Growth mechanisms and mechanical response of 3D superstructured cubic and hexagonal Hf1-xAlxN thin films

Transition metal aluminum nitrides are a technologically important class of multifunctional ceramics, however, the HfAlN system remains largely unexplored. We investigate phase stability, nanostructure design, and mechanical behavior of Hf1-xAlxNy thin films deposited on MgO(001) substrates using ion-assisted reactive magnetron sputtering. Compared to growth temperature and ion assistance, backscattered Ar neutrals are shown to have a dominant influence on the film structure. The Al-rich (x > 0.41) films form a nanocrystalline morphology consisting of Hf- and Al-rich nanodomains in a wurtzite-hexagonal(h) 0001 fiber-texture exhibiting about 22 GPa hardness, considerably higher than that of a binary AlN. For low Al contents, x < 0.30, surface-driven spinodal decomposition by energetic Ar neutrals during deposition in combination with quenching of sub-surface diffusion results in an unusual – and unique for nitrides - three-dimensional checkerboard superstructure of AlN- and HfN-rich nanodomains in the single-crystal rocksalt-cubic (c) phase. Lattice-resolved scanning transmission electron microscopy complemented with x-ray and electron diffraction reveals that the superstructure periodicity extends along <100> directions and the size increases linearly from 9 to 13 Å with rising Al content. Consequently, the nanoindentation hardness increases sharply from 26 GPa for HfNy, to ∼38 GPa for c-Hf1-xAlxNy, due to dislocation pinning at the superstructure strain fields. Micropillar compression of c-Hf0.93Al0.07N1.15 shows a considerably higher yield stress compared to HfNy and controlled brittle fracture occurs via {110}<011> slip systems, attributed to superstructure inhibited dislocation motion. In contrast, nanocrystalline h-Hf0.59Al0.41N1.23 exhibits a high yield stress and limited plasticity before strain burst failure.