Outstanding Elastic Limit of a Thin Film Composed of Nickel Nanosprings.

This paper presents experimental results of vertical loading using an atomic force microscope (AFM) performed on a thin film consisting of nickel helical nanoelements (nanosprings) formed by glancing angle deposition (GLAD) technique. As a helical element has large reversible deformation limit in general, a characteristic behavior is expected on the yielding of the film. From the load versus displacement curves, we find the outstanding elastic limit of nickel nanosprings film. The apparent yield strain is evaluated as ε′ Y = 5.2˜6.2 × 10−2, which is around 200 times of that in bulk nickel (ε Y = 0.29˜0.44 × 10−3). However, comparing the maximum shear stress in the helical spring and the solid film, the shape effect (helical shape) is only around 10˜20 times stemmed from the difference in the stress condition (torsion). The origin of difference is attributed to the size effect of nanosprings, as nano-scale metals have higher yield strain than that of bulk counterpart because of the difference in the understructure morphology. The combination of shape effect and size effect brings about the giant elastic limit on the film.