Evolution of defect size and strength of porous alumina during sintering

The evolution of fracture strength with increasing density in ceramics, using alumina as a model system, is discussed in terms of the interplay between a defect serving as stress concentrator, a crack lying in its enhanced stress field and the fracture toughness of the porous ceramic. Introduction of crack-free fracture-causing artificial pores of various sizes allows detailed measurement of their shrinkage with ongoing densification, while fractography describes the location and type of fracture initiation. A fracture mechanics model, describing growth of a semicircular crack emanating from the pore until instability, yields good agreement with experiment. In particular, the result that the radius of the artificial, spherical defect in a size regime between 25 and 120 μm has only a small influence on fracture strength for samples with an average grain size smaller than 1μm, can be explained.