Vacancies as a key mechanism for tuning thermodynamic stability and mechanical properties of nitride-based superlatticesWednesday (26.09.2018) 11:15 - 11:30 S1/03 - 23 Part of:
Hard coatings are used to protect engineering components from demanding application conditions, including severe external loads. In order to meet industrial requirements, coatings are nano-engineered as multilayer multicomponent systems. Especially superlattice architecture, i.e., an alternation of coherently stacked materials with only a few nanometer thicknesses, represents a powerful concept for tuning electronic, mechanical, optical or magnetic properties of materials. Moreover, the presence of the interface can give rise to unexpected physical phenomena, such as stabilisation of metastable phases, or enhancing materials properties far beyond the limit of its single layered components.
Owing to their unique combination of properties, such as extreme hardness, high melting temperatures, chemical stability, and good electrical and thermal conductivity, transition metal nitrides (TMNs) serve as promising building blocks of multilayered coatings. Exceptional performance and hardness enhancement at small bi-layer periods has already been confirmed for a series of (T)MNs-based superlattices, e.g., TiN/AlN, TiN/CrN, TiN/NbN, TiN/VN, TiN/TaN or AlN/CrN. Nevertheless, a number of fundamental questions remain, which are particularly related to the role of point defects unavoidably present in Physical Vapour Deposited (PVD) systems. Our combined experimental and computational work thus aims to reveal the impact of vacancies on thermodynamic stability and mechanical properties of superlattices based on metastable cubic phases, e.g., MoN/TaN, TiN/TaN, or TiN/NbN. According to DFT calculations, the bi-axial coherency stresses in the superlattice break the cubic symmetry beyond simple tetragonal distortions, leading to a new tetragonal ζ-phase. The ζ-structured MoN, TaN, and NbN are energetically preferred over their cubic counterparts and show fewer imaginary phonon frequencies. In agreement with our experimental findings, some of the vacancy-containing superlattices have much lower formation energies than the perfect configurations. Dynamical stability as well as elastic properties of defected superlattices are influenced also by a specific vacancy distribution (e.g., clustering of vacancies at the interface) as well as by the bi-layer period. Importantly, some of the highly defected configurations lead to high elastic moduli and improved ductility, as compared to respective monolithic phases.