100 latrix PyCal 2 B(treated Nicalon fibre)after failure. TEM longitudinal Fig. 7. Material G(untreated Nicalon fibre). TEM cross-section(after a cohesive failure mode of the multilayered interpha failure) showing the fibre/multilayer debonding anywhere in the with mu ion (inset: schematic deflection in strong interface). fibres, which has been already described for single pyr- first interface which is very smooth, geometrically well carbon interphase. Owing to the high number of cracks defined and also very weak on a chemical view point 5 produced during this deflection, this damaging mode appears as a highly dissipating mechanism and results in a 3.4.2. Composites with treated fibres higher toughness. 2 It has been observed either for the Matrix microcrack deflections exhibit very different composite with a single thick carbon layer or for those features for the composites fabricated with the treated with(PyC/SiC)n multilayered interphases in which the fibres. First, it was never seen any matrix microcrack pyrocarbon sublayers are much thinner propagating up to the first interface, i.e. the fibre/PyC The second interface (PyCl/SiC1) owing Its interface. The fibre remains bonded over its full length smoothness was often observed to be the interface at (except very near the matrix microcracks) even at the (or near to) which the mode I/mode Il deflection occur- matrix crack saturation step; the fibre-matrix being red, as shown in Fig 9. The fibre is not debonded. Some- never uncoupled in a net manner(as observed in the times, microcracks were seen to be deflected at an other material family). As the fibre is being strained interface of much higher order. As an example, Fig. 10 under loading, debonding and then sliding (if there is shows the case of a deflection within the last carbon sub till any) no long occur along a well geometrically layer, far away from the fibre(favourable for protection defined surface but in a diffuse manner. Microcracks against oxidation) seem to burst into an infinity of nanometric-scale cracks Finally, a very common feature of the multilayer as as they are deflected in a pyrocarbon layer, parallel to deflector was the multideflection mode as seen for the fibre-axis. This was clearly identified to a shearing example in Fig. 8. The matrix microcrack underwent a failure mode in the case of a simple pyrocarbon inter- mode I/mode II deflection and then a mode Il/mode I layer 15 This is illustrated for material B in Fig. 8. and so on.. This multideflection mode was abundantly Deflecting within the whole thickness of the interphase seen in the case of the treated fibre. This is the tough (and not only as a single debonded surface) is a key ening-based mechanism suspected for that class of feature of composites fabricated with treated Nicalon material together with the matrix multiple cracking®rst interface which is very smooth, geometrically well de®ned and also very weak on a chemical view point.15 3.4.2. Composites with treated ®bres Matrix microcrack de¯ections exhibit very di€erent features for the composites fabricated with the treated ®bres. First, it was never seen any matrix microcrack propagating up to the ®rst interface, i.e. the ®bre/PyC1 interface. The ®bre remains bonded over its full length (except very near the matrix microcracks) even at the matrix crack saturation step; the ®bre-matrix being never uncoupled in a net manner (as observed in the other material family). As the ®bre is being strained under loading, debonding and then sliding (if there is still any) no long occur along a well geometrically de®ned surface but in a di€use manner. Microcracks seem to burst into an in®nity of nanometric-scale cracks as they are de¯ected in a pyrocarbon layer, parallel to the ®bre-axis. This was clearly identi®ed to a shearing failure mode in the case of a simple pyrocarbon inter￾layer.15 This is illustrated for material B in Fig. 8. De¯ecting within the whole thickness of the interphase (and not only as a single debonded surface) is a key feature of composites fabricated with treated Nicalon ®bres, which has been already described for single pyr￾ocarbon interphase. Owing to the high number of cracks produced during this de¯ection, this damaging mode appears as a highly dissipating mechanism and results in a higher toughness.12 It has been observed either for the composite with a single thick carbon layer11 or for those with (PyC/SiC)n multilayered interphases in which the pyrocarbon sublayers are much thinner. The second interface (PyC1/SiC1) owing to its smoothness, was often observed to be the interface at (or near to) which the mode I/mode II de¯ection occur￾red, as shown in Fig. 9. The ®bre is not debonded. Some￾times, microcracks were seen to be de¯ected at an interface of much higher order. As an example, Fig. 10 shows the case of a de¯ection within the last carbon sub￾layer, far away from the ®bre (favourable for protection against oxidation). Finally, a very common feature of the multilayer as de¯ector was the multide¯ection mode as seen for example in Fig. 8. The matrix microcrack underwent a mode I/mode II de¯ection and then a mode II/mode I and so on...This multide¯ection mode was abundantly seen in the case of the treated ®bre. This is the tough￾ening-based mechanism suspected for that class of material together with the matrix multiple cracking. Fig. 8. Material B (treated Nicalon ®bre) after failure. TEM longitudinal section exhibiting a cohesive failure mode of the multilayered interphase with multide¯ection (inset: schematic de¯ection in strong interface). Fig. 7. Material G (untreated Nicalon ®bre). TEM cross-section (after failure) showing the ®bre/multilayer debonding anywhere in the material. S. Bertrand et al. / Journal of the European Ceramic Society 20 (2000) 1±13 7