S. Tariolle et al /Journal of the European Ceramic Sociery 25 (2005)3639-3647 An apparent fracture toughness has been calculated using was measured For each specimen, the maximum deviation the maximum load withstand by the composite. The fracture and the mean deviation for each interface or interlayer were toughness was measured using SENB method by Eq (6) given and correlated with the real fracture toughness by Eq. (7) Indeed. the value measured with SenB method overestimates the value of the fracture toughness when the notch root radius 3. Description of each type of composite: increases. A correction of the senb value has to be done macrostructure and mechanical properties kNB=va∑A Different types of interlayers or interfaces were elabo- rated. The denomination of each composite is indicated in Table 1. Whatever the material, the layers had uniform thick where ness and were parallel each other. The dense layers are sim- A0=1.9+0.075 ilar in the different composites. The toughness of a material made of dense layers is equal to 2.9 MPa m-and a work of rupture equal to 23 Jm-3 A1=-3.39+0.08 3.1. Weak interlayers A2=15.4-0.2175 3.1.1. Macrostructures and fractographies of the composites containing weak interlayers Different weak interlayers were studied, some had poros- A3=-2624+0.282 ity obtained with corn starch(CS), others were obtained us- ing no sintering aid (NSA), others mixing boron carbide and L nitride(B4C-BN). The grain size in the porous layers was A4=26.38-0.145 similar to that of the dense ones(0.8 um). The porosity in the layers with corn starch was interconnected and had a mean size of 10 um. In the porous layer without sintering aid, the KIC= KSENB tanh(2 V pe (7) material is under-sintered and the porosity was finer(around the micrometer). The weak layers obtained by a mixture of where pe is the notch root radius, ae the size of critical defect boron carbide and boron nitride have a skeleton of boron and y a geometrical factor equal to 1. 12 for a sharp crack carbide containing boron nitride grains. Macrostructures Lengths of crack deviation were measured on fractogra- of the different composites elaborated were represented in phies using the method developed by Kovar et al. 2 This Fig4 method consists in obtaining the delamination distances mea- Considering the characterization of the reinforcement tak suring the distance between through-thickness crack in adja- ing place in these composites, the typical results of 3 point- cent dense layers. An example of the method used is given bending tests were represented for each type of composite in Fig 3. The length of deviation at each interface or interlayer Figs. 5-7. Reinforcement by crack deflection was observed in the case of interlayers with corn starch for a porosity larg than 051(CS55)(Fig. 5)and for the interlayers made with a mixing of B C-BN(Fig. 7). In addition, as can be observed on the load-displacement curves, in these two cases, there was a friction stress due to the succession of crack deflections that induced a load resistance. In opposition, no crack deflection was observed in the composite(NSA)where interlayers are under-sintered. The rupture was brittle(Fig. 6) 3. 1.2. Weak porous interlayers in composites CS The influence of the porosity in the porous layers and the relative thickness between the dense and the porous layers on the work of rupture and on the lengths of crack deflection was studied in the composites(CS) with weak interlayers with corn starch 500 um 3.1.2.1. Infuence of the porosity on crack defection in CS. Fig 3. Fractography of a lamellar composite( CS55), the white lines corre- First, let us see the results concerning the influence of the bond to different crack deviations that have been measured level of porosity on the crack deflection properties. When theS. Tariolle et al. / Journal of the European Ceramic Society 25 (2005) 3639–3647 3641 An apparent fracture toughness has been calculated using the maximum load withstand by the composite. The fracture toughness was measured using SENB method11 by Eq. (6) and correlated with the real fracture toughness by Eq. (7). 11 Indeed, the value measured with SENB method overestimates the value of the fracture toughness when the notch root radius increases. A correction of the SENB value has to be done. KSENB IC = σr √ae 4 i=0 Ai ae W  i (6) where A0 = 1.9 + 0.075 L W , A1 = −3.39 + 0.08 L W , A2 = 15.4 − 0.2175 L W , A3 = −26.24 + 0.2825 L W , A4 = 26.38 − 0.145 L W . KIC = KSENB IC tanh  2Y ac ρe  (7) where ρe is the notch root radius, ac the size of critical defect and Y a geometrical factor equal to 1.12 for a sharp crack. Lengths of crack deviation were measured on fractogra￾phies using the method developed by Kovar et al.12 This method consists in obtaining the delamination distances mea￾suring the distance between through-thickness crack in adja￾cent dense layers. An example of the method used is given Fig. 3. The length of deviation at each interface or interlayer Fig. 3. Fractography of a lamellar composite (CS55), the white lines corre￾spond to different crack deviations that have been measured. was measured. For each specimen, the maximum deviation and the mean deviation for each interface or interlayer were given. 3. Description of each type of composite: macrostructure and mechanical properties Different types of interlayers or interfaces were elabo￾rated. The denomination of each composite is indicated in Table 1. Whatever the material, the layers had uniform thick￾ness and were parallel each other. The dense layers are sim￾ilar in the different composites. The toughness of a material made of dense layers is equal to 2.9 MPa m−1/2 and a work of rupture equal to 23 kJ m−3. 3.1. Weak interlayers 3.1.1. Macrostructures and fractographies of the composites containing weak interlayers Different weak interlayers were studied, some had poros￾ity obtained with corn starch (CS), others were obtained us￾ing no sintering aid (NSA), others mixing boron carbide and nitride (B4C-BN). The grain size in the porous layers was similar to that of the dense ones (0.8
m). The porosity in the layers with corn starch was interconnected and had a mean size of 10
m. In the porous layer without sintering aid, the material is under-sintered and the porosity was finer (around the micrometer). The weak layers obtained by a mixture of boron carbide and boron nitride have a skeleton of boron carbide containing boron nitride grains. Macrostructures of the different composites elaborated were represented in Fig. 4. Considering the characterization of the reinforcement tak￾ing place in these composites, the typical results of 3 point￾bending tests were represented for each type of composite in Figs. 5–7. Reinforcement by crack deflection was observed in the case of interlayers with corn starch for a porosity larger than 0.51 (CS55) (Fig. 5) and for the interlayers made with a mixing of B4C-BN (Fig. 7). In addition, as can be observed on the load–displacement curves, in these two cases, there was a friction stress due to the succession of crack deflections that induced a load resistance. In opposition, no crack deflection was observed in the composite (NSA) where interlayers are under-sintered. The rupture was brittle (Fig. 6). 3.1.2. Weak porous interlayers in composites CS The influence of the porosity in the porous layers and the relative thickness between the dense and the porous layers on the work of rupture and on the lengths of crack deflection was studied in the composites (CS) with weak interlayers with corn starch. 3.1.2.1. Influence of the porosity on crack deflection in CS. First, let us see the results concerning the influence of the level of porosity on the crack deflection properties. When the