designed and controlled on a singular scale level. Natural materials (e.g. bamboo), however, consist of complicated structural elements on the several scale level and achieve the excellent combinations of mechanical properties (e.g. flexibility, toughness and stiffness). In the recent studies, synergy ceramics has been proposed and some artificial hyper-organized structure controlling ceramics were fabricated [4]. Nano ceramics composites, for example, exhibited better fracture strength than conventional ceramics [5,6] and the fracture toughness of Si3N4 was enhanced by elongated coarse grains [7]. The idea of nano-composites and coarse grain bridging was based on the mixture of dual scale particles. On the other hand, the mixture of dual scale microstructures is also attractive for design of synergy ceramics. Simple example is the composites where clusters of ceramics A are dispersed in matrix of ceramics B. In this case, if the cluster bridging occurs, the toughening by the cluster bridging would be more effective than that by the grain bridging in the conventional ceramics composites because cluster size is much larger than fine singular grain. Though the coarse singular gains dispersed composites can be expected to get the same effect of the toughening by cluster bridging, it usually shows significant decrease in fracture strength caused by inter/trans granular fracture along/in coarse grains. Polycrystalline alumina clusters dispersed composites, however, could maintain the same fracture strength of conventional fine ceramics because the clusters consist of fine singular grains. In this study, alumina polycrystalline powder was obtained by crushing pre-sintered alumina body, mixed with alumina-zirconia composites or Pylex glass. Polycrystalline clusters dispersed composites were obtained by atmospheric sintering and measurements of mechanical properties, observation of microstructure and phase detection by XRD were performed. EXPERIMENTAL PROCEURE Commercially available alumina powder (Sumitomo Chemical Co., Ltd., AES-11) was used for polycrystalline powder preparation. Alumina powder was pressed by cold isostatic press under 200 MPa and sintered in air at 1873K for 2h. Sintered alumina body was crashed by a stamp-mill and a automated mortar and sieved through the mesh of which opening size is 32µm. Obtained polycrystalline alumina powder was mixed with virgin alumina powder and commercially available zirconia powder (Tosho Co., Ltd., TZ-3Y) by a wet ball-mill process in ethanol for 24h and dried by a rotary evaporator. The dried powder was pressed and sintered under the same condition mentioned above. The volume fraction of polycrystalline alumina, virgin alumina and virgin zirconia powder was shown in Table1. The volume fraction of total amount of alumina powder (virgin and polycrystalline power) and zirconia powder was set to 80 vol% : 20 vol%. Commercially available Pylex glass powder (Furuya Metal Co., Ltd., 80.9 mol% SiO2, 12.7 mol% B2O3 and 2.3 mol% Al2O3) was used for processing of alumina particulate glass matrix composites. The average diameter of Pylex glass powder was 2 µm. Polycrystalline or virgin alumina powder was mixed with Pylex glass powder, dried and pressed and under the same condition mentioned above. The pressed body was sintered in air for 9h. Volume fraction and sintering temperature were shown in Table2. Obtained sintered body was cut to the specimen of 3 x 4 x 40mm and four-point bending test was done. The bending test was performed using a universal testing machine with a cross-head speed of 0.5 mm/min, an upper span of 10 mm and a lower span of 30mm. The microstructure of polished and etched surface of specimens was observed a scanning electron microscope (SEM). Specimens of alumina-zirconia composites were also served for fracture toughness measurement. Vicker’s indenter was loaded on the specimen surface under 30kgf for 30sec to induce a crack perpendicular to long axis of a specimen. The four-point bending test was performed to the indented specimens, which were loaded tensile stress on indented surface and the fracture strength was measured. The length and depth of the indented crack on fracture surface was measured by optical microscope and the fracture toughness was calculated using Newman’s equation [8]. The path of indented crack on the surface was also observed by SEM. The phases in specimens of alumina-glass composites were identified by X-ray diffraction analysis (XRD) using Cu K radiation.
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