| Nickel-sapphire interfaces were used as a model metal-ceramic system to investigate the fracture mechanisms and toughening mechanisms of constrained metal layers. Fracture tests were performed using the double-cleavage drilled compression geometry, and measurements of the fracture energy release rate were obtained from load and crack length data. Interface strength was varied by segregating sulfur to the interface and by eliminating stress corrosion effects. Atomic Force Microscopy and Scanning Tunneling Microscopy were used to quantify features on fracture surfaces and to relate plasticity to fracture and toughening mechanisms. Additionally, the effect of constraint on fracture resistance was investigated by quantifying deformation near free surfaces at the interface.; The fracture mechanism was found to be that of brittle fracture: decohesion at the crack tip. This mechanism was invariant despite a four-fold change in the energy release rate. Brittle fracture of pure Ni arises from the constraint imposed by sapphire, which limits deformation at the interface. Toughening mechanisms included ligaments bridging the crack and crack tip blunting, both of which are caused by oxide particulates of various chemical compositions. The number of oxide particulates could be controlled by annealing, and were found to be responsible for the intermittent nature of crack advance. In addition to deformation on the {dollar}mu{dollar}m scale, plasticity features were quantified on smaller scales. Slip steps and surface morphology were observed with heights on the nanometer scale and were found to be associated with the inherent fracture resistance of the interface. The spatial character of surface morphology on the nm scale was invariant with fracture resistance, although its height scales with energy release rate.; Deformation was observed to increase near sample free edges over a distance equal to 40-60% of the layer thickness, and deformation at interfacial pores was found to be dependent upon both pore shape and position. Away from these free surfaces, deformation was not observed, demonstrating the effect of constraint in limiting the fracture resistance of metal layers embedded in a ceramic. |
