Outstanding Property in Fatigue Strength of Thin Film Comprising of Copper Helical Nano-Elements Grown by Glancing Angle Deposition

Abstract

In order to investigate the fatigue strength of thin film comprising of copper (Cu) nano-elements which is fabricated by means of the glancing angle deposition (GLAD) technique, cyclic loading tests are carried out on the specimens where the thin film is sandwiched between dissimilar materials. The thin film does not fracture at 10⁶ cycles in the strain range of 9.2×10^-2, which is 30 and 90 times higher than the fatigue strength at 10⁶ cycles of Cu solid thin film (grain size: 1000 nm) and bulk, respectively. This fatigue strength is brought about by superimposing the increase of allowable strain due to the shape of helical nano-elements (shape effect) and the improvement of material strength due to small grain size (size effect). The rates of strength increase due to the shape effect and the size effect are estimated to be 20 (film) - 40 (bulk), of which order has an agreement with the experimental results. In the relationship between the strain range and the number of cycles to failure (Δε - N_f curve), the gradient of the curve of the thin film comprising of Cu helical nano-elements is significantly smaller than those of the solid thin film and the bulk. This indicates that the thin film comprising of Cu nano-elements has brittle nature, which is induced by fine grains in the nano-elements.