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 atmo￾sphere 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 multilay￾ered ceramics in a porous body, i.e. a fiber preform. Its effectiveness has been assessed both for ‘micro￾composites’ 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 periodi￾cally 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 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 . 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-