level the roughness of the substrate on which it has been first of all on the fibre surface bonding strength(fibre/ itself deposited). Apart the fibre/PyCl interface Py CI bonding strength). As received fibres are coated by obtained with the pristine fibre(which is the smoothest a thin oxide layer which produces a weak bonding. In interface) the second interface(PyC, SiC1) was system- contrast, with the treated fibres, the fibre surface bond atically that with the highest smoothness whatever the ing strength is raised. The detailed analysis of the nature of the material (since PyCl has been deposited on microcrack propagation paths fully supports the classi the fibre surface known to be smooth) fication of the materials in two families previously done The interface related to a pyrocarbon-deposit onto a on the basis of the mechanical behaviour and corre Sic-layer(e.g. SiCn-1/Py Cn)exhibited a high roughness sponding to the use of untreated or treated fibres, when the thickness of the Sic layer was large(owing to whatever the nature of the interphase (number and the tendency of Sic to grow as large-faceted crystals, as thickness of sublayers) hown in Fig. 5). It can be even discontinuous when the Sic layer is very thin. Under such conditions, there 3.4.1. Composites with untreated fibres occurs direct (and relatively strong) bonding between The microcrack paths exhibit two main features: (i)all the PyCn-I and PyCn layers(Fig. 2) the microcracks propagate up to the Nicalon fibre sur i) in the0°-fibr 3.4. Microcrack propagation path within a microscale there is no longer any bonding between the fibre and the multilayered interphase interphase, i.e. the fibre is debonded over its full length as shown in Fig. 7. These features occur whatever the As previously observed for single carbon interlayer, 5 nature of the interphase but as long as the fibres have matrix microcracks present in specimens loaded to fail- not been treated. Finally, for this first material family, ure(and especially crack-deflection-mechanisms)depend debonding and then sliding occurred mostly along the Fibre Matrix microcrack I Mode I Mode Il m Fig. 6. Multilayered interphase(material G with untreated Nicalon fibre) loaded to failure: (a) SEM micrograph on a polished longitudinal section showing the propagation path of a matrix microcrack (arrow)deflected in mode ll on fibre surface;(b) schematic showing the large residual crack opening and the final deflection at the fibre surface(open arrows indicate the loading direction).level the roughness of the substrate on which it has been itself deposited). Apart the ®bre/PyC1 interface obtained with the pristine ®bre (which is the smoothest interface) the second interface (PyC1/SiC1) was system￾atically that with the highest smoothness whatever the nature of the material (since PyC1 has been deposited on the ®bre surface known to be smooth). The interface related to a pyrocarbon-deposit onto a SiC-layer (e.g. SiCnÿ1/PyCn) exhibited a high roughness when the thickness of the SiC layer was large (owing to the tendency of SiC to grow as large-faceted crystals, as shown in Fig. 5). It can be even discontinuous when the SiC layer is very thin. Under such conditions, there occurs direct (and relatively strong) bonding between the PyCnÿ1 and PyCn layers (Fig. 2). 3.4. Microcrack propagation path within a microscale multilayered interphase As previously observed for single carbon interlayer,15 matrix microcracks present in specimens loaded to fail￾ure (and especially crack-de¯ection-mechanisms) depend ®rst of all on the ®bre surface bonding strength (®bre/ PyC1 bonding strength). As received ®bres are coated by a thin oxide layer which produces a weak bonding. In contrast, with the treated ®bres, the ®bre surface bond￾ing strength is raised. The detailed analysis of the microcrack propagation paths fully supports the classi- ®cation of the materials in two families previously done on the basis of the mechanical behaviour and corre￾sponding to the use of untreated or treated ®bres, whatever the nature of the interphase (number and thickness of sublayers). 3.4.1. Composites with untreated ®bres The microcrack paths exhibit two main features: (i) all the microcracks propagate up to the Nicalon ®bre sur￾face, as shown in Fig. 6 and (ii) in the 0-®bre-bundles, there is no longer any bonding between the ®bre and the interphase, i.e. the ®bre is debonded over its full length, as shown in Fig. 7. These features occur whatever the nature of the interphase but as long as the ®bres have not been treated. Finally, for this ®rst material family, debonding and then sliding occurred mostly along the Fig. 6. Multilayered interphase (material G with untreated Nicalon ®bre) loaded to failure: (a) SEM micrograph on a polished longitudinal section showing the propagation path of a matrix microcrack (arrow) de¯ected in mode II on ®bre surface; (b) schematic showing the large residual crack opening and the ®nal de¯ection at the ®bre surface (open arrows indicate the loading direction). 6 S. Bertrand et al. / Journal of the European Ceramic Society 20 (2000) 1±13