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)00TEM 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