G Boitier et al./ Composites: Part A 33(2002)1467-1470 M M 200nm Fig. 3. TEM micrographs of the pyrocarbon interphase showing a microcrack deviation mode 1- mode II in a longitudinal section(parallel to the stress direction)(a), and a bridging of a microcrack by carbon ribbons(b), for SiCr-SiBC specimens creep tested at 1473 K, under 200 MPa and in A layer of PyC, from 10 to 100 nm thick, and oriented globally microcracks and, consequently, permits a correct damage parallel to these interfaces. The larger the interphase is, the tolerance. To propose a creep mechanism one must more pronounced the debonding between the matrix layers accurately analyze the carbon plane orientations, their evolution, and nano- and micro-mechanisms occurring at In the case of the pyrocarbon layer(between the silicon the different microstructural scales. Materials based on self- carbide fibers and the matrix), close to the matrix the carbon sealing matrix exhibit a significant improvement in structure is turbostratic type over a thickness of about 12 comparison with previous Cr-Sic or SiCr-SiC: they can 15 nm, i.e. 30-35 carbon atomic planes. Further away, the be considered as a class of material for gas turbine jet carbon structure is globally isotropic oriented, according to engines with, for example, lifetime higher than 100 h under the Despres nomenclature [15]. In this zone the carbon is 170 MPa at 1473 K [17] constituted of rollings made of about 10 carbon planes with some porosities and some amorphous carbon, which correspond to the texture class I [15. This interphaseAcknowledgements tructure is, at room temperature, in favor of phenomenon of debonding and sliding close to the interphase/matrix This work has been supported by Snecma Propulsion interfaces, due to the nature of the bounds of carbon planes Solide(Saint Medard en Jalles, France), by CNRS and of van der Waals type; such phenomenon is also operating Region de Basse-Normandie(SD). We wish to warmly during creep at high temperature. It is this geometry of thank Drs M.Bourgeon, E. Pestourie, J.M. Rouges, for microstructure, as in the case of Cr-SiC, which permits both fruitful discussions and for providing the specimens a mode I- mode Il crack deviation(Fig 3a)and a bridging some cracks by nano-ribbons of carbon planes(Fig. 3b), which are not an artifact as their diffraction patterns correspond to carbon: both features are nano-mechanisms References which consume energy and, then, permit a certain damage tolerance Cmm如xwNB几 Schneider H, editors. Hi油 Germany: Wiley-VCH; 2001. P. 731-43 [2 Luthra KL, Corman GS. In: Krenkel w, Naslain R, Schneider H 4. Discussion and conclusion editors. High temperature ceramic matrix composites, HT-CMC4 Weinheim, Germany: Wiley-VCH: 2001.P. 744-53 In this short paper one has shown that microscopic [3] Renz R, Krenkel w. Composites: from fundamentals to exploitatio ECCM 9, Brighton, UK. ECCM 9 CD ROM C 2000. IOM observations and analysis of thermo-mechanical or crept Communication Ltd: 4-7 June 2000 CMCs are the only way to access the different micro- [4] Renz R, Heidenreich B, Krenkel w, Schoppach A, Richter F. In: mechanisms occurring, for example when creep tests are Krenkel W, Naslain R, Schneider H, editors. High temperature performed. Creep tests give only a value of some ceramic matrix composites, HT-CMC4. Weinheim, Germany: mechanical parameters and concern only a macroscopic wley-VCH;2001.p.839-45 [5] Kerans RJ, Hay RS, Pagano NJ, Parthasarathy TA. Am Ceram Soc approach. For these materials, the pyrocarbon interphase Bull198968:429-42. play a key role for the development of the matrix [6 Kuntz M, Grathwohl G. Advd Engng Mater 2001; 3: 371-9layer of PyC, from 10 to 100 nm thick, and oriented globally parallel to these interfaces. The larger the interphase is, the more pronounced the debonding between the matrix layers appears. In the case of the pyrocarbon layer (between the silicon carbide fibers and the matrix), close to the matrix the carbon structure is turbostratic type over a thickness of about 12– 15 nm, i.e. 30–35 carbon atomic planes. Further away, the carbon structure is globally isotropic oriented, according to the Despre`s nomenclature [15]. In this zone the carbon is constituted of rollings made of about 10 carbon planes with some porosities and some amorphous carbon, which correspond to the texture class I [15]. This interphase structure is, at room temperature, in favor of phenomenon of debonding and sliding close to the interphase/matrix interfaces, due to the nature of the bounds of carbon planes of van der Waals type; such phenomenon is also operating during creep at high temperature. It is this geometry of microstructure, as in the case of Cf–SiC, which permits both a mode I ! mode II crack deviation (Fig. 3a) and a bridging of some cracks by nano-ribbons of carbon planes (Fig. 3b), which are not an artifact as their diffraction patterns correspond to carbon: both features are nano-mechanisms which consume energy and, then, permit a certain damage tolerance. 4. Discussion and conclusion In this short paper one has shown that microscopic observations and analysis of thermo-mechanical or crept CMCs are the only way to access the different micro￾mechanisms occurring, for example when creep tests are performed. Creep tests give only a value of some mechanical parameters and concern only a macroscopic approach. For these materials, the pyrocarbon interphases play a key role for the development of the matrix microcracks and, consequently, permits a correct damage tolerance. To propose a creep mechanism one must accurately analyze the carbon plane orientations, their evolution, and nano- and micro-mechanisms occurring at the different microstructural scales. Materials based on self￾sealing matrix exhibit a significant improvement in comparison with previous Cf–SiC or SiCf–SiC: they can be considered as a class of material for gas turbine jet engines with, for example, lifetime higher than 100 h under 170 MPa at 1473 K [17]. Acknowledgements This work has been supported by Snecma Propulsion Solide (Saint Me´dard en Jalles, France), by CNRS and Re´gion de Basse-Normandie (SD). We wish to warmly thank Drs M. Bourgeon, E. Pestourie, J.M. Rouge`s, for fruitful discussions and for providing the specimens. References [1] Christin F. In: Krenkel W, Naslain R, Schneider H, editors. High temperature ceramic matrix composites, HT-CMC4. Weinheim, Germany: Wiley–VCH; 2001. p. 731–43. [2] Luthra KL, Corman GS. In: Krenkel W, Naslain R, Schneider H, editors. High temperature ceramic matrix composites, HT-CMC4. Weinheim, Germany: Wiley–VCH; 2001. p. 744–53. [3] Renz R, Krenkel W. Composites: from fundamentals to exploitation. ECCM 9, Brighton, UK. ECCM 9 CD ROM C 2000. IOM Communication Ltd; 4–7 June 2000. [4] Renz R, Heidenreich B, Krenkel W, Scho¨ppach A, Richter F. In: Krenkel W, Naslain R, Schneider H, editors. High temperature ceramic matrix composites, HT-CMC4. Weinheim, Germany: Wiley–VCH; 2001. p. 839–45. [5] Kerans RJ, Hay RS, Pagano NJ, Parthasarathy TA. Am Ceram Soc Bull 1989;68:429–42. [6] Kuntz M, Grathwohl G. Advd Engng Mater 2001;3:371–9. Fig. 3. TEM micrographs of the pyrocarbon interphase showing a microcrack deviation mode I ! mode II in a longitudinal section (parallel to the stress direction) (a), and a bridging of a microcrack by carbon ribbons (b), for SiCf–SiBC specimens creep tested at 1473 K, under 200 MPa and in Ar. G. Boitier et al. / Composites: Part A 33 (2002) 1467–1470 1469