C. Reynaud et al. /Joumal of the European Ceramic Sociery 25 (2005)589-59 ing laminar composites mimics either that of CslC or that cant delaminations were observed. However, the behaviour of GPlc of the present composites has been shown to be consis- Let us now compare our materials with laminar com- tent with the behaviour observed for SiC/SiC-graphite4 and posites in the Si3N4 system (i.e, materials densified by Si3 N4/Si3N4-BN3 composites. Our opinion is that poros iquid phase sintering through the addition of 6 vol. of ity levels higher than the critical value proposed by Blanks tria)that were studied by Kovar et al. Five grades of et al. 7 are needed for crack deflection to occur reliably in weak layers were obtained from mixtures of Si3N4 and dense/porous laminar composites hexagonal BN: 20, 50, 75. 90 and 100 vol. of boron ni- tride. The load-deflection curves showed brittle failure up to 50 vol%bn. For this last bn content the delamina- tions are reported to be extremely short, less than 100 um References Substantial crack deflection and delamination started to be observed for bn content in the weak interlayers equal to 1. Clegg, W.J., Kendall, K, Alford, N. M, Birchall, D and Button, 75 voL% Due to the low cohesive force between graphite and Sic 455-457. or between BN and Si3N4 grains, it could be assumed that 2. Liu, H. and S. M. fracture behavior of multilayer silicon nitride/boron nitride ceramics. Am. Ceram. Soc. 1996.79. 2452- the graphite or BN platelets play a role similar to the pores resulting from the pyrolysis of the graphite platelets in the 3. Kovar, D, Thouless, M. D and Halloran, J. W, Crack deflection and GPLC specimens. In this respect, it appears that crack de- propagation in layered silicon nitride/boron nitride ceramics.J.Am. flection reliably occurs only for volume content significantly Ceram.Soc.1998,81,1004-1012. greater than the 37 vol % derived by Blanks et al. Unfortu- Mawdsley, J.R., Kovar, D. and Halloran, J. W, Fracture behavior nately, we were not able to process layers with a porosity f alumina/monazite multiplayer laminates. J. Am. Ceram. Soc. 2000 83,802-808 higher than 47% because the quantity of gas evolving from T. T. and Cooper, R. F, Ambient-temperature mechanical re- he burn out of the pore forming agent was too large and laminates. J. Am. Ceram. Soc. 1994 led to cracking and swelling of the materials. However. in 77,16991705 the light of the above comparison with SiC/SiC-graphite 6. Zhang, G.J., Yue, X. M. and Watanabe, T, High-temperature mul- and Si3N4/Si3N4-BN composites, our opinion is that it is bilayer composites with superplastic interlayers. J. Am. Ceram. Soc. 999,82,3257-3259 unlikely that the difference in the densification mechanism 7. Blanks, K.S., Kristoffersson, A, Carlstrom, E and Clegg, W.J could be the reason why Blanks' specimens showed crack Crack deflection in ceramic laminates using porous interlayers. J. Eur. deflection and not ours. Unfortunately, a more in-depth com- Ceran.Soc.1998,18,1945-1951 parison of the two sets of dense/porous laminates could not 8. Davis, J. B, Kristoffersson, A, Carlstrom, E. and Clegg, w. J be performed because of the lack of information on the mi- Fabrication and crack deflection in ceramic laminates with porous crostructure of the materials fabricated by Blanks and his interlayers. J 4m. Ceram. Soc. 2000, 83, 2369-2374 9. Howard, S.J., Stewart, R. A. and Clegg, w. J, Delamination of co-workers ceramic laminates due to residual thermal stresses In Key Engineering Materials, Vols 116-117, ed. T. w. Clyne. Trans. Tech Publications Switzerland, 1996, pp. 331-350 6. Conclusion 10. He, M -Y. and Hutchinson, J. W, Kinking of a crack out of interface.J.App. Mech. 1989, 56, 270-278 I1 Gong, S.-X. and Horil, H, General solution to the problem of mi- SiC materials have been fabricated by stacking tape cast crocracks near the tip of a main crack. J. Mech. Plns. Solids 1989 sheets and densification of the stack by liquid phase sin- 37(1),27-46 tering(YAG-alumina eutectic). Controlled porosity was in- 12. Alford, N M, Birchall, J.D. and Kendall,K, High strengthe troduced by the incorporation of pore forming agent in the slurry. Two types of samples have been prepared: mono- 13. Reynaud, C, Thevenot, F. and Chartier, T, Processing and m laminar composites. Int. Refract. Met. Hard Mater lithic blocks to determine the mechanical properties and al- ternate dense/porous laminates to test the ability to increase 14. Slamovich, E. B. an weak interlay pture by promoting crack deflection in the of ultrasonic e dependence on porosity of the mechanical properties solids in the frequency range 100 to 500 MHZ. J. Acoust. Soc. Am (Youngs modulus, modulus of rupture, toughness and frac 16. Damani, R. J, Gstrein, R. and Danzer, R, Critical notch-root radius ture energy)has been found to be well fitted on the entire effect in SENB-S fracture toughness testing. J. Eur: Ceram Soc. 1996, range of porosity(0-42%)by the set of equations proposed 16.695-70 by Wagh et al. 19,22 from a model that takes into account the 17. Jordan, Y, Elaboration et caracterisation de composites Dispersoides tortuosity of the porosity ENSM-SE/INSA Lyon, Saint-Etienne, France, 1991 Though laminates with weak interlayers having a poros- ity higher than the critical level required for crack deflec en Carbure de silicium Ph. D. thesis, ENSM-SE/INPG, Saint-Etien tion according to Clegg's group were fabricated, no signifi-596 C. Reynaud et al. / Journal of the European Ceramic Society 25 (2005) 589–597 ing laminar composites mimics either that of CSLC or that of GPLC. Let us now compare our materials with laminar com￾posites in the Si3N4 system (i.e., materials densified by liquid phase sintering through the addition of 6 vol.% of yttria) that were studied by Kovar et al.3 Five grades of weak layers were obtained from mixtures of Si3N4 and hexagonal BN: 20, 50, 75, 90 and 100 vol.% of boron ni￾tride. The load–deflection curves showed brittle failure up to 50 vol.% BN. For this last BN content the delamina￾tions are reported to be extremely short, less than 100 m. Substantial crack deflection and delamination started to be observed for BN content in the weak interlayers equal to 75 vol.%. Due to the low cohesive force between graphite and SiC or between BN and Si3N4 grains, it could be assumed that the graphite or BN platelets play a role similar to the pores resulting from the pyrolysis of the graphite platelets in the GPLC specimens. In this respect, it appears that crack de- flection reliably occurs only for volume content significantly greater than the 37 vol.% derived by Blanks et al. Unfortu￾nately, we were not able to process layers with a porosity higher than 47% because the quantity of gas evolving from the burn out of the pore forming agent was too large and led to cracking and swelling of the materials. However, in the light of the above comparison with SiC/SiC–graphite and Si3N4/Si3N4–BN composites, our opinion is that it is unlikely that the difference in the densification mechanism could be the reason why Blanks’ specimens showed crack deflection and not ours. Unfortunately, a more in-depth com￾parison of the two sets of dense/porous laminates could not be performed because of the lack of information on the mi￾crostructure of the materials fabricated by Blanks and his co-workers. 6. Conclusion SiC materials have been fabricated by stacking tape cast sheets and densification of the stack by liquid phase sin￾tering (YAG-alumina eutectic). Controlled porosity was in￾troduced by the incorporation of pore forming agent in the slurry. Two types of samples have been prepared: mono￾lithic blocks to determine the mechanical properties and al￾ternate dense/porous laminates to test the ability to increase the work of rupture by promoting crack deflection in the weak interlayers. The dependence on porosity of the mechanical properties (Young’s modulus, modulus of rupture, toughness and frac￾ture energy) has been found to be well fitted on the entire range of porosity (0–42%) by the set of equations proposed by Wagh et al.19,22 from a model that takes into account the tortuosity of the porosity. Though laminates with weak interlayers having a poros￾ity higher than the critical level required for crack deflec￾tion according to Clegg’s group were fabricated, no signifi- cant delaminations were observed. However, the behaviour of the present composites has been shown to be consis￾tent with the behaviour observed for SiC/SiC–graphite24 and Si3N4/Si3N4–BN3 composites. Our opinion is that poros￾ity levels higher than the critical value proposed by Blanks et al.7 are needed for crack deflection to occur reliably in dense/porous laminar composites. References 1. Clegg, W. J., Kendall, K., Alford, N. M., Birchall, D. and Button, T. W., A simple way to make tough ceramics. Nature 1990, 347, 455–457. 2. Liu, H. and Hsu, S. M., Fracture behavior of multilayer silicon nitride/boron nitride ceramics. J. Am. Ceram. Soc. 1996, 79, 2452– 2457. 3. Kovar, D., Thouless, M. D. and Halloran, J. W., Crack deflection and propagation in layered silicon nitride/boron nitride ceramics. J. Am. Ceram. Soc. 1998, 81, 1004–1012. 4. Mawdsley, J. R., Kovar, D. and Halloran, J. W., Fracture behavior of alumina/monazite multiplayer laminates. J. Am. Ceram. Soc. 2000, 83, 802–808. 5. King, T. T. and Cooper, R. F., Ambient-temperature mechanical re￾sponse of alumina-fluoromica laminates. J. Am. Ceram. Soc. 1994, 77, 1699–1705. 6. Zhang, G.-J., Yue, X. M. and Watanabe, T., High-temperature mul￾tilayer composites with superplastic interlayers. J. Am. Ceram. Soc. 1999, 82, 3257–3259. 7. Blanks, K. S., Kristoffersson, A., Carlström, E. and Clegg, W. J., Crack deflection in ceramic laminates using porous interlayers. J. Eur. Ceram. Soc. 1998, 18, 1945–1951. 8. Davis, J. B., Kristoffersson, A., Carlström, E. and Clegg, W. J., Fabrication and crack deflection in ceramic laminates with porous interlayers. J. Am. Ceram. Soc. 2000, 83, 2369–2374. 9. Howard, S. J., Stewart, R. A. and Clegg, W. J., Delamination of ceramic laminates due to residual thermal stresses. In Key Engineering Materials, Vols 116–117, ed. T. W. Clyne. Trans. Tech. Publications, Switzerland, 1996, pp. 331–350. 10. He, M.-Y. and Hutchinson, J. W., Kinking of a crack out of an interface. J. Appl. Mech. 1989, 56, 270–278. 11. Gong, S.-X. and Horii, H., General solution to the problem of mi￾crocracks near the tip of a main crack. J. Mech. Phys. Solids 1989, 37(1), 27–46. 12. Alford, N. M., Birchall, J. D. and Kendall, K., High strength ceramics through colloidal control to remove defects. Nature 1987, 330, 51–53. 13. Reynaud, C., Thevenot, F. and Chartier, T., Processing and microstruc￾ture of SiC laminar composites. Int. J. Refract. Met. Hard Mater. 2000, 19, 425–435. 14. Slamovich, E. B. and Lange, F. F., Densification of large pores: I. Experiments. J. Am. Ceram. Soc. 1992, 75, 2498–2508. 15. Mc Skimmin, H. J., Measurement of ultrasonic wave velocities for solids in the frequency range 100 to 500 MHz. J. Acoust. Soc. Am. 1960, 34, 404–409. 16. Damani, R. J., Gstrein, R. and Danzer, R., Critical notch-root radius effect in SENB-S fracture toughness testing. J. Eur. Ceram. Soc. 1996, 16, 695–702. 17. Jordan, Y., Elaboration et Caractérisation de Composites Disperso¨ıdes Base Alumine-zircone à Vocation Thermodynamique. Ph.D. thesis, ENSM-SE/INSA Lyon, Saint-Etienne, France, 1991. 18. Reynaud, C., Céramiques Lamellaires Monolithiques et Composites en Carbure de Silicium. Ph.D. thesis, ENSM-SE/INPG, Saint-Etienne, France, 2002.