S. Bertrand et al. /Journal of the European Ceramic Society 20(2000)1-13 4.1.4. Nanostructure of the Sic-based sublayers fracture. The first interface remains meanwhile the most XRD analyses have been performed on planar important which controls the whole interphase beha deposits (i.e.(Py C/SiC)n multilayer deposited by P- viour. I Fig. 14 compares the brightfield and the carbon CVD). They show the coexistence of a and B Sic vari 002 dark field images of the same interphase. The latter ies as previously reported by Heurtevent 4 The is obtained as shown in the diffraction pattern, by measurement of the L111 from the spectra, is 18.8 nm. selecting the information carried by the 002 reflection by lectron probe mi cro-ana lysis means of an aperture. By this technique only the car- (EPMA), have shown that the Sic-based sublayers, are bon is in contrast. Despite their smooth aspect, this in fact, C+ SiC codeposits, as previously mentioned. 20 projection evidences the roughness of the sublayers. The In microcomposites, 4 the nanostructure of the Sic- multiple deflection is therefore expected to be limited based sublayers changes according to the thickness of the layer. Over 30 nm, a slight columnar microstructure 4.2. Crack propagation in the interphase apparent. The first Sic-based deposit (when e(sic) <30 nm)is nanocrystallised, while, beyond 30 nm, a micro- These multilayered(Py C/SiC)n thin films have been crystallised layer is superimposed on this first amorphous utilised as interphase in SiC/SiC composites. Uniaxial tension tests were performed at room temperature on n minicomposites, the nanostructure of the Sic- the micro, mini- and 2D-composites. 4, 20 After failure, based layers changes with the distance from the fibre. the materials were studied by TEM and SeM to char The first layers are nanocrystallised, while the layers, acterise the deflection mode and the interfacial beha near the matrix, are microcrystallised vIol The matrix microcrack deflection mode depends on 4.1.5. The interfaces le treatment of the hi-Nicalon fibre The multilayered structure is developed to multiply When the fibre is not treated a strong radial shrink- the number of interfaces in order to increase the work of age occurs during the CvI processing at high tempera ture.> TEM low magnification images show clearly that a debonding occurs on most of the interfaces (arrows in Fig. 15). Even if the chemical interfacial bonding is strong, the strong fibre contraction when observed produces a high amount of debonding. As a Fibre Fig. 14. Interphase(Py Cxo/SiCso)o in a minicomposite with treated Fig. 13. Interfaces in treated Hi-Nicalon fibre-reinforced Sic compo- Hi-Nicalon fibres:(a) brightfield TEM image and(b)carbon 002-dark site. Inset is a 002 darkfield of the same interface showing three carbon field. Inset is the electron diffraction pattern of the multilayer with the layers at the treated Hi-Nicalon/(Py C2o/SiCso)o interface objective aperture position.4.1.4. Nanostructure of the SiC-based sublayers XRD analyses have been performed on planar deposits (i.e. (PyC/SiC)n multilayer deposited by P￾CVD). They show the coexistence of  and  SiC vari￾eties as previously reported by Heurtevent.14 The measurement of the L111 from the spectra, is 18.8 nm. Microanalyses, by electron probe micro-analysis (EPMA), have shown that the SiC-based sublayers, are in fact, C+SiC codeposits, as previously mentioned.20 In microcomposites,14 the nanostructure of the SiC￾based sublayers changes according to the thickness of the layer. Over 30 nm, a slight columnar microstructure is apparent. The ®rst SiC-based deposit (when e(SiC)<30 nm) is nanocrystallised, while, beyond 30 nm, a micro￾crystallised layer is superimposed on this ®rst amorphous layer. In minicomposites, the nanostructure of the SiC￾based layers changes with the distance from the ®bre. The ®rst layers are nanocrystallised, while the layers, near the matrix, are microcrystallised. 4.1.5. The interfaces The multilayered structure is developed to multiply the number of interfaces in order to increase the work of fracture. The ®rst interface remains meanwhile the most important which controls the whole interphase beha￾viour.11 Fig. 14 compares the bright®eld and the carbon 002 dark ®eld images of the same interphase. The latter is obtained, as shown in the di€raction pattern, by selecting the information carried by the 002 re¯ection by means of an aperture. By this technique, only the car￾bon is in contrast. Despite their smooth aspect, this projection evidences the roughness of the sublayers. The multiple de¯ection is therefore expected to be limited. 4.2. Crack propagation in the interphase These multilayered (PyC/SiC)n thin ®lms have been utilised as interphase in SiC/SiC composites. Uniaxial tension tests were performed at room temperature on the micro-, mini- and 2D-composites.14,20 After failure, the materials were studied by TEM and SEM to char￾acterise the de¯ection mode and the interfacial beha￾viour. The matrix microcrack de¯ection mode depends on the treatment of the Hi-Nicalon ®bre. When the ®bre is not treated, a strong radial shrink￾age occurs during the CVI processing at high tempera￾ture.25 TEM low magni®cation images show clearly that a debonding occurs on most of the interfaces (arrows in Fig. 15). Even if the chemical interfacial bonding is strong, the strong ®bre contraction when observed produces a high amount of debonding. As a Fig. 13. Interfaces in treated Hi-Nicalon ®bre-reinforced SiC compo￾site. Inset is a 002 dark®eld of the same interface showing three carbon layers at the treated Hi-Nicalon/(PyC20/SiC50)10 interface. Fig. 14. Interphase (PyC20/SiC50)10 in a minicomposite with treated Hi-Nicalon ®bres: (a) bright®eld TEM image and (b) carbon 002-dark ®eld. Inset is the electron di€raction pattern of the multilayer with the objective aperture position. 10 S. Bertrand et al. / Journal of the European Ceramic Society 20 (2000) 1±13