Effect of interfacial adhesion on toughness of metal/ceramic laminates

Diffusion bonded laminates have been produced, composed of aluminium layers ranging from 50 mu m to almost 1 mm thick and alumina layers 500 mu m thick. Their fracture behaviour has been investigated in bending and in tension, with and without notches. The main toughening mechanism has been identified as the formation of a bridging zone, within which large amounts of energy can be absorbed by the plastic stretching of ductile ligaments. A previously developed theoretical model for prediction of the energy absorbed during propagation of a dominant crack normal to the plane of the layers has been applied to the cases examined. This is shown to give good agreement with experiment over a range of metal volume fractions, Sol testing at both room temperature and at 300 degrees C. Studies are presented on the effect of changing the strength of the interfacial bond by the introduction of a fine dispersion of graphitic material into the interface. Interfacial debonding is expected to relax the constraint on initial yielding of the ligament, reducing the traction needed at the start of plastic deformation, and to increase the length of the bridging ligament (for a given crack opening displacement). The first of these effects will tend to reduce the work of fracture, and the second will tend to increase it. The reduction in initial yielding traction induced by the weak interfaces is apparently small, hut there is a significant increase in the extension of individual ligaments. The net result is to increase slightly the work of fracture. However, these results relate to the case of a single dominant crack. The effective toughness of these materials can be substantially enhanced by the occurrence of multiple cracking which is favoured by high metal contents. Results presented were indicate that multiple cracking is enhanced during bending of unnotched specimens by a suitable degree of interfacial weakening. During tensile loading of notched specimens there is little or no multiple clacking but a weak interface can lead to much greater bridging lengths and hence greater toughness. For tensile lending of unnotched specimens, multiple cracking is extensive over a range of bond strengths, although the crack spacing is increased with weaker interfaces.