species: propane (Qc Hs), hydrogen(QH and MTs sublayers has been limited to 0. 1 um or less for oxidation (QMS) resistance considerations(which will be not developed MTS, being a liquid at room temperature, is evapo- here). rated in a boiler set in a drying oven and then, the MTS(g is mixed with hydrogen before being injected in 2. 2. Pressure pulsed-CVl. nanometric scale multilayers the infiltration chamber. The composition of the gas phase used in the infiltration of Sic is characterised by a The nanometric scale multilayered interphases have dilution a-ratio defined as been fabricated by pressure pulsed CVD/CVI (P-CVD/ P-CVI) process. The apparatus used for the fabrication PH, QH of the materials and the experimental conditions have been described in another article 20 In this process, the operating pressure is pi acting gas is allowed by an upper inlet pneumatic valve, up to where Pi and Qi (with 1= H2 or MTS)are the partial the operating pressure. Then the furnace is closed dur- pressure and gas flowrate of species i, respectively. The ing a residence time, Ir and, finally, it is evacuated experiments have been carried out under conditions through an outlet pneumatic valve, and cooled traps by typical of the I-CVi process, which have been discussed using a rotary pump. A computer is used to monitor elsewhere valves'opening and closing, safety devices and the total Two different series of 2D-SiC/SiC composites have amount of pulses been prepared. In the first series, the Nicalon fabrics Hi-Nicalon bundles(from Nippon Carbon Company were used as-received (materials A, C, G, and K in Ltd, Tokyo, Japan), were used for the fabrication of Table 1)whereas, in the second series(materials B, D, SiC/Sic minicomposites(a minicomposite is a model H, and L), the Nicalon fabrics have received a treatment ID composite with one single fibre tow). In order to (proprietary treatment performed by SEP, Bordeaux) change the interfacial bonding strength two series of prior to the infiltration of the multilayered interphase, reinforcement were systematically utilised: in the first in order to improve the fibre-matrix bonding. series, tows were used as-received (i.e. non treated The nature of the various multilayered interphases fibres) whereas, in the second series, the tows were pre- deposited by I-CVI on the fibre surface is shown in viously submitted to a treatment (so-called treated Table 1. The interphases exhibit the following features: fibres) performed at the Laboratory, prior to the infil (i the first sublayer (i.e. that in contact with the fibre tration (or deposition) of the multilayered interphase surface)is always a pyrocarbon sublayer and the inter- The nature of the various multilayered interphase is facial sequence can be written as(PyC/SiC)n, (ii)when hown in table 2 n>1, the Sic of the last sequence is that of the matrix, (ii) the overall thickness of the multilayered interphase 2.3. Microstructural characterisation is constant and equal to 0.5 um and (iv) the thicknesses of the C and Sic sublayers are either maintained con- Microstructure of the multil stant(materials evolutive(materials K, L) ssessed by Transmission Electron Microscopy(TEM within the interphase. Finally, the thickness of the Pyc CM30ST/PEELS from Philips). TEM analyses were performed on sample cross-sections, perpendicular to the axis of the fibres as well as longitudinal sections Table TEM specimen sampling has been previously detailed in Material references and nature of the multilayered interphases of the 2D Nicalon/ SiC composites, processed by I-CVI a companion paper Optical microscopy in polarised light(MeF3 from Materials Nature of Nature of the C-SiC sequence in Reichert-Jung) was used to measure the pyrocarbon the fabrics the interphase and thickness(in um) anisotropy following the extinction angle technique F/C/SiC/C/Md (Ae), fully described elsewhere. 21 Ae was observed to 0.10.30.1 fall between I2°and14° corresponding to sm laminar(SL) and rough laminar(RL) pyrocarbon 0.10.10.10.10.1 X-Ray Diffraction (XRD, Siemens D5000)analysis 0.050.10050.10.050.10.05 was performed in order to evaluate the apparent crys tallite size in the [111] crystallographic direction (Lul) 0.050050.050.10.050.150.05 of the Sic grains by means of the Scherrer equation NT: not treated b T treated Auger Electron Spectroscopy (AES) microprobe equipped with an Ar+ sputtering gun(VG 310F)was used to record composition-depth profilesspecies: propane (QC3H8 ), hydrogen (QH2 ) and MTS (QMTS). MTS, being a liquid at room temperature, is evapo￾rated in a boiler set in a drying oven and then, the MTS…g† is mixed with hydrogen before being injected in the in®ltration chamber. The composition of the gas phase used in the in®ltration of SiC is characterised by a dilution -ratio de®ned as:  ˆ PH2 PMTS ˆ QH2 QMTS …3† where Pi and Qi (with i ˆ H2 or MTS) are the partial pressure and gas ¯owrate of species i, respectively. The experiments have been carried out under conditions typical of the I-CVI process, which have been discussed elsewhere.16±18 Two di€erent series of 2D-SiC/SiC composites have been prepared. In the ®rst series, the Nicalon fabrics were used as-received (materials A, C, G, and K in Table 1) whereas, in the second series (materials B, D, H, and L), the Nicalon fabrics have received a treatment (proprietary treatment performed by SEP, Bordeaux) prior to the in®ltration of the multilayered interphase, in order to improve the ®bre±matrix bonding.19 The nature of the various multilayered interphases deposited by I-CVI on the ®bre surface is shown in Table 1. The interphases exhibit the following features: (i) the ®rst sublayer (i.e. that in contact with the ®bre surface) is always a pyrocarbon sublayer and the inter￾facial sequence can be written as (PyC/SiC)n, (ii) when n > 1, the SiC of the last sequence is that of the matrix, (iii) the overall thickness of the multilayered interphase is constant and equal to 0.5 mm and (iv) the thicknesses of the C and SiC sublayers are either maintained con￾stant (materials G, H) or evolutive (materials K, L) within the interphase. Finally, the thickness of the PyC sublayers has been limited to 0.1 mm or less for oxidation resistance considerations (which will be not developed here). 2.2. Pressure pulsed-CVI: nanometric scale multilayers The nanometric scale multilayered interphases have been fabricated by pressure pulsed CVD/CVI (P-CVD/ P-CVI) process. The apparatus used for the fabrication of the materials and the experimental conditions have been described in another article.20 In this process, the operating pressure is pulsed. First, admission of reacting gas is allowed by an upper inlet pneumatic valve, up to the operating pressure. Then the furnace is closed dur￾ing a residence time, tr and, ®nally, it is evacuated through an outlet pneumatic valve, and cooled traps by using a rotary pump. A computer is used to monitor valves' opening and closing, safety devices and the total amount of pulses. Hi-Nicalon bundles (from Nippon Carbon Company Ltd., Tokyo, Japan), were used for the fabrication of SiC/SiC minicomposites (a minicomposite is a model 1D composite with one single ®bre tow). In order to change the interfacial bonding strength two series of reinforcement were systematically utilised: in the ®rst series, tows were used as-received (i.e. non treated ®bres) whereas, in the second series, the tows were pre￾viously submitted to a treatment (so-called treated ®bres) performed at the Laboratory, prior to the in®l￾tration (or deposition) of the multilayered interphase. The nature of the various multilayered interphase is shown in Table 2. 2.3. Microstructural characterisation Microstructure of the multilayers was essentially assessed by Transmission Electron Microscopy (TEM, CM30ST/PEELS from Philips). TEM analyses were performed on sample cross-sections, perpendicular to the axis of the ®bres as well as longitudinal sections. TEM specimen sampling has been previously detailed in a companion paper.15 Optical microscopy in polarised light (MeF3 from Reichert-Jung) was used to measure the pyrocarbon anisotropy following the extinction angle technique (Ae), fully described elsewhere.21 Ae was observed to fall between 12 and 14 corresponding to smooth laminar (SL) and rough laminar (RL) pyrocarbon. X-Ray Di€raction (XRD, Siemens D5000) analysis was performed in order to evaluate the apparent crys￾tallite size in the [111] crystallographic direction (L111) of the SiC grains by means of the Scherrer equation (k=0.9). Auger Electron Spectroscopy (AES) microprobe equipped with an Ar+ sputtering gun (VG 310F) was used to record composition-depth pro®les. Table 1 Material references and nature of the multilayered interphases of the 2D Nicalon/SiC composites, processed by I-CVI Materials Nature of the fabrics Nature of the C-SiC sequence in the interphase and thickness (in mm) A NTa Fc /C/SiC/C/Md B Tb 0.1 0.3 0.1 C NT F/C/C/SiC/M D T 0.1 0.1 0.1 0.1 0.1 G NT F/C/SiC/C/SiC/C/M H T 0.05 0.1 0.05 0.1 0.05 0.1 0.05 K NT F/C/SiC/C/SiC/C/M L T 0.05 0.05 0.05 0.1 0.05 0.15 0.05 a NT: not treated. b T: treated. c F: ®bre. d M: matrix. S. Bertrand et al. / Journal of the European Ceramic Society 20 (2000) 1±13 3