Fatigue failure involves fatigue crack initiation and propagation processes. In bulk metals, the fatigue crack initiation process is strongly affected by a dissimilar surface layer, while the fatigue crack propagation process is less affected. In the case of the metallic thin films, which are a kind of small-scale metallic materials, have a large surface-area-to-volume ratio, and the fatigue crack propagation process is dominated by the formation of out-of-plane surface fatigue damage, intrusion/extrusion, which is observed in fatigue crack initiation process in bulk metals. Therefore, even a surface layer with a thickness on the order of nanometers would have a significant influence on fatigue crack propagation behavior of metallic thin films. In this study, in situ field emission scanning electron microscope (FESEM) observation experiments of fatigue crack propagation were conducted on freestanding copper thin films with a thickness of 500 nm with a thickness-controlled surface oxide layer in the range of ~0 nm to ~20 nm. The results showed that the fatigue crack propagation properties were influenced by the thickness of the surface oxide layer: acceleration of fatigue crack propagation, transitions of fatigue crack propagation mechanisms (from intrusion/extrusion mode to tensile-dominant mode) and unstable fracture occurred at smaller stress intensity factor range ΔK for the films with a thicker surface oxide layer than that for the films with a thinner surface oxide layer. Moreover, the fatigue crack propagation rate da/dN was smaller in the region near the fatigue crack propagation threshold and thus the threshold stress intensity factor range ΔKth increased in the films with a thicker surface oxide layer. In situ FESEM observations showed that fatigue damage area around the fatigue crack became narrower in the films with thicker surface oxide layer, and the fatigue crack reached the threshold when intrusions/extrusions were rarely formed ahead of the crack tip. This suggests that the fatigue crack propagation threshold in metallic thin films is determined whether intrusions/extrusions are formed around the fatigue crack.
We developed an experimental technique for evaluating fatigue crack propagation properties of freestanding nano-films and conducted tensile and fatigue crack propagation experiments for about 500-nm-thick freestanding copper (Cu) films. We employed a sacrificial etching method for fabricating freestanding metallic nano-film specimens. A piezoelectric-actuator with long stroke and a load cell for small load were used for applying cyclic loading to nano-films. We developed original jigs for handling and aligning the nano-film specimens. Focused ion beam (FIB) was employed to introduce a single side-edge-notch in the nano-film specimens for fatigue crack propagation experiments. The results of tensile experiments revealed that the nano-films have resistance to plastic deformation comparable to cold-rolled Cu bulk, but have lower ductility. The results of fatigue crack propagation experiments revealed that a fatigue crack stably propagates in the nano-film by a uniaxial cyclic loading with constant maximum stress before unstable fracture. The fatigue crack propagation rate da/dN of the Cu nano-films is higher than that of Cu bulk counterpart in higher stress intensity factor range, ΔK. Fatigue crack started to propagate from the notch tip below the threshold value of Cu bulk with coarse grains, and the Cu nano-films show similar fatigue crack propagation properties to Cu bulk with ultrafine grains in lower ΔK. Morphology of the fracture surface transited from microstructure-sensitive rough surface to microstructure-insensitive ductile surface or chisel point fracture as fatigue crack propagated. This suggests that the dominant fracture mechanism changes from the accumulation of cyclic deformation to the monotonic tensile-dominant fracture.
We conducted crack propagation experiments on freestanding copper films with thicknesses ranging from about 800 to 100nm deposited by electron beam evaporation to clarify the size effect on fracture toughness in the nano-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, showing a clear size effect where thinner films have smaller fracture toughness. 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 will be related to a transition in deformation and fracture morphology near the crack tip.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
