Size effect on fracture toughness of freestanding copper nano-films

Abstract

► Fracture toughness experiments were conducted for freestanding copper nano-films. ► The crack propagates stably before unstable fracture. ► A clear size effect is observed where thinner films have smaller toughness. ► Fracture morphology transits from ductile to brittle with decreasing film thickness. We conducted fracture toughness experiments on freestanding copper films with thicknesses ranging from about 800 to 100 nm deposited by electron beam evaporation to elucidate the size effect on fracture toughness in the nano- or submicron-scale. It was found that initially, the crack propagated stably under loading, and then the crack propagation rate rapidly increased, resulting in unstable fracture. The fracture toughness K C was estimated on the basis of the R -curve concept to be 7.81 ± 1.22 MPa m 1/2 for the 800-nm-thick film, 6.63 ± 1.05 MPa m 1/2 for the 500-nm-thick film and 2.34 ± 0.54 MPa m 1/2 for the 100-nm-thick film. Thus, a clear size effect was observed. The fracture surface suggested that the crack underwent large plastic deformation in the thicker 800-nm and 500-nm films, whereas it propagated with highly localized plastic deformation in the thinner 100-nm film. This size effect in fracture toughness might be related to a transition in deformation and fracture morphology near the crack tip.