iropean Ceramic Society 24(2004)825-831 present results are in good agreement with that obtained value than that of the monolithic sample due to the by Clegg et al. on Al2O3 I and that by Blanks et al. on introduction of porosity in the component. On the other hand, it is also noted that for systems with crack deflection mechanism operating, i.e., the layered systems 3.3. Optimum configuration in layered systems with 39.8, 48.7, 57.6 and 65.2% of porous volume frac- tion in the porous interlayers, the fracture energy In the previous sections, the fabrication of a layered increases as the amount of porosity increases in the system with porous interlayer and the verification of the porous interlayer. However, when the porosity is further crack deflection criteria have been discussed. In the increased to 65.2%, the fracture energy of the system present section, the mechanical performance of the decreased again. Hence, there exists an optimum overall layered system with the different configuration is amount of porosity in the porous interlayers, or an investigated. The fracture energies of the layered sys- optimum value of R/Rm, to achieve the best fracture tems with different volume fraction of porosity in the toughness for such systems. This is attributed to the fact porous interlayers were quantified by four point bend- that as the amount of porosity in the porous interlayers ing tests. Fig. 4 shows the load-displacement curve for increases, despite the promotion of crack deflection the layered system(60% porosity in interlayer) and that mechanism, the overall mechanical strength of the of the monolithic alumina sample. It can be seen that component has been weakened. This strength weaken with effective crack deflection, the component possesses ing effect from the porosity hence becomes a competi higher fracture toughness. Fig. 5 shows the results of the tive factor to the toughening mechanism effect resulting racture energies of the layered systems with various from crack deflection. Eventually, as the porosity in the volume fraction of porosity in the porous interlayers. It porous interlayers exceeds a certain threshold where the can be further deduced that, in general, the layered sys- component becomes too weak, the overall fracture tems without crack deflection during the bending test energy of the system will deteriorate. This phenomenon possessed a lower value of fracture energy compared to can also be verified by estimating the total energy that with crack deflection occurring in the system. In absorbed from that contributed by crack deflection and fact, such system even possessed lower fracture energy that contributed by porosity, using the experimental Table 3 Fracture energy ratios between the dense and porous interlayers for cred systems with different amount of porosity volume fraction the porous interlayers Fig. 4. Load-displacement curve of a layered system with 60% Crack Deflection 0 100 Volume fracture of ceramic in interlayer Volume Fraction of Porosity (%) bols represent data fro Fig. 5. Fracture energy of various layered systems as a function of volume fraction porosity in the porous interlayerspresent results are in good agreement with that obtained by Clegg et al. on Al2O3 11 and that by Blanks et al. on SiC.2 3.3. Optimum configuration in layered systems In the previous sections, the fabrication of a layered system with porous interlayer and the verification of the crack deflection criteria have been discussed. In the present section, the mechanical performance of the overall layered system with the different configuration is investigated. The fracture energies of the layered sys￾tems with different volume fraction of porosity in the porous interlayers were quantified by four point bend￾ing tests. Fig. 4 shows the load-displacement curve for the layered system (60% porosity in interlayer) and that of the monolithic alumina sample. It can be seen that with effective crack deflection, the component possesses higher fracture toughness. Fig. 5 shows the results of the fracture energies of the layered systems with various volume fraction of porosity in the porous interlayers. It can be further deduced that, in general, the layered sys￾tems without crack deflection during the bending test possessed a lower value of fracture energy compared to that with crack deflection occurring in the system. In fact, such system even possessed lower fracture energy value than that of the monolithic sample due to the introduction of porosity in the component. On the other hand, it is also noted that for systems with crack deflection mechanism operating, i.e., the layered systems with 39.8, 48.7, 57.6 and 65.2% of porous volume frac￾tion in the porous interlayers, the fracture energy increases as the amount of porosity increases in the porous interlayer. However, when the porosity is further increased to 65.2%, the fracture energy of the system decreased again. Hence, there exists an optimum amount of porosity in the porous interlayers, or an optimum value of Ri/Rm, to achieve the best fracture toughness for such systems. This is attributed to the fact that as the amount of porosity in the porous interlayers increases, despite the promotion of crack deflection mechanism, the overall mechanical strength of the component has been weakened. This strength weaken￾ing effect from the porosity hence becomes a competi￾tive factor to the toughening mechanism effect resulting from crack deflection. Eventually, as the porosity in the porous interlayers exceeds a certain threshold where the component becomes too weak, the overall fracture energy of the system will deteriorate. This phenomenon can also be verified by estimating the total energy absorbed from that contributed by crack deflection and that contributed by porosity, using the experimental Table 3 Fracture energy ratios between the dense and porous interlayers for layered systems with different amount of porosity volume fraction in the porous interlayers Porosity Vp (%) Ri/Rm 30.2 0.60 39.8 0.29 48.7 0.18 57.6 0.13 65.2 0.09 Fig. 3. Relationship between relative fracture energy between the dense and adjacent porous layers and the porosity in the porous interlayers. Circle symbols represent data from Clegg et al., and delta symbols are from present work. Filled symbols indicate cracked deflection is observed. Fig. 4. Load–displacement curve of a layered system with 60% porosity interlayer and that of the monolithic sample. Fig. 5. Fracture energy of various layered systems as a function of volume fraction porosity in the porous interlayers. J. Ma et al. / Journal of the European Ceramic Society 24 (2004) 825–831 829