Micromechanisms of crack extension in ceramics

Abstract

The micromechanisms of crack extension at room temperature in ceramic single crystals are compared with those in bicrystals and in polycrystals. The toughness of polycrystals is found to depend upon their macroscopic ductility, their single crystal cleavage strength, and their intergranular cohesion. For some cubic materials plastic deformation may contribute to the measured toughness, whereas for brittle anisotropic ceramics over a wide range of grain sizes, the intergranular cohesion determines the toughness. The roles of thermal expansion anisotropy, of grain size and shape, and of porosity are considered in some detail, particularly with regard to their effect on lowering the intergranular cohesion. Porosity is of primary importance in determining toughness since the crack seeks the path of minimum solid area, but it can also toughen since it lowers the elastic modulus and increases the compliance. In the same way microcracking lowers the elastic modulus and at the same time increases the absorption of energy by increasing the total area of fracture surface. A process zone of microcracking enables slow crack growth to be stabilized and this is believed to be responsible for the increased toughness found in Al2O3 and Si3N4 to which critical volume fractions of ZrO2 have been added. A further mechanism of toughening occurs in certain partially stabilized zirconias, in which metastable coherent precipitates of tetragonal ZrO2 transform in the stress field of the crack, absorbing energy as they do and giving rise to compressive stress fields which further inhibit the crack. So far it has proved possible to increase the fracture surface energy of carefully prepared and treated alloys by a factor of about five.