Mechanical properties and deformation mechanism of nanoscale Ag/Ni multilayers

Abstract

Multilayers with periodicity in the nanoscale range have attracted a lot of attention due to their increased mechanical properties compared to the bulk counterparts. The enhancement has been explained by the dislocation-pile-up based Hall-Petch model. It is required to describe mechanical behavior based on a full understanding of the operative deformation mechanism for the application of nanoscale multilayers. Recently, several studies have reported a breakdown from the Hall-Petch behavior when the bilayer thickness is of the order of a few nanometers, but these softening mechanisms have not been clearly identified. In molecular dynamic (MD) simulation with nanocrystalline materials, the unusual mechanical behavior below the critical size is attributed to the grain boundary sliding and Coble creep. However, there are few experimental evidences to support the deformation mechanism in nanoscale multilayers. To elucidate underlying deformation mechanism, nanoindentation creep tests on nanoscale Ag/Ni multilayers are carried out with characterization of creep behaviors in regime below and above the critical bilayer thickness. Ag/Ni multilayers with different bilayer thickness (λ) between 3 and 100 nm produced by DC magnetron sputtering have been studied by cross-sectional TEM and nanoindentation. TEM micrographs show a perfect layered structure with planar, sharp interfaces between two phases. Each layer consists of a polycrystalline columnar structure with the in-plane grain dimension on the order of the layer thickness. Using small angle X-ray scattering technique, it is confirmed that the measured periodicities are well matched with the bilayer thickness expected in fabrication process. The hardness of the multilayers was measured with a nanoindenter (TriboScope, Hysitron). The hardness increases with decreasing bilayer thickness, but below a bilayer thickness of 8nm, hardness decreases with further decrease in bilayer thickness. The hardness enhancement shows a...