Influence of grain boundary sliding on fracture toughness of nanocrystalline ceramics

A theoretical model is proposed describing a new physical microscopic mechanism of increased fracture toughness of nanocrystalline ceramics. According to this model, when a ceramic with a microcrack is deformed, intensive grain boundary sliding occurs near the crack tip under certain conditions. This sliding is accompanied by the formation of an array of disclination dipoles (rotational defects) producing elastic stresses. These stresses partially compensate the high local stresses concentrated near the microcrack tip and thereby hamper the microcrack growth. The proposed model is used to theoretically estimate the increase in the critical microcrack length (the length above which the catastrophic growth of microcracks occurs) caused by the formation of disclination dipoles during grain boundary sliding in nanoceramics. The increase in the critical microcrack length is a quantitative characteristic of the increased fracture toughness of nanoceramics.