polycrystalline alumina powder could be related to the coarsening of polycrystalline alumina clusters due to double sintering mentioned above. On the other hand, the addition up to 20 mol% of polycrystalline alumina powder enhanced or kept constant of the bending strength of the samples regardless of coarsening of polycrystalline alumina clusters. This result indicates that the dual structure formed in alumina-zirconia samples may contribute on the improvement of bending strength. Figure 3 shows the fracture toughness of alumina-zirconia samples. Fracture toughness was apparently increased with the addition of polycrystalline alumina powder. Figure 4 shows a crack path of samples induced by Vicker’s indentation. In the case of the samples using only virgin powder, intergranular fracture largely dominated a crack path. In the case of the sample adding polycrystalline alumina powder, however, both of intergranular and transgranular fracture were observed in the cluster region of polycrystalline alumina grains and intergranular fracture largely dominated in the other region. It is noted that a crack deflection along polycrystalline alumina clusters had never been observed. This fact points out that the cluster bridging by polycrystalline alumina grains unfortunately did not occur in any alumina-zirconia samples although the grain bridging by alumina and zirconia particles were often observed in the mixture region of alumina and zirconia. Possible reason why the fracture toughness was enhanced by the addition of polycrystalline alumina powder is the change of the internal stress distribution in composites. The thermal expansion coefficient of zirconia is higher than that of alumina and consequently the tensile stress occur in the zirconia particles due to thermal residual stress in the case of the samples where zirconia particles are uniformly distributed in the alumina 5.0 4.0 3.0 2.0 30 20 10 0 Vp / % 700 600 500 σb / MPa 400 30 20 10 0 Vp / % KIC / MPa 1/2 Figure 3: Relationship between fracture toughness (KIC) and volume fraction (Vp) of polycrystalline alumina powder. Figure 2: Relationship between bending strength (σb) and volume fraction (Vp) of polycrystalline alumina powder. Figure 4: Indented crack path on the etched surface of the sample adding 20 vol% of polycrystalline alumina powder.
RkJQdWJsaXNoZXIy MjM0NDE=