Tensile and fracture behavior of free-standing copper films

We have studied tensile deformation and crack growth behavior of free-standing, electron beam evaporated copper films suspended over windows etched in a silicon substrate. In-situ straining experiments were performed on these films using a transmission electron microscope. External tests were conducted on a piezoelectric-driven microtensile system. Macroscopically, these copper films exhibited very low ductility ( < 1%). Dislocation activity was limited in regions far from propagating cracks. We discuss this in terms of how limited film thickness and fine grain size restrict dislocation glide distances and source operation during uniform loading. Near stable growing cracks, we observed considerable local plasticity. We saw evidence of slip activity both within grain interiors and in grain boundaries near such cracks. Although some dislocations moved very fast, others showed rates much lower than those typically measured for bulk copper. Fracture was both intergranular and transgranular, and occurred by linking of microcracks. Microcracks formed within one grain diameter or so of the main crack, usually within a grain boundary. Linking then took place by the easiest available path. A thickness-limited fracture toughness relation adequately describes the observed behavior.