S. Tariolle et al /Journal of the European Ceramic Sociery 25(2005)3639-3647 500um Fig. 4.(a-c) Macrostructures of the different composites: (a) interlayers with corn starch(CS50); (b) interlayers under-sintered (NSA),(c)interlayers of B4 C-BN. Black layers: porous, grey layers: dense porosity increased in the interlayers, an important increase of deflection in such type of materials. In boron carbide com- the work of rupture(Fig 8)and of the length of crack deflec- posites, deflection appeared from a porosity of 0.51. There tion( Fig. 9)was observed for a porosity of 0.51. Moreover is a great difference between our observations and those pre- the presence of a friction stress indicated some reinforcement dicted by the theory of Clegg. This difference can be explain in the case of a porosity equal to 0.46 and 0.51(Table 2). The by studying the energetic criterion that is fully developed apparent fracture toughness was also increased with porosity in a further article. I (Table 2). More the interlayers were porous, so brittle, better was the reinforcement of the composite. Then, the porosity 3. 1.2.2. Infuence of the relative thickness of the layers in in the porous interlayers is an important criteria for crack de- CS55. The influence of the relative thickness of the dense and flection in composites. In the introduction, a porosity of 0.37 of the porous layers ea/ep was studied in composites contain- was required by Clegg and coworkers+,to observe crack ing a porosity of 0.51 in the porous layers(CS55 ). This pa- rameter was varied between 0.27 and 257. Macrostructure Table 2 of these types of composites were represented in Fig. 10 Values of apparent toughness and friction stress in function of the porosity As we can observe for work of rupture(Fig. 11), lengths in the interlayers in composites( CS)with interlayers with corn starch of crack deflection(Fig. 12)and also for the apparent friction Composites Porosity in porous KIC(MPam 2) Fr(MPa) stress and the apparent fracture toughness, there was a great dispersion of the values in function of ed/ep. This criterion CS45 1.54±0.16 seemed to have no significant influence on reinforcement by CS50 195±0.34 crack deflection. All the composites tested( CS55) presented CS55 3.44±0.95 reinforcement3642 S. Tariolle et al. / Journal of the European Ceramic Society 25 (2005) 3639–3647 Fig. 4. (a–c) Macrostructures of the different composites: (a) interlayers with corn starch (CS50); (b) interlayers under-sintered (NSA); (c) interlayers of B4C-BN. Black layers: porous; grey layers: dense. porosity increased in the interlayers, an important increase of the work of rupture (Fig. 8) and of the length of crack deflec￾tion (Fig. 9) was observed for a porosity of 0.51. Moreover the presence of a friction stress indicated some reinforcement in the case of a porosity equal to 0.46 and 0.51 (Table 2). The apparent fracture toughness was also increased with porosity (Table 2). More the interlayers were porous, so brittle, better was the reinforcement of the composite. Then, the porosity in the porous interlayers is an important criteria for crack de- flection in composites. In the introduction, a porosity of 0.37 was required by Clegg and coworkers4,5 to observe crack Table 2 Values of apparent toughness and friction stress in function of the porosity in the interlayers in composites (CS) with interlayers with corn starch Composites Porosity in porous interlayers KIC (MPa m1/2) Ff (MPa) CS45 0.42 1.54 ± 0.16 0 CS50 0.46 1.95 ± 0.34 1.26 ± 0.78 CS55 0.51 3.44 ± 0.95 1.81 ± 0.59 deflection in such type of materials. In boron carbide com￾posites, deflection appeared from a porosity of 0.51. There is a great difference between our observations and those pre￾dicted by the theory of Clegg. This difference can be explain by studying the energetic criterion,8 that is fully developed in a further article.13 3.1.2.2. Influence of the relative thickness of the layers in CS55. The influence of the relative thickness of the dense and of the porous layers ed/ep was studied in composites contain￾ing a porosity of 0.51 in the porous layers (CS55). This pa￾rameter was varied between 0.27 and 2.57. Macrostructures of these types of composites were represented in Fig. 10. As we can observe for work of rupture (Fig. 11), lengths of crack deflection (Fig. 12) and also for the apparent friction stress8 and the apparent fracture toughness8, there was a great dispersion of the values in function of ed/ep. This criterion seemed to have no significant influence on reinforcement by crack deflection. All the composites tested (CS55) presented reinforcement