Strain rate sensitivity of a Cu/Al2O3 multi-layered thin film

To study the size and strain rate dependency of copper polycrystalline microstructures, a multi-layered copper/Al2O3 thin film was deposited on a Si substrate using a hybrid deposition system (combining physical vapor and atomic layer deposition). High temperature treatment was applied on the "As Deposited" material with ultrafine-grained structure to increase the average grain size, resulting in a "Heat Treated" state with microcrystalline structure. Focused ion beam milling was employed to create square shaped micropillars with two different sizes, that were subjected to compressive loading at various (0.001/s-1000/s) strain rates. Differences in the strain rate sensitivity behavior manifesting at low and high strain rates are discussed in the context of the pillar diameters and the grain size of the deformed samples. The Al2O3 interlayer studied by transmission electron microscopy showed excellent thermal stability and grain boundary pinning by precipitation, also resulting in the homogeneous deformation of the pillars and preventing shear localization. Geometrically necessary dislocation densities estimated by high (angular) resolution electron backscatter diffraction presented inhomogeneous dislocation distribution within the deformed pillar volumes, that is attributed to the proximity of the sample edges. Finally, the Al2O3 interlayers successfully suppressed any possible recrystallization processes, contributing to the excellent film stability, that makes the proposed coating ideal to be operating under extreme conditions.