Microstructure-controlled depth gradients of mechanical properties in thin nanocrystalline films: Towards structure-property gradient functionalization

Although the influence of the grain size on the mechanical properties of polycrystalline materials is well understood, the occurrence of depth-gradients of grain size, microstructure, and residual stresses in nanocrystalline thin films and their effect on the functional properties is a phenomenon which has not yet been studied in detail. Hence in this work, single-layered polycrystalline and mosaic epitaxial as well as multilayered CrN thin films were characterized by a combination of averaging as well as depth-resolved experimental techniques, such as cross-sectional X-ray nanodiffraction and small-angle cross-sectional nanoindentation. The results reveal the fundamental relationship between gradients of grain size, microstructure, and stresses, controlled by the film growth conditions, and gradients of hardness and elastic modulus. The effect of the compressive stress and structural defects associated with high particle energy on the mechanical properties of nanocrystalline thin films was found to be dominant over the grain size and crystallographic texture. These findings open the way to functionalize structure-property gradients in nanocrystalline thin films through microstructural design as demonstrated for multilayered CrN films.