J. Ma et al. Journal of the European Ceramic Society 24(2004)825-831 Table 4 Total energy absorbed estimation in various layered systems Crack deflectio Energy absorbed vp(%) energy Ed (J/m) Ea( /m) flection(L Lo) E(/m) Dense 398 85.1 97.3 163.3 decrease in the fracture energy of the layered system from 57.6% to 65.2% of interlayer porosity, the frac ture energy of the system at 65. 2% interlayer porosity is still much higher than that of the monolithic sample (400% higher). As a result, in general, it can be con cluded that crack deflection mechanism significantly enhances the fracture toughness of the component It is also noted in the process that the porous inter- layer systems studied in the present work are different from that introduced by Clegg et al. as in the pre- sent systems, there exists two contributing mechan Fig. 6. Final energy absorbed in the layered systems as a function of nterlayer porosity estimated after considering the effect from crack isms, namely, crack deflection and porosity. Although deflection length and porosity the porosity has shown to be fracture energy dete- riorating factor. it should be noted that it could be an essential parameter when strength to weight ratio is concerned results obtained. The averaged crack deflection length for various porosity laminates were first estimated using the SEM micrographs, and the factor increase com- pared to that without crack deflection can then be 4. Conclusions omputed. Using results from Table 2, the reduction in energy due to porosity for each porosity laminate sys e present work, the addition of PMMA in raw tem can also be evaluated by considering the amount of ceramic powders is found to be an easy and effective porous volume fraction in each system. The final energy way to generate uniform porosities in porous ceramic absorbed for the various systems was calculated and materials. Ceramic layered systems with interlayers of presented in Table 4 and Fig. 6. It can be seen that the different porosities were successfully fabricated. Theo energy variation trend estimated is in good agreement retical models on crack deflection criteria for layered with that from the experimental determined fracture systems reported in the literature were studied and energy values. With no crack deflection, the 30.2% compared with the present experimental results. It is interlayer porosity sample experienced a decrease in found that pore interaction effect in the porous inter- fracture energy compared to the monolithic sample. layers cannot be neglected. It is also shown that an However, as crack deflection started to occur, the increase of porosity in the porous interlayers promotes enhancement in energy from longer crack path has crack deflection, and hence the fracture toughness of the resulting in an overall improvement in systems fracture system. However, as the porosity in the porous inter nergy. Nevertheless, the increasing trend stops after layers increases beyond a critical volume fraction, the 57.6 porosity interlayer system, as beyond this point, the overall system will be weakened due to the large amount amount of crack deflection shown almost no increase. of porosity introduced and finally result in a decrease in As a result, the reduction of the energy due to higher the fracture toughness. Despite that, it is noted that porosity took over in significance, resulted in sub systems that promote crack deflection will possess sequent decrease in overall fracture energy level. It higher fracture toughness than that of the monolithic hould be noted that despite the observation of a sampleresults obtained. The averaged crack deflection length for various porosity laminates were first estimated using the SEM micrographs, and the factor increase com￾pared to that without crack deflection can then be computed. Using results from Table 2, the reduction in energy due to porosity for each porosity laminate sys￾tem can also be evaluated by considering the amount of porous volume fraction in each system. The final energy absorbed for the various systems was calculated and presented in Table 4 and Fig. 6. It can be seen that the energy variation trend estimated is in good agreement with that from the experimental determined fracture energy values. With no crack deflection, the 30.2% interlayer porosity sample experienced a decrease in fracture energy compared to the monolithic sample. However, as crack deflection started to occur, the enhancement in energy from longer crack path has resulting in an overall improvement in system’s fracture energy. Nevertheless, the increasing trend stops after 57.6 porosity interlayer system, as beyond this point, the amount of crack deflection shown almost no increase. As a result, the reduction of the energy due to higher porosity took over in significance, resulted in sub￾sequent decrease in overall fracture energy level. It should be noted that despite the observation of a decrease in the fracture energy of the layered system from 57.6% to 65.2% of interlayer porosity, the frac￾ture energy of the system at 65.2% interlayer porosity is still much higher than that of the monolithic sample (400% higher). As a result, in general, it can be con￾cluded that crack deflection mechanism significantly enhances the fracture toughness of the component. It is also noted in the process that the porous inter￾layer systems studied in the present work are different from that introduced by Clegg et al.1 ; as in the pre￾sent systems, there exists two contributing mechan￾isms, namely, crack deflection and porosity. Although the porosity has shown to be fracture energy dete￾riorating factor, it should be noted that it could be an essential parameter when strength to weight ratio is concerned. 4. Conclusions In the present work, the addition of PMMA in raw ceramic powders is found to be an easy and effective way to generate uniform porosities in porous ceramic materials. Ceramic layered systems with interlayers of different porosities were successfully fabricated. Theo￾retical models on crack deflection criteria for layered systems reported in the literature were studied and compared with the present experimental results. It is found that pore interaction effect in the porous inter￾layers cannot be neglected. It is also shown that an increase of porosity in the porous interlayers promotes crack deflection, and hence the fracture toughness of the system. However, as the porosity in the porous inter￾layers increases beyond a critical volume fraction, the overall system will be weakened due to the large amount of porosity introduced and finally result in a decrease in the fracture toughness. Despite that, it is noted that systems that promote crack deflection will possess higher fracture toughness than that of the monolithic sample. Table 4 Total energy absorbed estimation in various layered systems Porosity Vp (%) Crack length L (mm) Factor increase due to crack deflection (L/L0) Crack deflection energy Ed (J/m2 ) Energy reduction due to porosity El (J/m2 ) Energy absorbed Ea (J/m2 ) Dense 3.0 1.00 – – 62.3 30.2 3.2 1.07 66.5 11.1 55.4 39.8 4.1 1.37 85.1 20.0 65.1 48.7 5.8 1.93 120.5 23.2 97.3 57.6 9.1 3.03 189.0 24.5 164.5 65.2 9.1 3.03 189.0 25.7 163.3 Fig. 6. Final energy absorbed in the layered systems as a function of interlayer porosity estimated after considering the effect from crack deflection length and porosity. 830 J. Ma et al. / Journal of the European Ceramic Society 24 (2004) 825–831