202 M. Pavese et al. /Ceramics International 34 (2008)197-26 4. Conclusions Multilayered ceramics can be a suitable and low expensive way to obtain components apt for working at high temperature Tubular components of silicon carbide with a multilay structure were produced by tape casting and sintering without pressure. Compression tests indicated the presence of delamination phenomena that increase the multilayer toughness over that of a conventional ceramic. The layer thickness influences both the material strength and the oxidation resistance. Buckles containing thinner layers show an increased strength, even if their strength slightly decreases after oxidation 30 um at 1600C. Indentation and microdiffraction tests showed that residual stresses control the crack path. During long-term oxidation a continuous silica coating which acts as a barrier for Fig, lL. Indentation cracks on an as- prepared sample showing cracks moving a further oxygen penetration, forms. In spite of the oxidative preferentially in tangential direction and radially deflected reactions, fracture behaviour of the multilayer ceramic wa found unchanged, even after oxidation treatments carried out in very severe conditions (100 h at 1600C). Thermal shock tests showed that no loss in mechanical strength was observed and that the mechanism of crack deflection is still working even Buckles compression tests after thermal shock cycles after 50 cycles from 1070C No thermal cycling 10 cycles 50 cycles Compression strength(MPa) 287 Acknowledgement Part of this work has been performed within the framework 1070C does not have a profound effect on the mechanical of the Integrated European Project"ExtreMat"(contract NMP properties of such composites CT-2004-500253)with financial support by the Europear Indentation tests were carried out on cycled samples in order Community. It only reflects the view of the authors and the to verify if the crack deflection mechanism was yet active. A European Community is not liable for any use of the typical result is presented in Fig. 12, where it is possible to see information contained therein that the cracks travel only in the tangential direction while are stopped in the radial one. This confirms that residual stress is References still present even after 50 thermal shock cycles, and guide the []O. Dugne, S. Prouhet, A Guette, R. Naslain, R. Fourmeaux, Y. Khin,J. Sevely, J. P. Rocher, J. Cotteret, Interface characterization by TEM, AES and SIMS in tough SiC (ex-PCS)fibre-SiC(CVI) matrix composites with a BN interphase, J. Mater. Sci. 28(1993)3409-3422. [2] O. Lujt, Identification of carbon sublayer in a Hi-Nicalon/BN/SiC com- J. Mater.Sci.Lett.18(1999)1825-1828. B3] R.E. Mistler, E.R. Twiname, Tape Casting. Theory and Practice, The American Ceramic Society. USA, 2000 [4] P. Boch, T. Chartier, M. Huttepain, Tape casting of Al]O3/ZrO2 laminated mposites,. Am. Ceram Soc. 69(1986)C191-C192. [5]A I.Y. Tok, F.T.C. Boey, K.A. Khor, Tape casting of high .Eur. Ceram.Soc.2002000)1691-1697 [7 G.J. Zhang, X.M. Yue, T. Watanabe, AlO,TiC/(MoSig + Mo2Bs)multi- layer composite prepared by tape casting, J. Eur. Ceram. Soc. 19(1999) 2111-2116. [8] M.P. Albano, L B. Garrido, Influence of the slip composition on the properties of tape-cast alumina substrates, Ceram. Int. 31(2005)57-66. 50 Am. [9] C. Badini, P. Fino, A. Ortona, C. Amelio, G. Pasquale, Processing of multilayered SiC ceramic by tape casting and sintering, in: Proceedings of the EuroMat 2001, 2001, published on CD-ROM, ISBN: 88-85298-39-7 [10] M. Jimenez-Melendo, C. Clauss, A Dominguez-Rodriguez, Microstruc- Fig. 12. Indentation cracks on a thermally cycled sample, showing long ture and high-temperature mechanical behaviour of alumina/alumina- tangential cracks deflected toward the external surface Radial cracks are absent yttria-stabilized tetragonal zirconia multilayer composites, J. Am. Ceram. or very short Soe.80(1997)2126-2130.1070 8C does not have a profound effect on the mechanical properties of such composites. Indentation tests were carried out on cycled samples in order to verify if the crack deflection mechanism was yet active. A typical result is presented in Fig. 12, where it is possible to see that the cracks travel only in the tangential direction while are stopped in the radial one. This confirms that residual stress is still present even after 50 thermal shock cycles, and guide the crack path. 4. Conclusions Multilayered ceramics can be a suitable and low expensive way to obtain components apt for working at high temperature. Tubular components of silicon carbide with a multilayer structure were produced by tape casting and sintering without pressure. Compression tests indicated the presence of delamination phenomena that increase the multilayer toughness over that of a conventional ceramic. The layer thickness influences both the material strength and the oxidation resistance. Buckles containing thinner layers show an increased strength, even if their strength slightly decreases after oxidation at 1600 8C. Indentation and microdiffraction tests showed that residual stresses control the crack path. During long-term oxidation a continuous silica coating, which acts as a barrier for a further oxygen penetration, forms. In spite of the oxidative reactions, fracture behaviour of the multilayer ceramic was found unchanged, even after oxidation treatments carried out in very severe conditions (100 h at 1600 8C). Thermal shock tests showed that no loss in mechanical strength was observed and that the mechanism of crack deflection is still working even after 50 cycles from 1070 8C. Acknowledgements Part of this work has been performed within the framework of the Integrated European Project ‘‘ExtreMat’’ (contract NMP￾CT-2004-500253) with financial support by the European Community. It only reflects the view of the authors and the European Community is not liable for any use of the information contained therein. References [1] O. Dugne, S. Prouhet, A. Guette, R. Naslain, R. Fourmeaux, Y. Khin, J. Sevely, J.P. Rocher, J. Cotteret, Interface characterization by TEM, AES and SIMS in tough SiC (ex-PCS) fibre-SiC (CVI) matrix composites with a BN interphase, J. Mater. Sci. 28 (1993) 3409–3422. [2] O. Lujt, Identification of carbon sublayer in a Hi-Nicalon/BN/SiC com￾posite, J. Mater. Sci. Lett. 18 (1999) 1825–1828. [3] R.E. Mistler, E.R. Twiname, Tape Casting. Theory and Practice, The American Ceramic Society, USA, 2000. [4] P. Boch, T. Chartier, M. Huttepain, Tape casting of Al2O3/ZrO2 laminated composites, J. Am. Ceram. Soc. 69 (1986) C191–C192. [5] A.I.Y. Tok, F.T.C. Boey, K.A. Khor, Tape casting of high dielectric ceramic composite substrates for microelectronic application, J. Mater. Proc. Technol. 89/90 (1999) 508–512. [6] Z. Yuping, J. Dongliang, P. Greil, Tape casting of aqueous Al2O3 slurries, J. Eur. Ceram. Soc. 20 (2000) 1691–1697. [7] G.J. Zhang, X.M. Yue, T. Watanabe, Al2O3/TiC/(MoSi2 + Mo2B5) multi￾layer composite prepared by tape casting, J. Eur. Ceram. Soc. 19 (1999) 2111–2116. [8] M.P. Albano, L.B. Garrido, Influence of the slip composition on the properties of tape-cast alumina substrates, Ceram. Int. 31 (2005) 57–66. [9] C. Badini, P. Fino, A. Ortona, C. Amelio, G. Pasquale, Processing of multilayered SiC ceramic by tape casting and sintering, in: Proceedings of the EuroMat 2001, 2001, published on CD-ROM, ISBN: 88-85298-39-7. [10] M. Jime`nez-Melendo, C. Clauss, A. Domı`nguez-Rodrı`guez, Microstruc￾ture and high-temperature mechanical behaviour of alumina/alumina– yttria-stabilized tetragonal zirconia multilayer composites, J. Am. Ceram. Soc. 80 (1997) 2126–2130. 202 M. Pavese et al. / Ceramics International 34 (2008) 197–203 Fig. 11. Indentation cracks on an as-prepared sample, showing cracks moving preferentially in tangential direction and radially deflected. Table 2 Buckles compression tests after thermal shock cycles No thermal cycling 10 cycles 50 cycles Compression strength (MPa) 287 311 269 Fig. 12. Indentation cracks on a thermally cycled sample, showing long tangential cracks deflected toward the external surface. Radial cracks are absent or very short