Anisotropic hardness in TiB2±z thin films via depth-resolved crystallographic texture engineering

Abstract

he enhancement of protective coating properties, especially hardness, can be achieved not only through alloying or grain refinement but also by introducing structural variations (gradients) throughout the thickness of the thin film material without altering its chemical composition. In this regard, hexagonal structured transition metal diborides, such as TiB2±z thin films, serve as a prime example exhibiting a hardness-anisotropy regarding their crystallographic orientation. Therefore, this study explores a set of TiB2±z coatings containing different orientation gradients over the film thickness, generated by adjusting solely the deposition pressure during non-reactive sputtering of TiB2±z. To establish a correlation between the structure and mechanical properties of TiB2±z thin films dependent on depth, cross-sectional synchrotron X-ray nano-diffraction was combined with small-angle cross-section nanoindentation. Our results reveal a hardness gradient from approximately 27 GPa for regions with a mixed orientation up to 37 GPa for predominantly (0001) oriented areas. These results demonstrate, for the first time, that the anisotropic hardness of TiB2±z thin films can be tailored by controlling the crystallographic texture during deposition.