Anisotropic Hardness and Fracture Toughness of Highly Aligned YBa sub 2 Cu sub 3 O sub 7-- delta

The hardness and fracture toughness of aligned YBa sub 2 Cu sub 3 O sub 7-- delta obtained by melt processing was found to be highly anisotropic. Indentation measurements show that the (001), (100), and (010) planes are the preferred fracture planes in this material and that the critical stress intensity factor for propagating a crack on the (001) basal plane is the lowest, i.e. K exp 001 sub c < K exp 100 sub c or K exp 010 sub c . Indentation crack length measurements on the (001) basal plane with the impression diagonals oriented parallel to the \100\ and the \010\ directions, indicate that the fracture toughness of these planes is K exp air sub 100/010 approx 0.7 MPa/m exp 0.5 . The hardness in this orientation was found to be 6.7 GPa. Measurements on a plane perpendicular to the basal plane resulted in a lower hardness of approx 3.8 GPa. This reduction in hardness is influenced by the extensive preferential cleavage of the (001) basal plane boundaries. The extremely low values of the fracture toughness suggest that considerable toughening would have to be achieved in melt-textured 123 for any practical bulk application. The presence of 211 phase and Ag particles may serve to enhance the fracture toughness of aligned 123 by crack deflection and crack tip blunting, respectively.