The pursuit of materials that simultaneously exhibit both high hardness and high toughness remains one of the central challenges in the development of hard and wear-resistant protective coatings for extreme environments. Conventional ceramic materials, while exceptionally strong, often suffer from brittleness due to limited plastic deformation mechanisms. This dissertation addresses methods to simultaneously improve both hardness and toughness by design of advanced transition metal aluminum nitride thin films, multilayers, and superlattices based on TiN, ZrAlN, and HfAlN, synthesized by ion-assisted reactive magnetron sputtering. Systematic studies on structure–property relationships expand the knowledge of how film composition, microstructure, phase stability, and film architecture influence mechanical performance.

The work begins with multilayered TiN/Zr_0.37Al_0.63N_1.09 thin films, where the growth temperature (25-900°C) significantly affects crystallinity and decomposition behavior. While TiN layers transitioned from textured to epitaxial with increasing temperature, the ZrAlN layers evolved from amorphous to nanocrystalline to phase-separated domains. As a result, multilayers with simultaneous improvements in hardness, fracture stress and fracture toughness were found, for damage tolerant hard coatings with excellent thermal stability upon annealing to 900 °C.

A major focus has been on overstoichiometric HfNy single crystals, which challenge the long-standing trade-off between hardness and plasticity in ceramics. These films exhibit a rare atomic-scale three-dimensional (3D) checkerboard superstructure, arising from the ordered arrangement of Hf-vacancies and N-interstitials within the rocksalt lattice. This self-organized structure, revealed through high-resolution electron microscopy, X-ray and electron diffraction, and validated by ab initio modeling, changes the deformation behavior by enabling extensive dislocation glide on {111}<011> slip systems. Micropillar compression tests demonstrated extraordinary plasticity and pronounced strain hardening at compressive strains exceeding 50%, while preserving high hardness values up to 28 GPa. This unique synergy of strength and plasticity holds exceptional promise for novel damage-tolerant hard coatings. Further work investigated the relatively underexplored Hf_1-xAl_xN_y material system, where the structural evolution from cubic to hexagonal phases with increasing Al content was mapped. Spinodal decomposition of cubic Hf_1-xAl_xN_y results in a slightly larger 3D checkerboard superstructure, in this case composed of cube-like coherent HfN-rich and AlN-rich nanodomains. This periodic structure effectively hinders dislocation motion, increasing strength and hardness to ~38 GPa, but also results in a more brittle material with controlled strain burst fractures on the {110}<011> slip system. In contrast, hexagonal Hf_1-xAl_xN_y films exhibited fiber-textured nanocrystalline morphologies with moderate hardness (~22 GPa), and limited plasticity before fracture.

The plasticity of overstoichiometric HfNy and the hardness of Hf_1-xAl_xN_y were combined by designing epitaxial superlattices of HfN_1.33/Hf_0.76Al_0.24N_1.15 with bilayer periods between 6 nm and 20 nm. Each constituent material maintained its distinctive 3D checkerboard superstructure, confirmed by X-ray diffraction and reciprocal space mapping. This architecture enabled Koehler-type hardening, preserving the high hardness of the HfAlN layers (~36 GPa). Micropillar compression and cube-corner indentation demonstrated distributed cracking and (partial) recovery after fracture, indicating significantly improved toughness compared to Hf_0.76Al_0.24N_1.15 due to crack arrest and stress dissipation at the coherent, strained interfaces.

Beyond mechanical performance, the thesis also explored the optical properties of Hf_1-xAl_xN_y films for applications of high-reflection and anti-reflection coatings. A wide range of refractive indices and extinction coefficients were achieved by tuning the Al content and crystal structure, enabling design of multifunctional coatings with tailored optical and mechanical properties. The insights presented open up new avenues for developing robust, multifunctional transition metal aluminum nitride-based hard coatings and optical devices.