Growth Mechanisms and Mechanical Response of 3D Superstructured Cubic and Hexagonal Hf$_{1-x}$Al$_x$N Thin Films

TL;DR

This study explores the growth, nanostructure, and mechanical properties of HfAlN thin films, revealing unique superstructures and high hardness levels that depend on composition and processing conditions, with implications for advanced ceramics.

Contribution

It uncovers a novel three-dimensional superstructure in HfAlN films formed by spinodal decomposition, and links this to enhanced hardness and mechanical strength.

Findings

High hardness (~22 GPa) in Al-rich films due to nanodomains.

Unique 3D superstructure in low Al content films from spinodal decomposition.

Superstructure enhances dislocation pinning and yield stress.

Abstract

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 Hf$_{1-x}$Al$_x$N$_y$ 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 A with rising Al content. Consequently, the nanoindentation hardness increases sharply from 26 GPa for HfN$_y$, to \~38 GPa for c-Hf$_{1-x}$Al$_x$N$_y$, due to dislocation pinning at the superstructure strain fields. Micropillar compression of c-Hf$_{0.93}$Al$_{0.07}$N$_{1.15}$ shows a considerably higher yield stress compared to HfN$_y$ and controlled brittle fracture occurs via {110}<011> slip systems, attributed to superstructure inhibited dislocation motion. In contrast, nanocrystalline h-Hf$_{0.59}$Al$_{0.41}$N$_{1.23}$ exhibits a high yield stress and limited plasticity before strain burst failure.