S. Bertrand et al. /Journal of the European Ceramic Society 20(2000)1-13 M 100nm mOnm Fig. 10. Material D(treated Nicalon fibre). TEM longitudinal section along the 0 bundle showing a mode I/mode Il deflection in the last a matrix microcrack at the second interface. Note that the first interface ( fibre/PyC1)is not debonded (TEM brightfield, sample loaded to failure) 4. Results-nanometer-scale multilayers processed by 4.1.2. The fibre surface and the first interface P-CVI The first material deposited on the fibre surface is systematically a pyrocarbon layer. Results of AES 4.1. Structure of the multilayered interphases ormed on the untreated and treated fibres, prior to any P-CVI infiltration, were per 4.1.1. Regularity and continuity of the layers formed in order to assess the chemical composition near Fig. 11(a)shows an example of a multilayered inter- the fibre surface. For the untreated fibres, a Hi-Nicalon phase processed at nanometric scale. This micrograph is yarn has been set in the reactor, and then heated under a cross section in a minicomposite obtained with as- vacuum, in order to reproduce the conditions experi- received Hi-Nicalon fibres and a multilayered inter- enced by the fibre prior to interphase deposition phase:(PyC3/SiC3o)10(3 and 30 being the PyC and Sic The surface of the untreated fibre is composed of a Si- layer thicknesses in nm and 10 the number of PyC/ Sic C-O mixture, 15 nm in thickness, assumed to be a sequences in the multilayer). A higher magnification SiO2+ free-C phase mixture. 20 The silica is formed dur [Fig. 11(b)] shows that pyrocarbon and Sic-based sub- ing the fibre heating under vacuum. When observed by layers are regular and continuous. The P-CVI process 4 TEM after the multilayer has been deposited, the fibre enables to deposit thin layers, parallel to the surface of surface looks rather smooth and clean. Fig. 12 is a high the fibres. It enables also to control the a- gaseous ratio resolution TEM micrograph of the first interface in a H2/MTS known to control the deposit composition and composite fabricated with untreated fibres and a(Pyc3/ crystallinity(here, the Sic-based layers do not consist of SiC3o)10 multilayer. The first pyrocarbon sublayer, but of a nanocrystalline SiC +C mixture). 4 composed of 7/8 carbon fringes, is lying directly on the Sharp interfaces between hard and compliant materials SiC nanocrystals forming the free surface of the fibre. are now accessible with that process. This has to be The oxide layer evidenced by AES is not observed after compared to those obtained at higher scale with I-CVI P-CVI, in the final material ig. 2). Layer flatness and thereafter interface sharpness The surface of the treated Hi-Nicalon fibre is composed, are suspected to be key features for obtaining a layered essentially, of a free-C layer, approximately 50 nm-thick. material with improved toughness Fig. 13 shows the interface of a material processed with a4. ResultsÐnanometer-scale multilayers processed by P-CVI 4.1. Structure of the multilayered interphases 4.1.1. Regularity and continuity of the layers Fig. 11(a) shows an example of a multilayered inter￾phase processed at nanometric scale. This micrograph is a cross section in a minicomposite obtained with as￾received Hi-Nicalon ®bres and a multilayered inter￾phase: (PyC3/SiC30)10 (3 and 30 being the PyC and SiC layer thicknesses in nm and 10 the number of PyC/SiC sequences in the multilayer). A higher magni®cation [Fig. 11(b)] shows that pyrocarbon and SiC-based sub￾layers are regular and continuous. The P-CVI process14 enables to deposit thin layers, parallel to the surface of the ®bres. It enables also to control the -gaseous ratio H2/MTS known to control the deposit composition and crystallinity (here, the SiC-based layers do not consist of pure SiC but of a nanocrystalline SiC+C mixture).14 Sharp interfaces between hard and compliant materials are now accessible with that process. This has to be compared to those obtained at higher scale with I-CVI (Fig. 2). Layer ¯atness and thereafter interface sharpness are suspected to be key features for obtaining a layered material with improved toughness. 4.1.2. The ®bre surface and the ®rst interface The ®rst material deposited on the ®bre surface is systematically a pyrocarbon layer. Results of AES depth-pro®le analyses, performed on the untreated and treated ®bres, prior to any P-CVI in®ltration, were per￾formed in order to assess the chemical composition near the ®bre surface. For the untreated ®bres, a Hi-Nicalon yarn has been set in the reactor, and then heated under vacuum, in order to reproduce the conditions experi￾enced by the ®bre prior to interphase deposition. The surface of the untreated ®bre is composed of a Si￾C-O mixture, 15 nm in thickness, assumed to be a SiO2+free-C phase mixture.20 The silica is formed dur￾ing the ®bre heating under vacuum. When observed by TEM after the multilayer has been deposited, the ®bre surface looks rather smooth and clean. Fig. 12 is a high resolution TEM micrograph of the ®rst interface in a composite fabricated with untreated ®bres and a (PyC3/ SiC30)10 multilayer. The ®rst pyrocarbon sublayer, composed of 7/8 carbon fringes, is lying directly on the SiC nanocrystals forming the free surface of the ®bre. The oxide layer evidenced by AES is not observed after P-CVI, in the ®nal material. The surface of the treated Hi-Nicalon ®bre is composed, essentially, of a free-C layer, approximately 50 nm-thick. Fig. 13 shows the interface of a material processed with a Fig. 10. Material D (treated Nicalon ®bre). TEM longitudinal section along the 0 bundle showing a mode I/mode II de¯ection in the last interfacial carbon sublayer. Fig. 9. Material L (treated Nicalon ®bre). Mode I/mode II de¯ection of a matrix microcrack at the second interface. Note that the ®rst interface (®bre/PyC1) is not debonded (TEM bright®eld, sample loaded to failure). 8 S. Bertrand et al. / Journal of the European Ceramic Society 20 (2000) 1±13