C. Reynaud et al. /Joumal of the European Ceramic Sociery 25 (2005)589-592 Table 3 Laminar composites Pore forming agent PFA content(vol. % Porosity(vol %. Laminar composite sequences Corn starch 47(42户 l/-2/1-1/2 l/-2/-3/1-2/1-1/2 0545 43.5(40)3 Graphite platelets l/-2/-1/2 ce, the numbers refer to the number of tapes stacked to make one layer and indicate the relative thickness of the dense/porous layers. The first number concems the dense layers, the second one the porous ones(see Section 2.2) The values into brackets correspond to the porous monoliths whereas the others are the values in the porous interlayers of the composites samples. The cross-head displacement rate was varied from mens where the porous layers have a porosity equal to 40 0.5 to 0.004 mm/min to test a possible effect of the solici 43.5and47% tation rate. The role of the pore morphology was illustrated However, in the case of CSLC, whatever the porosity by comparing the behaviour of composites obtained using level, the cross-head speed or the architecture, the rupture corn starch(CSLC) and graphite platelets(GPLC) as pore was catastrophic with no increase in the work of fracture forming agent. The different materials and architectures are SEM micrographs of the side surfaces are shown in Fig. 9 listed in Table 3 For porosity levels up to 30%, the surface of rupture is flat Typical load-deflection curves for CSLC and GPLC are and is not affected by the alternation of dense/porous layer reported in Fig. 7. For all the materials, the load remains For larger porosity levels, whereas the crack propagates per- linear up to the peak load, at which point a crack initiates pendicularly through the dense layers, its direction changes at some surface defects and grows quickly in the through progressively in the porous layers, but, the lengths of de thickness direction flection are extremely short, of the order of 100 um or less, According to Clegg's group, 7.8 crack deflection should be except for the architecture with a dense to porous thickness observed when relation 2 is fulfilled, i.e., when Gp/Gs(1-P) ratio of 3 where deflections of 300 um are observed. Then, becomes lower than 0.57, and extensive deflection must oc- the crack kinks out of the porous interphase when it reaches cur when this ratio is lower than 0. 4, that corresponds to the porous/dense interface without inducing delamination porosity higher than 37 and 44%, respectively(see Fig 8 in In the case of GPLC, where the pores are elongated and Ref. 7). Using the parameters from Wagh's model (Table 2), lie roughly parallel to the interfaces, the occurrence of a sec- the corresponding values of porosity for the present materi- ondary load peak(see curves B in Fig. 7)indicates that the als are 19 and 33%, respectively. Then, from Fig. 8, where penetrating crack was effectively arrested, that allowed the Gp/Gs(1-P), calculated from the experimental data re- measured load to rise again until a new crack formed. The de- orted in Section 4, is plotted versus the porosity of the flection in curve Bl corresponds to the cross-head displace- porous layers, it should be the case at least for the speci- ment whereas the deflection in curve B2 corresponds to the splacement of the outer tensile surface(see the schematic illustration in Fig. 7). The difference between the deflection 18 B2 12 Deflection(mm) Porosity(vol%) res in 3-point bend test uence 1/1, P=43.5%); B: GPLC Fig. 8. GP/Gs(1-P)vS. porosity of the porous layers(corn starch). The ection for Bl corresponds to the solid line corresponds to Gp/Gs(1-P)=0.57(Eq (2). The shadowed splacement; the corresponds to the displacement of band divides the graph in an upper region where crack does not deflect tensile layer(see explanation at the end of Section 5.1). and a lower region where crack did deflect according to Blanks et al. 7594 C. Reynaud et al. / Journal of the European Ceramic Society 25 (2005) 589–597 Table 3 Laminar composites Pore forming agent PFA content (vol.%) Porosity (vol.%) Laminar composite sequencesa Corn starch 55 47 (42)b 1/1–2/1–1/2 50 43.5 (40)b 1/1–2/2–3/1–2/1–1/2 45 40 1/1 34 28 1/1 25 21 1/1 Graphite platelets 50 41 1/1–2/1–1/2 a For a given sequence, the numbers refer to the number of tapes stacked to make one layer and indicate the relative thickness of the dense/porous layers. The first number concerns the dense layers, the second one the porous ones (see Section 2.2). b The values into brackets correspond to the porous monoliths whereas the others are the values in the porous interlayers of the composites. samples. The cross-head displacement rate was varied from 0.5 to 0.004 mm/min to test a possible effect of the solici￾tation rate. The role of the pore morphology was illustrated by comparing the behaviour of composites obtained using corn starch (CSLC) and graphite platelets (GPLC) as pore forming agent. The different materials and architectures are listed in Table 3. Typical load–deflection curves for CSLC and GPLC are reported in Fig. 7. For all the materials, the load remains linear up to the peak load, at which point a crack initiates at some surface defects and grows quickly in the through thickness direction. According to Clegg’s group,7,8 crack deflection should be observed when relation 2 is fulfilled, i.e., when GP/GS(1−P) becomes lower than 0.57, and extensive deflection must oc￾cur when this ratio is lower than 0.4, that corresponds to porosity higher than 37 and 44%, respectively (see Fig. 8 in Ref. 7). Using the parameters from Wagh’s model (Table 2), the corresponding values of porosity for the present materi￾als are 19 and 33%, respectively. Then, from Fig. 8, where GP/GS(1 − P), calculated from the experimental data re￾ported in Section 4, is plotted versus the porosity of the porous layers, it should be the case at least for the speci- 0 20 40 60 80 100 120 140 0 0.1 0.2 0.3 0.4 0.5 Deflection (mm) Load (N) A B1 B2 B1 B2 Fig. 7. Typical load–displacement curves in 3-point bend test of laminar composites. A: CSLC (sequence 1/1, P = 43.5%); B: GPLC (sequence 1/1, P = 41%). The deflection for B1 corresponds to the cross-head displacement; that for B2 corresponds to the displacement of the outer tensile layer (see explanation at the end of Section 5.1). mens where the porous layers have a porosity equal to 40, 43.5 and 47%. However, in the case of CSLC, whatever the porosity level, the cross-head speed or the architecture, the rupture was catastrophic with no increase in the work of fracture. SEM micrographs of the side surfaces are shown in Fig. 9. For porosity levels up to 30%, the surface of rupture is flat and is not affected by the alternation of dense/porous layers. For larger porosity levels, whereas the crack propagates per￾pendicularly through the dense layers, its direction changes progressively in the porous layers, but, the lengths of de- flection are extremely short, of the order of 100 m or less, except for the architecture with a dense to porous thickness ratio of 3 where deflections of 300 m are observed. Then, the crack kinks out of the porous interphase when it reaches the porous/dense interface without inducing delamination. In the case of GPLC, where the pores are elongated and lie roughly parallel to the interfaces, the occurrence of a sec￾ondary load peak (see curves B in Fig. 7) indicates that the penetrating crack was effectively arrested, that allowed the measured load to rise again until a new crack formed. The de- flection in curve B1 corresponds to the cross-head displace￾ment whereas the deflection in curve B2 corresponds to the displacement of the outer tensile surface (see the schematic illustration in Fig. 7). The difference between the deflection Fig. 8. GP/GS(1 − P) vs. porosity of the porous layers (corn starch). The solid line corresponds to GP/GS(1 − P) = 0.57 (Eq. (2)). The shadowed band divides the graph in an upper region where crack does not deflect and a lower region where crack did deflect according to Blanks et al.7