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 nanosized 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.

Short-term breakdown and long-term failure in nanodielectrics: a review

“…NANOCOMPOSITES present a series of unique properties, such as electrics [1][2][3], mechanics [4][5], optics [6][7] and magnetics [8,9], due to nanoparticles with a giant specific surface area, quantum size effect and the special interface between particles and polymer matrix. Nanodielectrics have attracted a great attention since the first experimental data were reported publicly in 2002 [10][11].…”

LDPE/HDPE/Clay Nanocomposites: Effects of Compatibilizer on the Structure and Dielectric Response

“…There has been growing interest in polymer/nanoclay nanocomposites in recent years because of their outstanding properties at low loading levels as compared with conventional composites. It has been observed that adding small quantities of nanoclay to some thermoplastics as a reinforcing filler to form nanocomposite materials has not only led to more improved mechanical and thermal properties, but also to an enhancement of the dielectric strength and partial discharge resistance [1][2][3][4][5][6][7][8][9][10]. However, the understanding of the role of the interfaces of the nanofillers with the molecular mobility mechanism is still rather unsatisfactory.…”

Relation between functional properties of alumina-based nanocomposites and locations of dispersed particles

“…The dislocations within matrix could improve the fracture strength and fracture toughness because the residual stress in alumina decreased and the frontal process zone size increased. 10) On the other hand, the fracture strength and toughness of the α-alumina/1vol%Ni sample were approximately equivalent to those of monolithic alumina, due to the minute amount of Ni existing within the matrix particles compared with the volume of γ -alumina. Based on the results described above, we conclude that the existence of nanoparticles within the alumina matrix is very effective for improving both strength and toughness.…”