Fractography of Al2O3 Ceramics Strengthened by Precoarsening Treatments

Abstract

Role of precoarsening treatments in the strengthening mechanism were examined by scanning electron microscopy (SEM) observations on the fracture surface of Al2O3 ceramics, which were made by hot-pressing under various sintering conditions. Al2O3 polycrystals by general sintering procedure yielded the 4-point bending strength of 400-500MPa, while the precoarsening treatments increased the strength up to 750MPa. Since the fracture toughness kept the constant value of 3.5MPa√m regardless of the treatments, it is expected that the reduced flaw size resulted in the enhanced strength. From the observations of the fracture origin, it was found that the precoarsening treatments suppressed the stable growth of a flaw during the bending tests and gave the relatively small flaw size. The increased stability of grain boundary near the flaw is thought to be the reason for the reduced stable growth rate of the flaw. The decreasing tendency of fracture toughness with decreasing flaw size was not observed due to the small grain size, which causes the crack propagation without the development of process zone.