dc.description.abstract
Thin ceramic films based on transition metal nitrides have been successfully applied as protective coatings in the cutting industry and on forming tools. High pressure on enhancing their performance is pushing experimental as well as bottom-up computational material development towards optimised thermal stability, hardness, toughness, tribological properties, as well as wear, corrosion and oxidation resistance. A vital approach consists in tuningthe microstructure. In particular, synthesis of alternate nanolayers of different materials,called superlattices, presents an elegant strategy for the design of atomic-scale architectures that allow to stabilise metastable phases and to optimise typically antagonistic properties of ceramic materials—such as hardness and fracture toughness—via controlling the individual layer thicknesses. This work offers modelling insights into relationships between stability, structural, elastic, and fracture properties of superlattices based on transition metal nitrides. The primary method employed within the practical part is quantum-mechanical Density Functional Theory (DFT). DFT allows to screen many material systems, metastable or even unstable phases,as well as specific defect-containing configurations in an unbiased manner, under well-defined and controllable conditions, e.g. subject to purely uniaxial tensile loading in a desired direction, possessing exact chemistry, defect content, and interface orientation. The model material systems are MoN/TaN, TiN/WN, and AlN/TiN superlattices based on the cubicrocksalt phase. MoN/TaN combines inherently ductile materials (compared to the class of nitride, carbide, and boride ceramics), showing a strong driving force for lattice vacancies,considerable lattice mismatch, and fairly similar elastic moduli. In contrast, TiN/WN exhibits essentially zero lattice yet significant elastic mismatch of the layer components, TiN and WN, where TiN is very stable in the cubic phase but inherently brittle, while WN is ductile but its cubic structure lies on the edge of instability. AlN/TiN, the third model superlattice, presents a system for which some reference computational as well as experimental data already exist, in particular, cube corner indentation experiments suggest significantly enhanced toughness at ultra-low layer thicknesses.My own contribution to the field is demonstrated by 6 core publications. DFT calculations indicate high inherent ductility of MoN/TaN superlattices, combined with a remarkable versatility in chemistry and elastic properties via the strong affinity for vacancies. When layer thicknesses do not exceed few nanometers, however, vacancy formation in the superlattice seems partially hindered compared to the superlattice monolithic components. This can be rationalised by coherency strains inducing formation of the tetragonal ζ-phase near interfaces. Such effect, predicted also for TiN/WN superlattices and indirectly supported by experiment, could contribute to the fracture toughness enhancement recorded for magnetron-sputtered MoN/TaN and TiN/WN films at low bilayer periods of about 5 and 10 nm, respectively. In case of AlN/TiN, cleavage simulations reveal a strong dependence of the energy and stress for brittle cleavage on the distance from superlattice interfaces, suggesting that cracks most likely initiate exactly at interfaces or in the middle of TiN layers. Finally, high-throughput ab initio screening renders promising candidates for novel nitride and carbonitride superlattice films with superior ductility, strength, and fracture toughness. Among the predicted candidates are TiN/MoN, HfC/WN, TaC/MoN, VC/TaN and others.
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