# Point-defect engineering of MoN/TaN superlattice films: A first-principles and experimental study

## TL;DR

This study combines first-principles modeling and experiments to understand how point defects influence the structure, stability, and elasticity of MoN/TaN superlattice films, revealing defect-induced properties and optimal compositions.

## Contribution

It provides a comprehensive analysis of defect chemistry and stability in MoN/TaN superlattices using DFT calculations and experimental validation, highlighting the role of vacancies.

## Key findings

- No stable defect-free superlattice exists.
- MoN0.5/TaN is the most likely coating composition.
- Experimental and theoretical elastic properties agree.

## Abstract

Superlattice architecture represents an effective strategy to improve performance of hard protective coatings. Our model system, MoN/TaN, combines materials well-known for their high ductility as well as a strong driving force for vacancies. In this work, we reveal and interpret peculiar structure-stability-elasticity relations for MoN/TaN combining modelling and experimental approaches. Chemistry of the most stable structural variants depending on various deposition conditions is predicted by Density Functional Theory calculations using the concept of chemical potential. Importantly, no stability region exists for the defect-free superlattice. The X-ray Diffraction and Energy-dispersive $\text{X-ray}$ Spectroscopy experiments show that MoN/TaN superlattices consist of distorted fcc building blocks and contain non-metallic vacancies in MoN layers, which perfectly agrees with our theoretical model for these particular deposition conditions. The vibrational spectra analysis together with the close overlap between the experimental indentation modulus and the calculated Young's modulus points towards MoN$_{0.5}$/TaN as the most likely chemistry of our coatings.