Nanoindentation investigation of the mechanical behaviors of nanoscale Ag/Cu multilayers

The microstructure, hardness, elastic modulus, and indentation creep of Ag/Cu multilayers prepared by magnetron sputtering were investigated by x-ray diffraction, transmission electron microscopy, and nanoindentation. The hardness values obey the Hall-Petch relationship as the periodicity decreases to 20 nm. For multilayers with periodicity smaller than 20 nm, the Hall-Petch relationship breaks down and the hardness values saturate at about 4.6 GPa; moreover, there are shear bands formed around their indents and strain bursts occurring during the load-holding process of indentation creep. These results imply that there is a transition of the deformation mechanism in the region where the periodicity is equal to 20 nm. This transition of the deformation mechanism can be ascribed to grain-size-dependent competition between the dislocations-mediated plasticity and grain-boundary sliding-mediated plasticity.