0. Zan et al. Ceramics International 30(2004)441-446 h,)are plotted in Figs. 6 and 7, respectively, where w(2)Keeping the layer thickness ratio of about 10 and and Wo are the work of fracture of multilayered Si3N4/ increasing the number of layers, the bending strength BN and monolithic SI3 N4 ceramics respectively decreases slightly, and a maximal toughness was Comparing Figs 6 and 7 to Figs. 3 and 4, the similar obtained at NA30 curve trend was found, but different critical points and ()According to the mechanical model arid experi- absolute value of mechanical properties. That is because mental results, it is suggested that the number of layers of the following two reasons. On the one hand, in the and ratio of layer thickness are 30 and 10 in structural model calculation only the absorbed energy by the crack design. deflection is calculated. However, in fact many other toughening mechanisms in Si3 N4/BN multilayer materi- als were identified, such as bridging, frictional sliding, References whisker toughening, and so on [2, 3, 5]. A mass of energy absorbed during the fracture process by these [I] W.J. Clegg, K. Kendall, NMcN. Alford, T.W. Button, mechanisms. On the other hand. some defects will be J D. Birchall, A simple way to make tough ceramics, Nature unavoidably introduced into the multilayer materials in London)347(1990 fabrication processing; therefore the me 2]H. Liu, S.M. Hsu, Fracture behavior of multilayer silicon nitride/ canicE al boron nitride ceramics, J. Am. Ceram Soc. 79(1996)2452-2457. erties of the multilayer materials are lower than those 3 D. Kovar, M.D. Thouless, J.W. Halloran, Crack deflection and calculated results by the model propagation in layered silicon nitride/boron nitride ceramics According to the above four figures, synthetically Am. Ceran.Soc.81(1998)10041012 considering bending strength and work of fracture 4Y. Huang, H. Guo, Z P. Xie, The fine micro-structure of inter- optimal geometrical factors are selected as ih 10 and face layer for laminated Si]N4 ceramics, J. Mater. Sci. Lett. 17 (1998)569-571 5 C.A. Wang. Y. Huang, Q F Zan, H. Guo, S.Y. Cai, Biomimetic for Si3N4 layer and 10 um for bn layer) for work of ructure design-a possible approach to change the brittleness of fracture reaching optimum and bending strength t. Sci. Eng. C: Biomimetic and Supra- maining relative high level. molecular Systems 11(2000)9-12. 6 S.Y. Cai, PhD Dissertation, Tsinghua University, 1998 [7 C.A. Folsom, F.w. Zok, F.F. Lange, Flexural properties of brit- tle multilayer materials: I, modelling, J. Am. Ceram. Soc. 77 5. Conclusions (1994)689696 [8A.J. Phillipps, W.J. Clegg, T.w. Clyne, Fracture behavior of (1)At a given number of layers, with the increase of ramic laminates in bending--l. modeling of crack propagation, layer thickness ratio in a range, the toughness and Acta Metall. Mater. 41(1993)805-817. 9x.H. Guo, Q.H. Cai, C.A. Wang, Y Huang, A mechanics ana- bending strength both increase, and when it continues lysis of strengthening toughening design of multilayer structure increasing, the two curves both level off. ceramics. Acta Mech. Solid Sina 21(2000)313-324h1) are plotted in Figs. 6 and 7, respectively, where W and W0 are the work of fracture of multilayered Si3N4/ BN and monolithic S13N4 ceramics respectively. Comparing Figs. 6 and 7 to Figs. 3 and 4, the similar curve trend was found, but different critical points and absolute value of mechanical properties. That is because of the following two reasons. On the one hand, in the model calculation only the absorbed energy by the crack deflection is calculated. However, in fact many other toughening mechanisms in Si3N4/BN multilayer materi￾als were identified, such as bridging, frictional sliding, whisker toughening, and so on [2,3,5]. A mass of energy is absorbed during the fracture process by these mechanisms. On the other hand, some defects will be unavoidably introduced into the multilayer materials in fabrication processing; therefore the mechanical prop￾erties of the multilayer materials are lower than those calculated results by the model. According to the above four figures, synthetically considering bending strength and work of fracture, optimal geometrical factors are selected as lh
10 and N
30 (corresponding to a thickness of about 100 mm for Si3N4 layer and 10 mm for BN layer) for work of fracture reaching optimum and bending strength remaining relative high level. 5. Conclusions (1) At a given number of layers, with the increase of layer thickness ratio in a range, the toughness and bending strength both increase, and when it continues increasing, the two curves both level off. (2) Keeping the layer thickness ratio of about 10 and increasing the number of layers, the bending strength decreases slightly, and a maximal toughness was obtained at N
30. (3) According to the mechanical model arid experi￾mental results, it is suggested that the number of layers and ratio of layer thickness are 30 and 10 in structural design. References [1] W.J. Clegg, K. Kendall, N.McN. Alford, T.W. Button, J.D. Birchall, A simple way to make tough ceramics, Nature (London) 347 (1990) 455–457. [2] H. Liu, S.M. Hsu, Fracture behavior of multilayer silicon nitride/ boron nitride ceramics, J. Am. Ceram. Soc. 79 (1996) 2452–2457. [3] D. Kovar, M.D. Thouless, J.W. Halloran, Crack deflection and propagation in layered silicon nitride/boron nitride ceramics, J. Am. Ceram. Soc. 81 (1998) 1004–1012. [4] Y. Huang, H. Guo, Z.P. Xie, The fine micro-structure of inter￾face layer for laminated Si3N4 ceramics, J. Mater. Sci. Lett. 17 (1998) 569–571. [5] C.A. Wang, Y. Huang, Q.F. Zan, H. Guo, S.Y. Cai, Biomimetic structure design—a possible approach to change the brittleness of ceramics in nature, Mat. Sci. Eng. C: Biomimetic and Supra￾molecular Systems 11 (2000) 9–12. [6] S.Y. Cai, PhD Dissertation, Tsinghua University, 1998. [7] C.A. Folsom, F.W. Zok, F.F. Lange, Flexural properties of brit￾tle multilayer materials: I, modelling, J. Am. Ceram. Soc. 77 (1994) 689–696. [8] A.J. Phillipps, W.J. Clegg, T.W. Clyne, Fracture behavior of ceramic laminates in bending—I. modeling of crack propagation, Acta Metall. Mater. 41 (1993) 805–817. [9] X.H. Guo, Q.H. Cai, C.A. Wang, Y. Huang, A mechanics ana￾lysis of strengthening toughening design of multilayer structure ceramics, Acta Mech. Solid Sina 21 (2000) 313–324. 446 Q. Zan et al. / Ceramics International 30 (2004) 441–446