《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_multilayered interphases-1

E噩≈3S Journal of the European Ceramic Society 20(2000)1-13 TEM structure of(PyC/SiC)n multilayered interphases in SiC/SiC composites S. Bertrand * C. Droillard.R. Pailler. X. Bourrat.R. Naslain Laboratoire des Composites Thermostructuraux, UMR 580/ CNRS-SEP/SNECMA-UBl, Universite bordeaux, 1-3 allee de la boetie, Received 28 January 1999; accepted 14 March 1999 Two generations of multilayered interphases, composed of carbon and silicon carbide, have been developed to act as a mechan- ical fuse in SiC/Sic composites with improved oxidation resistance. Pyrocarbon is an ideal interfacial material, from the mechanical point of view, whereas Sic has a good oxidation resistance. In the multilayered interphase, the carbon mechanical fuse is split into hin sublayers, each being protected against oxidation by the neighbouring Sic-based glass former layers. A first generation of multilayers as synthesised by means of isobaric-CVI with sublayers with micrometric thickness. Then, in order to push forward the concept, pressure pulsed-CVI was involved to deposit nanometric scale sublayers. In this work, transmission electron microscopy was developed to characterise the two generations of materials. The microstructure of the layers and the influence of the fibrous preforms on the structure of the layers were studied. Examinations were then performed on the loaded samples and damaging mode haracterised at nanometric scale. C 1999 Elsevier Science Ltd. All rights reserved Keywords: Composites; Electron microscopy: Interphase: SiC; Carbon 1. ntroduction types of interphase involving alternating thin layers of two different materials have been suggested: the laminar It is now well established that the mechanical beha ceramics, 6-10 viour of ceramic matrix composites(CMCs) with con- A breakthrough was achieved by Droillard et al I1, I tinuous fibre reinforcement depends not only on the demonstrating that(PyC/SiC)n multilayered material intrinsic properties of the fibre and the matrix, but also in 2D woven Nicalon SiC composites, behaves as an n the fibre-matrix bonding. -To control the strength efficient interfacial materials, but only if their bonding of the fibre-matrix bonding in CMCs, an additional to the fibre surface is reinforced. Fig. 1, published in a phase referred to as the interphase is used which serves previous paper I shows the tensile tests realised on 2D as a compliant layer between the fibre and matrix. Two Nicalon/SiC composites with different multilayered main functions are devoted to the interphase: first, load combinations. All the materials could be grouped into transfer between matrix and reinforcement and sec- two distinct families: (i) materials reinforced with ondly, control of the crack deflection at the interface. 4 untreated fibres have a weak fibre bonding and are The interphase is deposited on the fibre surface prior to characterised by a relatively low strength and a low interphase materials are pyrocarbon(PyC) and boron possess a stronger interface and are characterised ba the deposition of the matrix. The most commonly used toughness, whereas (i) materials with treated fibre nitride(BN). However, both of them are not stable high strength and a high toughness. As a result, when under oxidising conditions at high temperatures. New stronger interfaces were introduced, strength and concepts have been proposed to produce interphase that toughness were increased; in the mean time more than have both oxidation resistance and mechanical proper- 50% of the carbon was removed from the interfacial ties required to yield tough composites. 4.5 Also, new zone. In contrast, when the interface was weak, only the first carbon sublayer was involved in the fracture mechanism and ultimate performances remained s Corresponding author 0955-2219/99/.see front C 1999 Elsevier Science Ltd. All rights reserved PII:S0955-2219(99)00

TEM structure of (PyC/SiC)n multilayered interphases in SiC/SiC composites S. Bertrand*, C. Droillard, R. Pailler, X. Bourrat, R. Naslain Laboratoire des Composites Thermostructuraux, UMR 5801 CNRS-SEP/SNECMA-UB1, Universite Bordeaux, 1±3 alleÂe de la BoeÂtie, 33 600 Pessac, France Received 28 January 1999; accepted 14 March 1999 Abstract Two generations of multilayered interphases, composed of carbon and silicon carbide, have been developed to act as a mechan￾ical fuse in SiC/SiC composites with improved oxidation resistance. Pyrocarbon is an ideal interfacial material, from the mechanical point of view, whereas SiC has a good oxidation resistance. In the multilayered interphase, the carbon mechanical fuse is split into thin sublayers, each being protected against oxidation by the neighbouring SiC-based glass former layers. A ®rst generation of multilayers as synthesised by means of isobaric-CVI with sublayers with micrometric thickness. Then, in order to push forward the concept, pressure pulsed-CVI was involved to deposit nanometric scale sublayers. In this work, transmission electron microscopy was developed to characterise the two generations of materials. The microstructure of the layers and the in¯uence of the ®brous preforms on the structure of the layers were studied. Examinations were then performed on the loaded samples and damaging mode characterised at nanometric scale. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Composites; Electron microscopy; Interphase; SiC; Carbon 1. Introduction It is now well established that the mechanical beha￾viour of ceramic matrix composites (CMCs) with con￾tinuous ®bre reinforcement depends not only on the intrinsic properties of the ®bre and the matrix, but also on the ®bre±matrix bonding.1±3 To control the strength of the ®bre±matrix bonding in CMCs, an additional phase referred to as the interphase is used which serves as a compliant layer between the ®bre and matrix. Two main functions are devoted to the interphase: ®rst, load transfer between matrix and reinforcement and sec￾ondly, control of the crack de¯ection at the interface.4 The interphase is deposited on the ®bre surface prior to the deposition of the matrix. The most commonly used interphase materials are pyrocarbon (PyC) and boron nitride (BN). However, both of them are not stable under oxidising conditions at high temperatures. New concepts have been proposed to produce interphase that have both oxidation resistance and mechanical proper￾ties required to yield tough composites.4,5 Also, new types of interphase involving alternating thin layers of two di€erent materials have been suggested: the laminar ceramics.6±10 A breakthrough was achieved by Droillard et al.11,12 demonstrating that (PyC/SiC)n multilayered materials, in 2D woven Nicalon/SiC composites, behaves as an ecient interfacial materials, but only if their bonding to the ®bre surface is reinforced. Fig. 1, published in a previous paper,11 shows the tensile tests realised on 2D Nicalon/SiC composites with di€erent multilayered combinations. All the materials could be grouped into two distinct families: (i) materials reinforced with untreated ®bres have a weak ®bre bonding and are characterised by a relatively low strength and a low toughness, whereas (ii) materials with treated ®bres possess a stronger interface and are characterised by a high strength and a high toughness. As a result, when stronger interfaces were introduced, strength and toughness were increased; in the mean time more than 50% of the carbon was removed from the interfacial zone. In contrast, when the interface was weak, only the ®rst carbon sublayer was involved in the fracture mechanism11 and ultimate performances remained low. 0955-2219/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(99)00086-2 Journal of the European Ceramic Society 20 (2000) 1±13 * Corresponding author

400 B H 300 OTHON INPRPACP a25星星4 K C 200 0 WIX TERFACE 02 0.4 0.6 Longitudinal tensile strain(c) Fig. I. Tensile stress-strain curves obtained for 2D-SiCSiC composites with various multilayered interphases: B, D, H and L fabricated with treated Nicalon fibre; A, C, G and K fabricated with as-received Nicalon fibre(according to Refs [4] and [liD Pasquier3 has investigated the potentialities of multi- tooling. The components of the multilayered(Py C/SiC) layered(PyC/SiC)n interphases at the micrometer scale in interphases and the Sic-matrix were infiltrated within the Nicalon/SiC composites in terms of oxidation resistance porous fibre preforms, according to the isothermal/iso- Then, Heurtevent4 has developed the nanoscale multi- baric chemical vapour infiltration(I-CVI)process, which layered(PyC/SiC)n interphases in Hi-Nicalon/ SiC micro- has been described elsewhere. 6-18 Pyrocarbon and silicon composites and studied their behaviour at high carbide were deposited from propane C3Hs and methyl temperatures in oxidative conditions trichlorosilane(MTS) CH3siCl3/H2, respectively, accord The aim of the present paper is to char ng to the following overall equations: tructure of micro- and nano-scaled(Py C/SiC)n multi layered interphases. The particular aspect related to the CH3 SiC3(e) SiC+3HClgy treatment is fully described in a companion paper, s ace first interface and the influence of the fibre surf C3Hxg)→3C(+4H2g 2. Experimental procedure in a hot-wall chamber (internal diameter: 130 mm; 2.1. 2D-SiC/SiC materials obtained by 1-CVI cooled r f. coil (maximum temperature capability c1500.C). The apparatus has been designed to work The 2D-SiC/SiC composites were prepared as rectan- under reduced pressures(0.5<P<10 kPa), the total gular plates (130x 100x5 mm )from 2D-preforms con- pressure being maintained at a constant value with a sisting of stacks of Nicalon fabrics(NLM 202 ceramic pressure sensor(type 127A from MKS)and a pressure grade from Nippon Carbon Company Ltd, Tokyo, regulator(type 252A from MKS). Mass flowmeters were Japan)maintained pressed together with a graphite used to measure the flowrates of the various gaseous

Pasquier13 has investigated the potentialities of multi￾layered (PyC/SiC)n interphases at the micrometer scale in Nicalon/SiC composites in terms of oxidation resistance. Then, Heurtevent14 has developed the nanoscale multi￾layered (PyC/SiC)n interphases in Hi-Nicalon/SiC micro￾composites and studied their behaviour at high temperatures in oxidative conditions. The aim of the present paper is to characterise the structure of micro- and nano-scaled (PyC/SiC)n multi￾layered interphases. The particular aspect related to the ®rst interface and the in¯uence of the ®bre surface treatment is fully described in a companion paper.15 2. Experimental procedure 2.1. 2D-SiC/SiC materials obtained by I-CVI The 2D-SiC/SiC composites were prepared as rectan￾gular plates (1301005 mm3 ) from 2D-preforms con￾sisting of stacks of Nicalon fabrics (NLM 202 ceramic grade from Nippon Carbon Company Ltd., Tokyo, Japan) maintained pressed together with a graphite tooling. The components of the multilayered (PyC/SiC)n interphases and the SiC-matrix were in®ltrated within the porous ®bre preforms, according to the isothermal/iso￾baric chemical vapour in®ltration (I-CVI) process, which has been described elsewhere.16±18 Pyrocarbon and silicon carbide were deposited from propane C3H8 and methyl￾trichlorosilane (MTS) CH3SiCl3/H2, respectively, accord￾ing to the following overall equations: CH3SiC3…g† ! H2 SiC…s† ‡ 3HCl…g† …1† C3H8…g† ! 3C…s† ‡ 4H2…g† …2† in a hot-wall chamber (internal diameter: 130 mm; height: 250 mm) heated isothermally with a water￾cooled r.f. coil (maximum temperature capability: 1500C). The apparatus has been designed to work under reduced pressures (0.5<P<10 kPa), the total pressure being maintained at a constant value with a pressure sensor (type 127A from MKS) and a pressure regulator (type 252A from MKS). Mass ¯owmeters were used to measure the ¯owrates of the various gaseous Fig. 1. Tensile stress±strain curves obtained for 2D-SiC/SiC composites with various multilayered interphases: B, D, H and L fabricated with treated Nicalon ®bre; A, C, G and K fabricated with as-received Nicalon ®bre (according to Refs.[4] and [11]). 2 S. Bertrand et al. / Journal of the European Ceramic Society 20 (2000) 1±13

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 profiles

species: 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

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

3.2. Nanostructure of the Sic-sublayers The first Sic sublayer was observed to be often dis- continuous, particularly when its thickness was low (i.e. The Sic present in the multilayered(PyC/SiC)n inter- 0.1 um), as shown in Fig. 2. As a result, "mechanical phase was always deposited onto pyrocarbon surfaces, bridges"were formed under such conditions, between the roughly flat, as supported by a comparison of the pyr- sublayers(inset in Fig. 2). Obviously, an optimising of the ocarbon C(002) lattice fringes TEM images (recorded at nucleation/growth processes should be further carried out about the same magnification) shown in Figs. 3 and 4. Fig. in order to achieve thinner and smoother sublayer 4 shows that there is probably e relation between the orientation of the carbon aromatic planes in the substrate 3.3. Multiple interfacing and that of the crystals in the Sic-deposit. Meanwhile, no precise relationship was found for the preferred growth The number of interfaces in the(PyC/SiC)n multi directions of SiC (i.e. 1 ll for the cubic B modification and layered interphase increases as n is raised: up to 8 when 00. 1 for the hexagonal a modification)with respect to the n=4(materials G, H, K and L)remembering that the C aromatic planes in the substrate. silicon carbide in the last sequence is the matrix itself SiC in the deposits was well crystallised, the size of the As discussed in a previous paper, the first interface has crystals being often limited by the thickness of the Sic a unique role in controlling the behaviour of the whole sublayer itself. The crystals are either of the cubic(3C)B interfacial sequence: fibre/Pyc, delle ng has to be modification or consisted of a sequence of disordered strong in order to allow multiple deflection at the dif polytypes, as shown in Fig. 5 and already reported by ferent other interfaces. Fibre surface bonding strength several authors.24. 25 The diffraction pattern(inset in Fig is related to the surface state of the fibre. In addition to 5)shows a straining of the reciprocal nodes along the the first interface(fibre/Py Ci) there exists two kinds of [lll] growth axis, i.e. perpendicular to the stacking interface characterised by a very different roughness fault plane henomena) in such multilayered(PyC/SiC)n inter The interface related to a Sic-deposit onto a Pyc c layer(e.g. PyCn-I/SiCn) was usually smooth, as already 00,2 mentioned, owing to the layered structure of pyr carbon and to its "covering capability"(tending to SIC t co0. 2 5c11 SiCAl 5n Fig. 5. Structure of the Sic-based sublayer. Inset: electron diffraction Fig 4. Growth of SiC on the surface of PyC layer: smoothness of the pattern centered on the contrasted crystal exhibiting one-dimensional

3.2. Nanostructure of the SiC-sublayers The SiC present in the multilayered (PyC/SiC)n inter￾phase was always deposited onto pyrocarbon surfaces, roughly ¯at, as supported by a comparison of the pyr￾ocarbon C(002) lattice fringes TEM images (recorded at about the same magni®cation) shown in Figs. 3 and 4. Fig. 4 shows that there is probably some relation between the orientation of the carbon aromatic planes in the substrate and that of the crystals in the SiC-deposit. Meanwhile, no precise relationship was found for the preferred growth directions of SiC (i.e. 111 for the cubic  modi®cation and 00.1 for the hexagonal  modi®cation) with respect to the C aromatic planes in the substrate. SiC in the deposits was well crystallised, the size of the crystals being often limited by the thickness of the SiC sublayer itself. The crystals are either of the cubic (3C)  modi®cation or consisted of a sequence of disordered polytypes, as shown in Fig. 5 and already reported by several authors.24,25 The di€raction pattern (inset in Fig. 5) shows a straining of the reciprocal nodes along the [111]c growth axis, i.e. perpendicular to the stacking fault plane. The ®rst SiC sublayer was observed to be often dis￾continuous, particularly when its thickness was low (i.e. 0.1 mm), as shown in Fig. 2. As a result, ``mechanical bridges'' were formed under such conditions, between the sublayers (inset in Fig. 2). Obviously, an optimising of the nucleation/growth processes should be further carried out in order to achieve thinner and smoother sublayers. 3.3. Multiple interfacing The number of interfaces in the (PyC/SiC)n multi￾layered interphase increases as n is raised: up to 8 when n=4 (materials G, H, K and L) remembering that the silicon carbide in the last sequence is the matrix itself. As discussed in a previous paper,11 the ®rst interface has a unique role in controlling the behaviour of the whole interfacial sequence: ®bre/PyC1 bonding has to be strong in order to allow multiple de¯ection at the dif￾ferent other interfaces.11 Fibre surface bonding strength is related to the surface state of the ®bre. In addition to the ®rst interface (®bre/PyC1) there exists two kinds of interface characterised by a very di€erent roughness (important features regarding debonding and friction phenomena) in such multilayered (PyC/SiC)n inter￾phases. The interface related to a SiC-deposit onto a PyC￾layer (e.g. PyCnÿ1/SiCn) was usually smooth, as already mentioned, owing to the layered structure of pyr￾ocarbon and to its ``covering capability'' (tending to Fig. 4. Growth of SiC on the surface of PyC layer: smoothness of the interface. Fig. 5. Structure of the SiC-based sublayer. Inset: electron di€raction pattern centered on the contrasted crystal exhibiting one-dimensional disordered polytypism. S. Bertrand et al. / Journal of the European Ceramic Society 20 (2000) 1±13 5