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It is known that thermal residual stress in particulate ceramics results from the mismatch of thermal expansion coefficients of particulates and matrix and contributes to toughening of ceramic composites. In this study, alumina-zirconia composites with agglomerated structures were prepared using alumina or alumina-zirconia powder to obtain large-sized compressive zones in particulate ceramics without degrading flexural strength. Agglomerated powder was obtained by pre-sintering. Then several samples used different fraction and size of agglomerated powder were prepared by pressureless sintering. Microstructure and crack paths of prepared samples were examined by scanning electron microscopy (SEM); flexural strength and fracture toughness of samples were evaluated by four-point flexural test and controlled surface flow method, respectively. Alumina-rich agglomerated structures and a zirconia-rich matrix were formed in samples that were produced using pre-sintered powder. Addition of zirconia to pre-sintered alumina powder prevented coarsening of alumina grains in agglomerated structures. Grain coarsening and cracking caused the decrease in flexural strength of samples with agglomerated structures. Agglomerated structures enhanced fracture toughness. In particular, a specimen using 21.1 vol% of pre-sintered alumina-rich powder of 32 to 150 mm exhibited increase in fracture toughness by approximately 30% without sacrificing average flexural strength. A SEM observation of crack paths showed that grain bridging did not occur in samples. Thereby, we inferred that the compressive residual stress zone in agglomerated structures played an important role in raising fracture toughness.

Preparation and Mechanical Properties of Alumina-Zirconia Composites with Agglomerated Structures Using Pre-Sintered Powder