ESTIMATION OF FRACTURE-RESISTANCE OF AL2O3 POLYCRYSTALS FROM SINGLE-CRYSTAL VALUES

A crack deflection model was constructed to explain the fracture behavior of Al2O3 polycrystals. From a consideration of the competition between penetration and deflection at grain boundaries, a three-dimensional fracture mechanism was modelled, where the crack path is affected by adjacent crack planes. The cracks in polycrystals were divided into three types: transgranular, intergranular and a mixture of the two crack types. The fracture toughness and the transgranular fracture ratio were calculated as a function of the grain boundary toughness. However, for anisotropy within a grain, experiments on single-crystal Al2O3 were carried out and the results were used for the deflection model. The maximum toughness obtainable in Al2O3 polycrystals is estimated to be about 4.3 MPa m(1/2), which is about three times that of single crystals, and the crack path restriction at triple or quadruple points was found to be the primary reason for the increased fracture toughness of polycrystals over that of single crystals. The calculated relationship between the fracture toughness and transgranular fracture ratio showed good agreement with the experimental results.