Gee et al. Enhanced fracture toughness by ceramic laminate design stress crack shielding. This optimisation assumes that References he additional toughening mechanism of crack bifurca- on is avoided. Ceramic laminates can be designed 1. w.J. Clegg, K. Kendall, N. MeN. Alford, T. w. Button and J D exhibit a range of fracture behaviour, including cata Birchall: Nature. 1990. 347. 455-457 strophic, multistage bifurcation and delamination. In n,E. Carlstrom and w.J. Clegg: J.Er. Ceran.Soc,1998,18,1945-195 the present authors opinion, residual stress crack 3. J. B. Davis, A. Kristoffersson, E Carlstrom and w.J. Cleg shielding offers the best potential for maximising J.Am. Ceran.Soc,2000,83,2369-2374 apparent fracture toughness D. Kovar. M. D. Thoules and J W. Halloran: J. An Ceram. Soc. 5. D-H. Kuo and Kriven: Mater. Sci. Eng. A, 1998. 241 241-250 Conclusions 6. D -H. Kuo and w. M. Kriven: J. Anm. Cera. Soc. 1995. 78 a brief overview has been presented of some proposed 7. J.R. Mawdsley, D Kovar and J w. Halloran: J. Am. CeramSoc. toughening mechanisms and laminate failure mechanisms 2000 balanced, multilayer ceramics, containing periodic 8. B. Hatton and P.s. Nicholson: J. Am. Ceram Soc., 2001, 84, residual stress patterns. Multilayer ceramic composites may be designed to exhibit some unique mechanical 9. M. P. Rao, A.J. Sanchez-Herencia, G. E Beltz, R. M. MeMeeking and F. F. Lange: Science. 1999. 286. 102-105 properties. Theoretical critical thickness calculations 10. Z. Lences, P lik, M. Toriyama, M. E. Brito and S Kanzaki have been reported to avoid tensile matrix cracking J.Eur. Ceran.Soc,2000,20.347-355 and to incorporate or exclude both edge cracking and I1. P. Z. Cai, D. J. Green and G. L. Messing: J. Eur. Ceram. Soc. bifurcation by design. The previously reported model 998,5,2025-2034 describing the phenomena of threshold strength in 12. M. P. Rao and F. F. Lange: J. Am. Ceran. Soc., 2002, 85. laminar composites has been discussed. The powerful 13. S Ho, C. Hillman. F F. Lange and Z Suo: J Am Ceram Soc theoretical tool of fracture mechanics weight func 1995,78,2353-2359 tion analysis has been introduced. Silicon nitride based 14 M. Oechsner, C. Hillman and FF. Lange: J. Am. Ceram. Soc. ceramics with strong interfaces have successfully been 996,79,183438 Mater. 2003 manufactured by stacking and hot pressing green tapes N4-30 wt-%TIN. Apparent fracture ,s 35, 3otsi, M. Lugovy and V. Slyunyayev: M. Lugovy, N. Orlovskaya, V Slyunyayev, G Gogotsi, J. Kuebl oughness KIe was observed to increase from Sanchez-Herencia: Compos. Sci. TechnoL, 2002, 6 4-26 MPa m 2 for monolithic Si3N4 to 17 MPa m 819-830 17. S Ho and Z Suo: J. Appl. Mech., 1993, 60, 890-894 for a non-optimised laminate. The R curve behaviour 18. FF. Lange: J.A Ceram. Soc.,1989, 72,3 f a Si3N4/Si3N4-30 wt-%Tin laminate with surface 19. H. F. Bueckner: Z. Angew. Math. Mech. 1970. 50. 529-546 compressive stresses has been investigated using SEVNB 20. T. Fett and D. Munz: J. Mater. Sci. Lett., 1990.9.1403-1406 and demonstrated to agree well with theoretical predic- 21. D. B. Marshall and M. v. Swain: J. A. Ceram Soc., 1988, 71, tions based on fracture mechanics weight function analysis. 22. R. Lakshminarayanan, D. Shetty and R. Cutler: J. Am. Ceram. Soc.,1996.79,79-87 23. A. J. Blattner, R. Lakshminarayanan and D. Shetty: Eng. Fract. Acknowledgement 24. R. Moon, M. Hoffman, J. Hilden, K. Bowman, K. Trumble and Rodel: J. Am. Ceram. Soc. 2002. 85. 1505-1511 This work was supported by the European Commission 25. R Moon, M. Hoffman, J Hilden, K Bowman, K Trumble and under the Copernicus 2 programme. It is part of the J. Rodel: Eng. Fract. Mech, 2002. 69. 1647-166 26. J. Kuebler: Fract. Mech. Ceram. 2002. 13. 437-4 roject"Silicon nitride based laminar and functionally 27. R. Larker, L-Y. Wei, M. Olsson and B.Loberg:Proc.4th Int gradient ceramics for engineering application, proposal Symp. on Ceramic Materials and Components for Engines, (ed R NICA2-1999-10109 Carlsson): 1992. Amsterdam, Elsevier Applied Science. Advances in Applied Ceramics 2005 VOL 104 No3 109stress crack shielding. This optimisation assumes that the additional toughening mechanism of crack bifurca￾tion is avoided. Ceramic laminates can be designed to exhibit a range of fracture behaviour, including cata￾strophic, multistage, bifurcation and delamination. In the present authors’ opinion, residual stress crack shielding offers the best potential for maximising apparent fracture toughness. Conclusions A brief overview has been presented of some proposed toughening mechanisms and laminate failure mechanisms in balanced, multilayer ceramics, containing periodic residual stress patterns. Multilayer ceramic composites may be designed to exhibit some unique mechanical properties. Theoretical critical thickness calculations have been reported to avoid tensile matrix cracking and to incorporate or exclude both edge cracking and bifurcation by design. The previously reported model describing the phenomena of threshold strength in laminar composites has been discussed. The powerful theoretical tool of fracture mechanics weight func￾tion analysis has been introduced. Silicon nitride based ceramics with strong interfaces have successfully been manufactured by stacking and hot pressing green tapes of Si3N4 and Si3N4–30 wt-%TiN. Apparent fracture toughness KIc was observed to increase from 4.26 MPa m1/2 for monolithic Si3N4 to 17 MPa m1/2 for a non-optimised laminate. The R curve behaviour of a Si3N4/Si3N4–30 wt-%TiN laminate with surface compressive stresses has been investigated using SEVNB and demonstrated to agree well with theoretical predic￾tions based on fracture mechanics weight function analysis. 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