S. Tariolle et al. Journal of Solid State Chemistry 177(2004)487-49 感感 Fig 3. Micrographs of multi-layered BC materials: (a) black porous layers made with 50vol% of corn starch, (b) black porous layers made with 55 vol% of corn starch, (c) dark grey porous layers made in absence of sintering aid. Table 2 Thickness and density of layers in different B C multi-layered materials Dense layer Porous layer Thickness Relative Thickness Relative a(50 vol% 0.50 Fig. 6. Pore shape made by graphite platelets (a) or by polyamide powder(b)in SiC laminates sintering aid) characteristic. Lengthened pores aligned in the plane of the layers (Fig. 6a) seem to be more efficient that circular pores(Fig. 6b) 3.2.2.B4C The pictures and graphs(Fig. 7)show the crack propagation and the associated strain versus displace- ment curves in the different cases of laminar B,C materials In cases (a) and (b) rupture of the samples remains purely brittle. is no reinforcement of Fig 4. Micrograph of multi-layered B,C with weak interfaces formed materials by graphite coating However, in cases(c)and (d), crack deflection took place at the interfaces. The rupture is no more brittle and the work of fracture is improved 4. Conclusions Different SiC and BC multi-layered materials repared by using tape casting and thermocompression Characteristics of specimens were in agreement with those required to initiate crack deflection according to Clegg et al. [4, 5]. Even though specimens had the required level of porosity, significant crack deflection did not occur in liquid phase sintered Sic specimens Fig. 5. Micrographs of Sic multi-layered materials:(a) porous layer The absence of crack deflection at the interfaces may be made with corn starch P=4647 vol%,(b) porous layer made with linked to the continuous variation of properties(rupture graphite platelets P=40-41 vol% energy) from dense to porous layers due to the presencecharacteristic. Lengthened pores aligned in the plane of the layers (Fig. 6a) seem to be more efficient that circular pores (Fig. 6b). 3.2.2. B4C The pictures and graphs (Fig. 7) show the crack propagation and the associated strain versus displace￾ment curves in the different cases of laminar B4C materials. In cases (a) and (b), the rupture of the samples remains purely brittle. There is no reinforcement of materials. However, in cases (c) and (d), crack deflection took place at the interfaces. The rupture is no more brittle and the work of fracture is improved. 4. Conclusions Different SiC and B4C multi-layered materials were prepared by using tape casting and thermocompression. Characteristics of specimens were in agreement with those required to initiate crack deflection according to Clegg et al. [4,5]. Even though specimens had the required level of porosity, significant crack deflection did not occur in liquid phase sintered SiC specimens. The absence of crack deflection at the interfaces may be linked to the continuous variation of properties (rupture energy) from dense to porous layers due to the presence ARTICLE IN PRESS (a) (b) (c) Fig. 3. Micrographs of multi-layered B4C materials: (a) black porous layers made with 50 vol% of corn starch, (b) black porous layers made with 55 vol% of corn starch, (c) dark grey porous layers made in absence of sintering aid. Table 2 Thickness and density of layers in different B4C multi-layered materials Dense layer Porous layer Thickness (mm) Relative density Thickness (mm) Relative density a (50 vol% corn starch) 150 0.94 100 0.55 b (55 vol% corn starch) 150 0.94 100 0.50 c(without sintering aid) 100 0.94 150 0.65 Fig. 4. Micrograph of multi-layered B4C with weak interfaces formed by graphite coating. (a) (b) Fig. 5. Micrographs of SiC multi-layered materials: (a) porous layer made with corn starch P=46–47 vol%, (b) porous layer made with graphite platelets P=40–41 vol%. (a) (b) 20µm 20µm Fig. 6. Pore shape made by graphite platelets (a) or by polyamide powder (b) in SiC laminates. 490 S. Tariolle et al. / Journal of Solid State Chemistry 177 (2004) 487–492