .Ž . 467. w x 20 S. Bertrand, C. Droillard, R. Pailler, X. Bourrat, R. Naslain, J. Eur. Ceram. Soc. 20 2000 1. Ž
