H Miyazaki et al. / Journal of the European Ceramic Sociery 26(2006)3539-3546 powder used in this study contained 0.02 wt silica It is likely that yttrium in the zirconia phase may have diffused into the alumina phase and affected the grain growth of alumina By con trast, the size of alumina grain in the powder-mixture composites decreased significantly with increasing the zirconia content. The inhibition of grain growth in matrix phase by addition of sec- ndary phase particles is well known as"pinning effect". 19-21 The grain growth of alumina in the powder-mixture compos ites was inhibited by the pining effect of the zirconia particles. The grain size of zirconia in the fibrous composites was almost the same as that of the monolithic zirconia. The size of 4.0 grain in the powder-mixture composites decreased with increas ing the alumina content because of the pinning effect by alumina O Powder- mixture composite particles. Both the monolithic alumina and monolithic zirconia were Volume fraction of ZrO phase fz(vol%) sintered to almost full density (Table 1), whereas the relative density of the composites was slightly lower than that of the Fig. 5. Dependence of the fracture toughness of the composites on the volume monolithic. The insufficient densification in these fibrous com- fraction of zirconia phase fz posites may arise from mismatch in both the total amount of sintering shrinkage and the shrinkage rate between the two 3.3. Fracture toughness 3.3.1. Effect of"stress-induced"transformation of zirconia 3.2. Youngs modulus on the fracture toughness Fig 5 shows the fracture toughness of both the composites Fig 4 shows the dependence of the Youngs modulus on the and the constituent monolithic ceramics. The fracture tough ness of the powder-mixture composites was almost the same as fz for both the fibrous composites and the powder-mixture com- that of the monolithic alumina when the fz was 10 vol %, and posites. The solid line in the Fig. 4 shows calculated value using the well-known Voigt rule-of-mixture: increased with fz, then became saturated at the higher fz. To correlate toughness with the tetragonal-to-monoclinic transfor E= Ezf ea(-f2 mation, the volume fraction of the transformed zirconia phase in (1) the composites due to fracture was measured. The result is shown where Ez and Ea are the Youngs modulus of zirconia and alu- in Fig. 6 along with the data for the fibrous composites.The mina and fz is the volume fraction of the zirconia phase. The volume fraction of transformed zirconia in the powder-mixture Youngs modulus, Ez and Ea, used for the calculation was mea- composites was very small when fz was 10 vol%, and increased sured with the alumina and zirconia monoliths, respectively. with fz. It is obvious that the improvement in fracture toughness From Fig. 4. it is clear that the Young's modulus of both the of the powder-mixture composites was originated mainly from fibrous composites and powder-mixture composites followed the"stress-induced"transformation of zirconia phase the Voigt rule-of-mixture. The measured value was slightly lower than the predicted value, which is attributable to the lower relative densities of the composites. ◆ Co-extruded composite O Powder-mixture composite ◆ Co-extruded comp Powder-mixture comp 59 FE 6080100 Volume fraction of ZrO2 phase fz(vol% Volume fraction of ZrO2 phasefz(vol%) Fig. 6. Volume fraction of zirconia transformed from sites in both co-extruded composites and powder-mixture semamgosites o a function and powder-mixture composites on the volume fraction of zirconia phase fz. of volume fraction of zirconia phase fz3542 H. Miyazaki et al. / Journal of the European Ceramic Society 26 (2006) 3539–3546 powder used in this study contained 0.02 wt% silica. It is likely that yttrium in the zirconia phase may have diffused into the alumina phase and affected the grain growth of alumina. By con￾trast, the size of alumina grain in the powder-mixture composites decreased significantly with increasing the zirconia content. The inhibition of grain growth in matrix phase by addition of sec￾ondary phase particles is well known as “pinning effect”.19–21 The grain growth of alumina in the powder-mixture compos￾ites was inhibited by the pining effect of the zirconia particles. The grain size of zirconia in the fibrous composites was almost the same as that of the monolithic zirconia. The size of zirconia grain in the powder-mixture composites decreased with increas￾ing the alumina content because of the pinning effect by alumina particles. Both the monolithic alumina and monolithic zirconia were sintered to almost full density (Table 1), whereas the relative density of the composites was slightly lower than that of the monolithic. The insufficient densification in these fibrous com￾posites may arise from mismatch in both the total amount of sintering shrinkage and the shrinkage rate between the two phases. 3.2. Young’s modulus Fig. 4 shows the dependence of the Young’s modulus on the fZ for both the fibrous composites and the powder-mixture com￾posites. The solid line in the Fig. 4 shows the calculated value using the well-known Voigt rule-of-mixture: E = Ezfz + Ea(1 − fz) (1) where Ez and Ea are the Young’s modulus of zirconia and alu￾mina and fZ is the volume fraction of the zirconia phase. The Young’s modulus, Ez and Ea, used for the calculation was mea￾sured with the alumina and zirconia monoliths, respectively. From Fig. 4, it is clear that the Young’s modulus of both the fibrous composites and powder-mixture composites followed the Voigt rule-of-mixture. The measured value was slightly lower than the predicted value, which is attributable to the lower relative densities of the composites. Fig. 4. Dependence of the Young’s modulus of both co-extruded composites and powder-mixture composites on the volume fraction of zirconia phase fZ. Fig. 5. Dependence of the fracture toughness of the composites on the volume fraction of zirconia phase fZ. 3.3. Fracture toughness 3.3.1. Effect of “stress-induced” transformation of zirconia on the fracture toughness Fig. 5 shows the fracture toughness of both the composites and the constituent monolithic ceramics. The fracture tough￾ness of the powder-mixture composites was almost the same as that of the monolithic alumina when the fZ was 10 vol%, and increased with fZ, then became saturated at the higher fZ. To correlate toughness with the tetragonal-to-monoclinic transfor￾mation, the volume fraction of the transformed zirconia phase in the composites due to fracture was measured. The result is shown in Fig. 6 along with the data for the fibrous composites. The volume fraction of transformed zirconia in the powder-mixture composites was very small when fZ was 10 vol%, and increased with fZ. It is obvious that the improvement in fracture toughness of the powder-mixture composites was originated mainly from the “stress-induced” transformation of zirconia phase. Fig. 6. Volume fraction of zirconia transformed from tetragonal to monoclinic in both co-extruded composites and powder-mixture composites as a function of volume fraction of zirconia phase fZ