Toughening mechanism in Nanocomposites A schematic diagram explaining crack extension resistances in polycrystalline ceramics with rising R-curve behavior is shown in Fig. Fig. indicates that the intrinsic fracture toughness, Ki, is related to the energy required to create the damaged FPZ at the crack tip, and that DKR is caused by the shielding effects of bridging in a process zone wake. Thus, there are two mechanisms for improving the fracture toughness in polycrystalline ceramics. One mechanism is the process zone toughening mechanism which creates a damaged zone in front of a crack tip. Therefore, to improve the intrinsic fracture toughness, the fracture energy consumed in the process zone must be increased. The other mechanism is the crack-surface bridging toughening mechanism operating in a process zone wake which produces an extrinsic increase in crack resistance after a certain extension of the crack from the initial crack length. The toughening mechanism in nanocomposites is mainly the process zone toughening mechanism.Fig. shows a schematic illustration of the toughen-ing mechanism of nanocomposites .Dispersed dislocations within the matrix grains after annealing for alumina/silicon carbide nanocomposites are described in this figure. In a matrix grain, sub-grain boundaries or dislocation networks are generated around the nano-sized silicon carbide particles and the sessile dislocations are dispersed in the matrix, shown in Fig. In this situation, when the tip of a propagating large crack reaches this area, these sessile dislocations in the matrix will operate as nano-crack nuclei in the vicinity of the propagating crack tip, shown in Fig. The highly stressed state in the FPZ is then released by nano-crack nucleation, and the nano-cracks expand the FPZ size, enhancing the intrinsic fracture toughness of the materials.