Abstract
We conducted crack propagation experiments on freestanding copper films with thicknesses ranging from about 60 to 800 nm deposited by electron beam evaporation to clarify the size effect on fracture toughness in the nano-scale. It was found that the fracture toughness K_C shows a clear size effect where thinner films have smaller fracture toughness. The fracture surface suggested that the crack underwent large plastic deformation accompanied by many multiple slips in the thicker 500-nm and 800-nm films whereas it propagated with highly localized plastic deformation and flat brittle fracture occurred in some region in the thinner 100-nm film. In situ TEM observation of the crack propagation in a 100-nm thick film revealed that the crack propagated via the processes of nucleation and coalescence of voids ahead of the crack tip, and the flat fracture was brought about by the void nucleation along grain boundaries or by highly localized slip deformation. The size effect in fracture toughness in the nano-scale is closely related to the transition in fracture mechanism.
