Thickness dependence of grain boundary strengthening effect on plasticity of submicrometer-to nanometer-thick freestanding copper thin films

Abstract

The grain sizes of polycrystalline metallic thin films deposited by physical vapor deposition often correspond to the film thickness; i.e., thinner films tend to have smaller grains. This correlation between film thickness and grain size hinders a detailed understanding of the intrinsic (microstructure size) and extrinsic (sample dimension) size effects of the mechanical properties of metallic thin films. In this study, submicrometer-to nanometer-thick freestanding Cu films with independently controlled grain sizes and film thicknesses were fabricated to investigate the individual effects of grain size and film thickness on plasticity. Microtensile experiments were conducted on nearly pristine freestanding Cu thin film microspecimens cut out from large-area freestanding Cu thin films with thicknesses of ∼500, ∼100, and ∼40 nm with various grain sizes. The Cu films exhibited a thickness effect where the yield stress increased as the film thickness decreased when compared at similar grain sizes. By contrast, the effectiveness of grain boundary strengthening decreased as the film thickness decreased from ∼500 nm to ∼100 nm. Furthermore, grain boundary weakening occurred as the film thickness decreased to ∼40 nm. The results indicate a transition in the mechanical role of grain boundaries during plastic deformation, shifting from strengthening to weakening as film thickness decreases from the submicrometer to the nanometer range. The findings of this study enhance the understanding of intrinsic and extrinsic size effects on dislocation-mediated and grain boundary-mediated plasticity, providing valuable guidelines for improving the mechanical strength of submicro/nanoscale metallic materials.