Relation between Strength, Fracture Toughness, and Critical Frontal Process Zone Size in Ceramics

Abstract

In order to improve the fracture toughness of inherently brittle ceramics, a new material design concept must be developed. One suitable concept involves the use of dislocation activities. Intra-type ceramic-based nanocomposites use dislocation activities to enhance strength and fracture toughness. The dislocations are generated by residual stresses induced during sintering process around the second-phase nanoparticles dispersed within the matrix grains. In this paper, first, we proposed an indirect technique for estimating a critical frontal process zone (CFPZ) size for ceramics and clarified the relation between the strength, fracture toughness, and CFPZ size. The fracture toughness of ceramics is closely related to the CFPZ size because ceramics with larger CFPZ size consume higher fracture energy during crack extension and consequently have higher fracture toughness. Second, we fabricated toughened alumina-nickel nanocomposites using a soaking method to construct an intra-type nanostructure, and found that the appropriate annealing conditions after sintering could achieve toughened nanocomposites. Finally, we discussed the relation between the experimentally obtained fracture toughness and the CFPZ size of monolithic alumina, as-sintered nanocomposites, and annealed nanocomposites. The results revealed that nanocomposites showed the largest CFPZ size and the highest fracture toughness after appropriate annealing: the sessile dislocations ahead of a crack tip in a nanocomposite are thought to serve as stress concentration, create many nanocracks in the CFPZ, and expand the CFPZ size.