Y.H. Ko et al. Journal of the European Ceramic Society 24(2004)699-703 Table I 4. Kovar. D, Thouless, M. D. and Halloran, J. H. Crack deflection Summarized mechanical properties of monolithic Si3N4 and fibrous and propagation in layered silicon nitride-boron nitride ceramics. monoliths with cell boundary thickness of 37 um J.Am. Ceran.Soc.,l998,81,10041012 5. She J, Inoue. T and Ueno, K. Damage resistance and R-curve Samples MOR(MPa) WOF (J/m2) KIc(MPa m/2) behavior of multilayer AlO/SiC ceramics. Ceramic Inter- Monolithic Sin4697±2 1 Negligible7.5±0.16 ationa,2000,26.801-805. Fibrous monolith 227-1l 1216±23310±0.19 6. Russo, C. J. Harmer. M. P. Chan. H. M. and Miller. G.A. Design of a laminated ceramic composite for improved strength and toughness. Am. Ceram. Soc. 1992.. 3396-3400 energy is stored in the material, the crack propagates 7. She, J, Inoue, T and Uneo, K, Multilayer Al,O3/SiC ceramics J. Eur. Ceram. Soc. 2000 through the cells and the cell boundaries without any sig- 20.1771-1775 nificant crack interactions. On the other hand when the 8. Liu, H and Hsu, S. M, Fracture behavior of multilayer silicon strength is too low, the work-of-fracture is low because nitride/boron nitride ceramics. J. Anm. Ceram. Soc., 1996, 79 the material can withstand only limited stress(see Fig. 5). 2-2457 fracture behavior is dependent on the stored energy 9. Ohji, T, Shigegaki, Y, Miyajima, T. and Kanzaki, S resistance behavior of multilayered silicon nitride. J. Am c before fracture initiation, as well as the change in inter Soc.1997,80,991-994. facial fracture resistance by different cell boundary thick- 10.Clegg, WJ,Kendall,X,Alford, KMcN.Button,TWand ness. Therefore, cell boundary thickness is one of the most critical factors for obtaining high wOF. The mechanical London),1990,357,455-457 properties of monolithic Si3 N4 and fibrous monolith with I1. Coblenz, w.S., Fibrous Monolithic Ceramic and Method for cell boundary thickness of 37 um are listed in Table 1. The 12. Kovar, D. King, B. H, Trice, R.w. and Halloran, J.H WOF and facture toughness increased remarkably for Fibrous monolithic ceramics. J. Am. Ceram. Soc.. 1997.80 2471- fibrous monolith with non-catastrophic failure 13. Baskaran, S. Nunn, S. D, Popovic, D. and Halloran, J Fibrous monolithic ceramics: 1. fabrication. microstructure and indentation behavior. Am. Ceram. Soc. 1993.76.2209-2216. 4. Conclusion 14. Baskaran, S and Halloran, J. H. Fibrous monolithic ceramic Il, flexural strength and fracture behavior of the silicon carbide/ brous monolithic ceramics consisting of strong graphite system. J. Am. Ceram Soc., 1993, 76, 2217-222 Si3N4 cells surrounded by weak BN cell boundaries with 15. Baskaran S and Halloran. J H. Fibrous monolithic various thicknesses were fabricated by hot-pressing Si3N4-polymer was extruded, and then coated with a 16. Baskaran. S. Nunn. S D and Halloran. J H. Fibrous mono- BN-containing slurry by dip-coating. Cell boundary thickness was controlled by adjusting the concentration of the alumina/nickel system. J. Am. Ceram. Soc., 1994, 77, 1256- of BN in the slurry. On increasing the cell boundary thickness, the density decreased and the fracture beha 17. Trice.R. W. and Halloran.J I temperature on the interfacial fracture energy of silicon nitride/ vior changed from a brittle pattern to one of the non- boron nitride fibrous monolithic ceramics.. dm. ceram. soc catastrophic failure type. Mechanical properties of these fibrous monoliths were significantly affected by these 18. Trice, R. w and Halloran, J.H. Elevated-temperature mechan- distinctive fracture behaviors. When the cell boundary ical properties of silicon nitride/ boron nitride fibrous monolithic thickness was increased, the flexural strength decreased ceramics. J. m. Ceram. Soc. 2000.83. 311-316. 19. Trice, R. W.and Halloran, J. H, Effect of sintering aid compo- due to the reduction in the volume fraction of the si3 N4 tion on the processing of Si3 N4/BN fibrous monolithic ceramics However, the apparent work-of-fracture and the frac J. Am. Ceram. Soc. 82.2943-2947 ture toughness increased significantly due to the exten 20. Hai, G, Yong. H and An, w. C, Preparation and ve crack interactions with the weak bn cell fibrous monolithic nics by in-situ synthesizing. J. Mater boundaries. these different fracture behaviors were Sci,1999,34,2455-2459 related to the energy stored before fracture initiation 21. She. J, Inoue. T, Suzuki, M. Sodeoka. S. and Ueno, K, ties and fracture behavior of fibrous AlO3/ and the change in the interfacial fracture resistance. Sic ceram ok. F. w. and Lange. F.F. Flexur of brittle multilayer materials: I, modeling. J. Am. Ceram. Soc References 1. Harmer, M. P. Chan. H. M. and Miller. G. A. Un ce between dissimilar elastic materials. Int. Solids. Struct tunities for microstructural engineering with duplex 1989,125,1053-1067 ceramic composites. J. Am. Ceram Soc., 1992, 75, I 24. Camus. G. Modeling of the Mechanical behavior and damage Evans, A G, Perspective on the development of high-toughness process of fibrous ceramic matrix composites: application to a 2- ceramics. J. Anm. Ceram. Soc. 1990. 73. 187-206. 3. Kerans, R.J. and Parthasarathy, T.A., Crack deflection in cera- 25. King, B. H, Influcence of Architecture on the Mechnaical Prop- mic composites and fiber coating design criteria. Composites. erties of Fibrous Monolithic Ceramics. PhD Thesis, University 1999,A30,521-524. Michigan. Ann Arbor. MI. 1997energy is stored in the material, the crack propagates through the cells and the cell boundaries without any sig￾nificant crack interactions. On the other hand, when the strength is too low, the work-of-fracture is low because the material can withstand only limited stress (see Fig. 5). Fracture behavior is dependent on the stored energy before fracture initiation, as well as the change in inter￾facial fracture resistance by different cell boundary thick￾ness. Therefore, cell boundary thickness is one of the most critical factors for obtaining high WOF. The mechanical properties of monolithic Si3N4 and fibrous monolith with cell boundary thickness of 37 mm are listed in Table 1. The WOF and facture toughness increased remarkably for fibrous monolith with non-catastrophic failure. 4. Conclusion Fibrous monolithic ceramics consisting of strong Si3N4 cells surrounded by weak BN cell boundaries with various thicknesses were fabricated by hot-pressing. Si3N4-polymer was extruded, and then coated with a BN-containing slurry by dip-coating. Cell boundary thickness was controlled by adjusting the concentration of BN in the slurry. On increasing the cell boundary thickness, the density decreased and the fracture beha￾vior changed from a brittle pattern to one of the non￾catastrophic failure type. Mechanical properties of these fibrous monoliths were significantly affected by these distinctive fracture behaviors. When the cell boundary thickness was increased, the flexural strength decreased due to the reduction in the volume fraction of the Si3N4. However, the apparent work-of-fracture and the frac￾ture toughness increased significantly due to the exten￾sive crack interactions with the weak BN cell boundaries. These different fracture behaviors were related to the energy stored before fracture initiation and the change in the interfacial fracture resistance. References 1. Harmer, M. P., Chan, H. M. and Miller, G. A., Unique oppor￾tunities for microstructural engineering with duplex and laminar ceramic composites. J. Am. Ceram. Soc., 1992, 75, 1715–1728. 2. Evans, A. G., Perspective on the development of high-toughness ceramics. J. Am. Ceram. Soc., 1990, 73, 187–206. 3. Kerans, R. J. and Parthasarathy, T. A., Crack deflection in cera￾mic composites and fiber coating design criteria. Composites, 1999, A30, 521–524. 4. Kovar, D., Thouless, M. D. and Halloran, J. H., Crack deflection and propagation in layered silicon nitride–boron nitride ceramics. J. Am. Ceram. Soc., 1998, 81, 1004–1012. 5. She, J., Inoue, T. and Ueno, K., Damage resistance and R-curve behavior of multilayer Al2O3/SiC ceramics. Ceramic Inter￾national, 2000, 26, 801–805. 6. Russo, C. J., Harmer, M. P., Chan, H. M. and Miller, G. A., Design of a laminated ceramic composite for improved strength and toughness. J. Am. Ceram. Soc., 1992, 75, 3396–3400. 7. She, J., Inoue, T. and Uneo, K., Multilayer Al2O3/SiC ceramics with improved mechanical behavior. J. Eur. Ceram. Soc., 2000, 20, 1771–1775. 8. Liu, H. and Hsu, S. M., Fracture behavior of multilayer silicon nitride/boron nitride ceramics. J. Am. Ceram. Soc., 1996, 79, 2452–2457. 9. Ohji, T., Shigegaki, Y., Miyajima, T. and Kanzaki, S., Fracture resistance behavior of multilayered silicon nitride. J. Am. Ceram. Soc., 1997, 80, 991–994. 10. Clegg, W. J., Kendall, X., Alford, K.McN., Button, T. W. and Birchall, J. D., A simple way to make tough ceramics. Nature (London), 1990, 357, 455–457. 11. Coblenz, W. S., Fibrous Monolithic Ceramic and Method for Production. US Patent, No. 4, 772524, 1998. 12. Kovar, D., King, B. H., Trice, R. W. and Halloran, J. H., Fibrous monolithic ceramics. J. Am. Ceram. Soc., 1997, 80, 2471– 2487. 13. Baskaran, S., Nunn, S. D., Popovic, D. and Halloran, J. H., Fibrous monolithic ceramics: I, fabrication, microstructure, and indentation behavior. J. Am. Ceram. Soc., 1993, 76, 2209–2216. 14. Baskaran, S. and Halloran, J. H., Fibrous monolithic ceramics: II, flexural strength and fracture behavior of the silicon carbide/ graphite system. J. Am. Ceram. Soc., 1993, 76, 2217–2224. 15. Baskaran, S. and Halloran, J. H., Fibrous monolithic ceramics: III, mechanical properties and oxidation behavior of the silicon carbide/ boron nitride system. J. Am. Ceram. Soc., 1994, 77, 1249–1255. 16. Baskaran, S., Nunn, S. D. and Halloran, J. H., Fibrous mono￾lithic ceramics: IV, mechanical properties and oxidation behavior of the alumina/nickel system. J. Am. Ceram. Soc., 1994, 77, 1256– 1262. 17. Trice, R. W. and Halloran, J. H., Influence of microstructure and temperature on the interfacial fracture energy of silicon nitride/ boron nitride fibrous monolithic ceramics. J. Am. Ceram. Soc., 1999, 82, 2502–2508. 18. Trice, R. W. and Halloran, J. H., Elevated-temperature mechan￾ical properties of silicon nitride/boron nitride fibrous monolithic ceramics. J. Am. Ceram. Soc., 2000, 83, 311–316. 19. Trice, R. W. and Halloran, J. H., Effect of sintering aid compo￾sition on the processing of Si3N4/BN fibrous monolithic ceramics. J. Am. Ceram. Soc., 1999, 82, 2943–2947. 20. Hai, G., Yong, H. and An, W. C., Preparation and properties of fibrous monolithic ceramics by in-situ synthesizing. J. Mater. Sci., 1999, 34, 2455–2459. 21. She, J., Inoue, T., Suzuki, M., Sodeoka, S. and Ueno, K., Mechanical properties and fracture behavior of fibrous Al2O3/ SiC ceramics. J. Eur. Ceram. Soc., 2000, 20, 1877–1881. 22. Folsom, C. A., Zok, F. W. and Lange, F. F., Flexural properties of brittle multilayer materials: I, modeling. J. Am. Ceram. Soc., 1994, 77, 689–696. 23. He, M.-Y. and Hutchinson, J. W., Crack deflection at an inter￾face between dissimilar elastic materials. Int. J. Solids. Struct., 1989, l25, 1053–1067. 24. Camus, G., Modeling of the Mechanical behavior and damage process of fibrous ceramic matrix composites: application to a 2- D SiC/SiC. Int. J. Solids. Struct, 2000, 37, 919–942. 25. King, B. H., Influcence of Architecture on the Mechnaical Prop￾erties of Fibrous Monolithic Ceramics. PhD Thesis, University of Michigan, Ann Arbor, MI, 1997. Table 1 Summarized mechanical properties of monolithic Si3N4 and fibrous monoliths with cell boundary thickness of 37 mm Samples MOR (MPa) WOF (J/m2 ) KIC (MPa m1/2) Monolithic Si3N4 69721 Negligible 7.50.16 Fibrous monolith 22711 1216233 100.19 Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 699–703 703