Evolution of defect size and strength of porous alumina during sintering

The evolution of fracture strength with increasing density in ceramics, usi ng alumina as a model system, is discussed in terms of the interplay betwee n a defect serving as stress concentrator, a crack lying in its enhanced st ress field and the fracture toughness of the porous ceramic. Introduction o f crack-free fracture-causing artificial pores of various sizes allows deta iled measurement of their shrinkage with ongoing densification, while fract ography describes the location and type of fracture initiation. A fracture mechanics model, describing growth of a semicircular crack emanating from t he pore until instability, yields good agreement with experiment. In partic ular, the result that the radius of the artificial, spherical defect in a s ize regime between 25 and 120 mum has only a small influence on fracture st rength for samples with an average grain size smaller than 1 mum, can be ex plained. (C) 2000 Elsevier Science Ltd. All rights reserved.