Fracture surfaces were examined by Scanning Electron 3.1. Pyrocarbon nanostructure Microscopy(FEG-SEM Hitachi $4500)at low voltage (3kv) Generally speaking, pyrocarbon resulting from the cracking of propane under the I-CVI conditions was characterised by a large value of the Lz-parameter, 3. Results--micrometer-scale multilayers as processed strong anisotropy and a low porosit by I-Cvi the lateral size of the aromatic carbon sheet in the tur bostratic stack as measured by Hr-tEm). Similar fea When seen in cross section, multilayers deposited by tures have been also reported for pyrocarbons resulting means of I-CVI exhibited rough and discontinuous from the cracking of propylene C3.23 sublayers(Fig. 2). The inset shows a low magnification The first pyrocarbon sublayer growth occurred of interphase"G" constituted by seven sublayers infil- directly onto the fibre surface whose composition was trated in an as-received 2d Nicalon nlm 202-based different for the two series of materials considered here preform. This sequenced ceramic material appears The bonding of carbon onto the fibre was seen to con- gh and disrupted. a close inspection at higher mag- trol the nature of the composite interface. 5Then, all nification revealed that carbon sublayers were system the subsequent pyrocarbon sublayers grew onto surfaces atically continuous, and that disruptions, when present, made of pure, well crystallised Sic which exhibited were related to the Sic sublayer crystallinity some roughness at the nanometric scale. As shown in Fig. 3, the Pyc deposit first filled the concave parts (formed by adjacent cone-like SiC crystals) of the SiC substrate. Then, at a distance, the PyC aromatic layers Material references and nature of the multilayered interphases of the tended to deposit parallel to the mean surface of the Hi-Nicalon/SiC minicomposites, processed by P-CVI coated fibre and exhibited a pronounced anisotropy Materials Nature of Nature of the C-SiC sequence in The analysis of the first carbon layers has not shown(on ne tows the interphase and thickness(in nm) the basis of the TEM images) any significant difference in the carbon organisation depending on the nature of 2050 the sublying Sic crystals F/(PyC/SiC)o/M 330 NT: non treated b T: treated Pyc sic 2 c2 10 naterial G (TEM contrasted brightfield): undulation of the layers related to the crystal finity of Sic. Inset is a low magnification of an equivalent area(same Fig. 3. Growth of the first pyrocarbon layers onto a well crystallised SiC surface: smoothing effect of carbon (high resolution TEM)Fracture surfaces were examined by Scanning Electron Microscopy (FEG-SEM Hitachi S4500) at low voltage (3 kV). 3. ResultsÐmicrometer-scale multilayers as processed by I-CVI When seen in cross section, multilayers deposited by means of I-CVI exhibited rough and discontinuous sublayers (Fig. 2). The inset shows a low magni®cation of interphase ``G'' constituted by seven sublayers in®l￾trated in an as-received 2D Nicalon NLM 202-based preform. This sequenced ceramic material appears rough and disrupted. A close inspection at higher mag￾ni®cation revealed that carbon sublayers were system￾atically continuous, and that disruptions, when present, were related to the SiC sublayer crystallinity. 3.1. Pyrocarbon nanostructure Generally speaking, pyrocarbon resulting from the cracking of propane under the I-CVI conditions was characterised by a large value of the L2-parameter, a strong anisotropy and a low porosity22 (L2 characterises the lateral size of the aromatic carbon sheet in the tur￾bostratic stack as measured by HR-TEM). Similar fea￾tures have been also reported for pyrocarbons resulting from the cracking of propylene C3H6. 23 The ®rst pyrocarbon sublayer growth occurred directly onto the ®bre surface whose composition was di€erent for the two series of materials considered here. The bonding of carbon onto the ®bre was seen to con￾trol the nature of the composite interface.15 Then, all the subsequent pyrocarbon sublayers grew onto surfaces made of pure, well crystallised SiC which exhibited some roughness at the nanometric scale. As shown in Fig. 3, the PyC deposit ®rst ®lled the concave parts (formed by adjacent cone-like SiC crystals) of the SiC￾substrate. Then, at a distance, the PyC aromatic layers tended to deposit parallel to the mean surface of the coated ®bre and exhibited a pronounced anisotropy. The analysis of the ®rst carbon layers has not shown (on the basis of the TEM images) any signi®cant di€erence in the carbon organisation depending on the nature of the sublying SiC crystals. Table 2 Material references and nature of the multilayered interphases of the Hi-Nicalon/SiC minicomposites, processed by P-CVI Materials Nature of the tows Nature of the C-SiC sequence in the interphase and thickness (in nm) 15 NTa Fc /(PyC/SiC)10/Md 54 Tb 20 50 45 NT F/(PyC/SiC)10/M 3 30 a NT: non treated. b T: treated. c F: ®bre. d M: matrix. Fig. 2. Cross-section of the interfacial sequence in material G (TEM contrasted bright®eld): undulation of the layers related to the crystal￾linity of SiC. Inset is a low magni®cation of an equivalent area (same technique). Fig. 3. Growth of the ®rst pyrocarbon layers onto a well crystallised SiC surface: smoothing e€ect of carbon (high resolution TEM). 4 S. Bertrand et al. / Journal of the European Ceramic Society 20 (2000) 1±13