Toughening mechanism and frontal process zone size of ceramics

Abstract

In order to improve the fracture toughness of intrinsically brittle ceramics, it is essential to expand the frontal process zone (FPZ) ahead of a crack tip. Nanocomposites proposed by Niihara utilize dislocations generated around the dispersed nano-sized particles in matrix and expand the FPZ. In this paper, we discussed the toughening mechanism of ceramics using our experimental data on alumina-based nanocomposites, focusing on fundamental theories of fracture mechanics, such as Griffith-Irwin energy equilibrium and the local fracture criterion. We estimated the FPZ size using an indirect technique proposed recently, and clarified that the FPZ expansion toughening mechanism is achieved in nanocomposites by means of dislocation activities. The results revealed that ceramics with a larger FPZ size had higher fracture toughness and properly annealed nanocomposites had the largest FPZ size. The FPZ expansion mechanism in nanocomposites was considered that sessile dislocations generated around a crack tip in matrix, served as nano-crack nuclei, and hence expanded the FPZ size. It is clarified that the critical local stress is the strength of an infinite plate with no crack, and must be the true strength with no size effect. The fracture toughness, the critical local stress, and the FPZ size are the mutually dependent material properties, and the relation among them is clarified.