S. Bertrand et al. /Journal of the European Ceramic Society 20(2000)1-13 continuous. Only this technique enables to pro References duce multilayers at a nanometric scale trough a proper control of the residence time and deposi 1. Evans. A.G., Zok, F.w. and Davis, J, The role of tion kinetics interfaces in fibre-reinforced brittle matrix composites os.Sci. Technol,1991,42(3),3-24 2. Silicon carbide crystallisation is the leading para- 2. Naslain, R, Fibre-matrix interphases and interfaces in meter which controls the continuity of the layers ceramic-matrix composites processed by CVI. Composite Interfaces, 1993, 1, 253-28 of I-CVI, thick layers(0.1 um)are discontinuous 3. Cao, H because too much crystallised (scattered nuclea A. G. Marshall.d. B. and brennan.j. j. effect of inter- faces on the properties of fibre-reinforced ceramics. J.Am tion). Conversely, P-CVI was efficient to deposit 90,73(6),1691 d continuous layers. But pr ressure 4. Naslain, R, The design of the fiber-matrix interfacial cycling does not produce any crystallisation break zone in ceramic matrix composites. Composites Part A CVI parameter and especially MTS/H2 ratio plays 998,29A,1145-1155 an important role ces m a als as i er hasan gi ic sid dl m: o 3. P-CVI of pyrocarbon allows a close control of the deposit thickness. Both processes produce a pyr 6. Boisver, R. P, Hutter, R. K. and Diefendorf, R. J. carbon which flattens the roughness of the sic Interface manipulation in ceramic matrix composites substrate. Thin sublayers of carbon are efficient to mproved mechanical performance. Proc. JP, US Conf get good mechanical properties ompos. Mater. 4, 1989, 789-798 4. Both types of multilayers (micro and nano) 7. Naslain, R, The concept of layered interphases in Sic SiC. Ceram. Trans. 1995. 58. 23-26 appeared efficient to deflect matrix microcracking 8. Clegg, W.J., The fabrication and failure of laminar in loaded composites at ambient temperature. It c composI has been shown in a previous paper that the mul 3093 tilayer can increase together, strength and tough 9. Folsom, G, Zok, F. W. and Lange, F. F, Flexural pro erties of brittle multilayer materials: II experiments. ness in Sic/SiC composites. The only condition is to get some residual radial tensile stress and a 10. Ignat, M, Nadal, M, Bernard, C, Ducarroir, M.and strong chemical bonding at the fibre surface. This is Teyssandier, F, Mechanical response and rupture modes the case for treated Nicalon nlm 202.4 The inter- of Sic/C CVD lamellar composites. J. Physique, Colloque C5,1989,50,259-267 face behaves with a cohesive failure mode and the 11. Droillard C. Lamon, J and Bourrat, X, Strong interface work to fracture is thus increased. On the contrary, in CMCs, a condition for efficient multilayered inter the pristine fibres display a too weak bonding and phases. Mat. Res. Soc. Proc., 1995, 365, 371-376 the interface behaves with an adhesive failure mode 12. Droillard, C. and Lamon, J, Fracture toughness of 2D controling the debond-sliding mechanism woven SiC/SiC CVI-composites with multilayered inter With the pristine Hi-Nicalon fibre, a strong bond phases. J. Am. Ceram Soc., 1996, 79(4), 849-858 13. Pasquier, S, Thermomechanical behaviour of SiC/Sic ing takes place in minicomposites fabricated by P- omposite with multilayered interphase--effect of the CVI process, but the important contraction of this environment. Ph. D. thesis, no. 1727, University of Bor fibre brings about an important debonding of deaux I, france, 1997(in French). most of the fibre surface. This debonding prevents 14. Heurtevent, F, Nanoscale-multilayered(PYC/SiC)n the multilayer to act as mechanical fuse iterials--application as interphases in thermostructural posites. Ph. D. thesis, no. 1476, University of Bor 6. In the case of treated Hi-Nicalon fibre. the con- deaux I, france, 1996(in French). traction is limited but the treatment leaves a 15. Droillard. C. Bourrat. X. and Naslain. R. Weak and poorly organised carbon at the surface of the fibre strong interface in ceramic matrix composites: TEM As a result. the bonding strength is too weak. The approach of fracture mechanics. J. Eur. Ceram. Soc., in yourab ole balance between the residual tensile 16. Naslain, R and Langlais, F, CVD-processing of ceramic- stress and chemical bonding enables to get an ite materials. In Tailoring Mult enhanced toughness in this case. This is emphasised ceramic composmics, ed. R.E. Tressler, G.L. In a companion paper. G. Pantano and R. e. Newnham um Press, New-York, 1986, pp. 145-164 17. Naslain, R, Langlais, F and Fedou, R, The CVI-pro- ceramic Acknowledgements ix composites. J. de physique Colloque C5,1989,50,191-207 18. Naslain, R, CVI-composites. In Ceramic-Matrix Con This work has been supported by CNrs and SEP posites, ed. R. Warren. Blackie, Glasgow and London through grants given to S. Bertrand and C. droillard 1992,pp.199-244 The authors wish to thank m. alrivie from lcts and 19. Jouin, J. M., Cotteret, J. and Christin, F, SiC/SiC inter phase: case history. In: Proc. 2nd European Colloquium on P. Garetta, from CREMEM (University of Bordeaux-1) Designing Ceramic Interfaces. CEE Joint Res. Center for their help in TEM samples preparation Petten(NL), 11-13 November 1991continuous. Only this technique enables to pro￾duce multilayers at a nanometric scale, trough a proper control of the residence time and deposi￾tion kinetics. 2. Silicon carbide crystallisation is the leading para￾meter which controls the continuity of the layers and sharpness of the PyC/SiC interfaces. By means of I-CVI, thick layers (0.1 mm) are discontinuous because too much crystallised (scattered nuclea￾tion). Conversely, P-CVI was ecient to deposit homogeneous and continuous layers. But pressure cycling does not produce any crystallisation break. CVI parameter and especially MTS/H2 ratio plays an important role. 3. P-CVI of pyrocarbon allows a close control of the deposit thickness. Both processes produce a pyr￾ocarbon which ¯attens the roughness of the SiC substrate. Thin sublayers of carbon are ecient to get good mechanical properties. 4. Both types of multilayers (micro and nano) appeared ecient to de¯ect matrix microcracking in loaded composites at ambient temperature. It has been shown in a previous paper11 that the mul￾tilayer can increase together, strength and tough￾ness in SiC/SiC composites. The only condition is to get some residual radial tensile stress and a strong chemical bonding at the ®bre surface. This is the case for treated Nicalon NLM 202.4 The inter￾face behaves with a cohesive failure mode and the work to fracture is thus increased. On the contrary, the pristine ®bres display a too weak bonding and the interface behaves with an adhesive failure mode controling the debond-sliding mechanism. 5. With the pristine Hi-Nicalon ®bre, a strong bond￾ing takes place in minicomposites fabricated by P￾CVI process, but the important contraction of this ®bre26 brings about an important debonding of most of the ®bre surface. This debonding prevents the multilayer to act as mechanical fuse. 6. In the case of treated Hi-Nicalon ®bre, the con￾traction is limited but the treatment leaves a poorly organised carbon at the surface of the ®bre. As a result, the bonding strength is too weak. The favourable balance between the residual tensile stress and chemical bonding enables to get an enhanced toughness in this case. This is emphasised in a companion paper.20 Acknowledgements This work has been supported by CNRS and SEP through grants given to S. Bertrand and C. Droillard. The authors wish to thank M. Alrivie, from LCTS and P. Garetta, from CREMEM (University of Bordeaux-1) for their help in TEM samples preparation. References 1. Evans, A. G., Zok, F. W. and Davis, J., The role of interfaces in ®bre-reinforced brittle matrix composites. Compos. Sci. Technol., 1991, 42(3), 3±24. 2. Naslain, R., Fibre-matrix interphases and interfaces in ceramic±matrix composites processed by CVI. Composite Interfaces, 1993, 1, 253±286. 3. Cao, H. C., Bischo€, E., Sbaizero, O., Ruhle, M., Evans, A. G., Marshall, D. B. and Brennan, J. J., E€ect of inter￾faces on the properties of ®bre-reinforced ceramics. J. Am. Ceram. Soc., 1990, 73(6), 1691±1699. 4. Naslain, R., The design of the ®ber-matrix interfacial zone in ceramic matrix composites. Composites Part A, 1998, 29A, 1145±1155. 5. Jacques, S., Guette, A., Langlais, F. and Naslain, R., C(B) materials as interphases in SiC/SiC model micro￾composites. J. Mater. Sci., 1997, 32, 983±988. 6. Boisver, R. P., Hutter, R. K. and Diefendorf, R. J., Interface manipulation in ceramic matrix composites for improved mechanical performance. Proc. JP, US Conf. Compos. Mater. 4, 1989, 789±798. 7. Naslain, R., The concept of layered interphases in SiC/ SiC. Ceram. Trans., 1995, 58, 23±26. 8. Clegg, W. J., The fabrication and failure of laminar cera￾mic composites. Acta. Met. Mat., 1992, 40(11), 3085± 3093. 9. Folsom, G., Zok, F. W. and Lange, F. F., Flexural prop￾erties of brittle multilayer materials: II experiments. J. Am. Ceram. Soc., 1994, 77(8), 2081±2087. 10. Ignat, M., Nadal, M., Bernard, C., Ducarroir, M. and Teyssandier, F., Mechanical response and rupture modes of SiC/C CVD lamellar composites. J. Physique, Colloque C5, 1989, 50, 259±267. 11. Droillard, C., Lamon, J. and Bourrat, X., Strong interface in CMCs, a condition for ecient multilayered inter￾phases. Mat. Res. Soc. Proc., 1995, 365, 371±376. 12. Droillard, C. and Lamon, J., Fracture toughness of 2D woven SiC/SiC CVI-composites with multilayered inter￾phases. J. Am. Ceram. Soc., 1996, 79(4), 849±858. 13. Pasquier, S., Thermomechanical behaviour of SiC/SiC composite with multilayered interphaseÐe€ect of the environment. Ph.D. thesis, no. 1727, University of Bor￾deaux I, France, 1997 (in French). 14. Heurtevent, F., Nanoscale-multilayered (PYC/SiC)n materialsÐapplication as interphases in thermostructural composites. Ph. D. thesis, no. 1476, University of Bor￾deaux I, France, 1996 (in French). 15. Droillard, C., Bourrat, X. and Naslain, R., Weak and strong interface in ceramic matrix composites: TEM approach of fracture mechanics. J. Eur. Ceram. Soc., in press. 16. Naslain, R. and Langlais, F., CVD-processing of ceramic￾ceramic composite materials. In Tailoring Multiphase and Composite Ceramics, ed. R. E. Tressler, G. L. Messing, C. G. Pantano and R. E. Newnham. Mater. Sci. Res., Ple￾num Press, New-York, 1986, pp. 145±164. 17. Naslain, R., Langlais, F. and Fedou, R., The CVI-pro￾cessing of ceramic matrix composites. J. de Physique, Colloque C5, 1989, 50, 191±207. 18. Naslain, R., CVI-composites. In Ceramic±Matrix Com￾posites, ed. R. Warren. Blackie, Glasgow and London, 1992, pp. 199±244. 19. Jouin, J. M., Cotteret, J. and Christin, F., SiC/SiC inter￾phase: case history. In: Proc. 2nd European Colloquium on Designing Ceramic Interfaces. CEE Joint Res. Center, Petten (NL), 11±13 November 1991. 12 S. Bertrand et al. / Journal of the European Ceramic Society 20 (2000) 1±13