Estimates of Interfacial Properties in Copper/Nickel Multilayer Thin Films using Hardness and Internal Stress Data

Modeling the defect structure and mechanical properties of metallic multilayer thin films requires estimates of dislocation parameters such as interfacial line energy, interfacial barrier strength, and resistance to confined layer slip (CLS). A method is presented to estimate these parameters using experimental measurements of hardness and internal stress vs. individual layer thickness, h. Cu/Ni multilayers of varying bilayer thickness (20 nm ≤ Λ ≤ 60 nm) and volume fraction (25 ≤ %Ni ≤ 60%) were fabricated via sputtering and characterization performed using x-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Internal stresses for the samples were calculated via peak positions from inplane XRD and second order elastic constants. The experimental techniques of nanoindentation and micropillar compression data were used to look at flow stresses, hardness, and strain rate sensitivities for the Cu/Ni multilayers. Internal stress was seen to increase with decreasing bilayer thickness and decreasing layer thickness for both layer types. In addition, hardness was seen to increase with decreasing bilayer thickness and decreasing Cu layer thickness.;The data acquired via characterization and experimentation was used as inputs within a CLS model in order to extract quantities for interfacial properties. It was seen that that separate values of line energy operate in the Cu and Ni layers and that a single effective line energy for the multilayer is inappropriate. This indicates that dislocation loops will encounter a different resistance at the shared interface depending on whether the dislocation loop originates in Ni or Cu. Analytical models for line energy overestimated the line energy in Cu and underestimated the line energy in Ni. It was seen that the interfacial barrier to dislocation motion increased with increasing bilayer thickness and misfit strain. A maximum value of 444 MPa was extrapolated for the interfacial barrier to dislocation transmission. Pinning stresses increased initially with increasing layer thickness due to the increasing misfit strain. The pinning stresses reached a maximum of ~ 240 MPa in Ni before decreasing for layer thicknesses > 20 nm. Finally, it was seen that coherency stresses derived from lattice parameter mismatch did not completely explain the hardness found in the multilayers. Atomistics had predicted that strength was dominated by the contribution of coherency stresses while these experiments indicated that some contribution from pinning and interfacial stresses was required to fully explain the multilayer strength. The results of the modeling in this dissertation suggest that multilayer strength can be increased by methods other than reducing bilayer thickness. These methods include selecting a seed or buffer layer to place the multilayer in a state of net in-plane compression and increasing the barrier to dislocation transmission by fabricating the compressive phase in thinner quantities compared with the tensile phase.