0. Zan et al. Ceramics International 30(2004)441-446 180 (d) Fig. 2. SEM photographs of crack propagating paths in Si3N4/BN multilayer ceramics:(a) crack deflection;(b)crack bridging: (c) friction and sliding:(d)matrix-layer pull-out the preparation procedure becomes difficult, would lead to a defect in the interlayer increasing and lowering the strength of the multilayer material; on the other hand the too thin interlayer weakens the abil ity of deflecting crack, when the multilayer material is loaded, crack is difficult to deflect and propagate in the interlayer, in other words, the material is easy to frac ture catastrophically and low work of fracture obtained. these are the reasons that the two curves tend to level off when the layer thickness ratio is large enough Considering both bending strength and work of frac The Layer Thickness Ratio ture, a suitable value of layer thickness ratio is sug- Fig 3. Bending strength and work of fracture with various layer gested as about 10 in the multilayer structure design. thickness ratios(.. bending strength; - work of fracture) 3.3. Efect of layer number (N crack propagation, hence it will lower both the bending ih is too large (i.e. the matrix layer is too thick and e In the case of approximately fixed thickness ratio, for strength and work of fracture On the other hand if the mple about 10 for thickness ratio designed in the interlayer too thin), it is favorable to the strength present work, the effect of matrix-layer thickness on bviously. but, because of the interlayer being too thin, mechanical properties of the laminated Si3 n4/Bn cera-crack propagation, hence it will lower both the bending strength and work of fracture. On the other hand, if the lh is too large (i.e. the matrix layer is too thick and interlayer too thin), it is favorable to the strength obviously. But, because of the interlayer being too thin, the preparation procedure becomes difficult, which would lead to a defect in the interlayer increasing largely and lowering the strength of the multilayer material; on the other hand the too thin interlayer weakens the abil￾ity of deflecting crack, when the multilayer material is loaded, crack is difficult to deflect and propagate in the interlayer, in other words, the material is easy to frac￾ture catastrophically and low work of fracture is obtained. These are the reasons that the two curves tend to level off when the layer thickness ratio is large enough. Considering both bending strength and work of frac￾ture, a suitable value of layer thickness ratio is sug￾gested as about 10 in the multilayer structure design. 3.3. Effect of layer number (N) In the case of approximately fixed thickness ratio, for example, about 10 for thickness ratio designed in the present work, the effect of matrix-layer thickness on mechanical properties of the laminated Si3N4/BN cera￾Fig. 3. Bending strength and work of fracture with various layer thickness ratios (- - - bending strength; ––– work of fracture). Fig. 2. SEM photographs of crack propagating paths in Si3N4/BN multilayer ceramics: (a) crack deflection; (b) crack bridging; (c) friction and sliding; (d) matrix-layer pull-out. Q. Zan et al. / Ceramics International 30 (2004) 441–446 443