matrix. On the other hand, the mixture region of alumina and zirconia has higher macroscopic thermal expansion coefficient than the cluster region of polycrystalline alumina and thus the compressive stress occur in the polycrystalline alumina cluster in the samples where polycrystalline alumina clusters are distributed in the mixture structure of alumina and zirconia. SEM observation indicates that a crack propagated into the polycrystalline alumina cluster. If the compressive stress prevents crack propagation in the polycrystalline alumina cluster, the fracture toughness would be enhanced. Of course, further study is needed because grain size distribution, crack deflection, crack bowing and the other factors should be considered to explain the toughening mechanism by the addition of polycrystalline alumina powder. Alumina Particulate Glass Matrix Composites (Alumina-glass) Figure 5 shows typical XRD pattern of the alumina-glass samples. The samples using virgin alumna powder shows that the formation of mullite occurred when sintering temperature was higher than 1473K. On the other hand, the samples using polycrystalline alumina powder shows the mullite formation occurred when sintering temperature was higher than 1373K. The peak ratio of mullite (peak B) to alumina (peak A) exhibits 0.25 for the samples using virgin alumina and 0.46 for the samples using polycrystalline alumina powder when sintering temperature was 1473K. The addition of polycrystalline alumina powder, therefore, enhances mullite formation. Possible reason of enhancement of mullite formation is impurities mixed at preparation of polycrystalline powder. Figure 6 shows microstructure of the samples with mullite formation. Fibrous mullite B A (a) (b) Figure 6: Microstructure of the etched surface of alumina particulate Pylex glass sintered at 1473K. (a) the sample using virgin alumina powders (b) the sample using polycrystalline alumina powder. Figure 5: XRD pattern of alumina particulate Pylex glass using polycrystalline alumina powder (sintering temperature is 1473K).
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