FRACTURE OF CERAMIC-TO-METAL INTERFACES

Abstract
The methods used for studying fracture at metal-ceramic interfaces are based on fracture mechanical principles. Sandwich-like specimens consisting of two rectangular pieces of ceramic solid-state bonded by a intermediate metal layer and notched or precracked at one interface between the two materials are fractured in a three- or four-point bend test device. The interface fracture energy [MATH]_C is obtained from the measured loaddeflection curve, the fracture load, the dimensions of the specimen and by use of a correction function Y_G which pays regard to the elastic properties of the materials involved and the geometry of the specimen. The fracture behaviour of ceramic-to-metal joints with and without intermediate reaction layers is studied. Internal stresses due to differences in the thermal expansion of the constituents are reduced by microcrack formation. For polycrystalline material transitions a mixed failure mode is observed including both adhesive and cohesive fracture contributions. The fracture energy is influenced by impurity phases and by the microstructure of the interface region. Experiments with combinations of single crystalline niobium and sapphire indicate that the interface fracture energy depends on the crystallographic orientationship of the two crystal surfaces bonded together. It is shown that the work of adhesion is only a very small fraction of the interface fracture energy. The contributions of energy dissipation processes to the fracture energy are discussed in more detail.