Metastable cubic transition metal aluminium nitride and oxynitride coatings: Theoretical phase stability and defect structure predictions and verification by industrial-scale growth experiments

In the first part of this thesis, the crystallite size-dependent metastable phase formation of nanocrystalline TiAlN, an industrial benchmark coating material, is demonstrated through correlative ab initio calculations and advanced material characterization at the nanometer scale. By relating calculated surface and volume energy contributions to the total energy, the chemical composition-dependent phase boundary between the two metastable solid solution phases of cubic and wurzite Ti1-xAlxN is predicted. This phase boundary is characterized by the critical crystallite size dcritical. Crystallite size-dependent phase stability predictions are in very good agreement with experimental phase formation data where x was varied by utilizing combinatorial vapor phase condensation. The wide range of maximum Al solubilities for metastable cubic Ti1-xAlxN from xmax = 0.4 to 0.9 reported in literature and the sobering disagreement thereof with density functional theory predictions can be rationalized based on the here identified crystallite size-dependent metastable phase formation. Furthermore, it is evident that phase stability predictions are flawed, if the previously overlooked surface energy contribution to the total energy is not considered. In the second part, Ti-Al-O-N coatings are synthesized by cathodic arc and high power pulsed magnetron sputtering. The effect of oxygen incorporation on stress-free lattice parameters and Young’s moduli of Ti-Al-O-N coatings is investigated by X-ray diffraction and nanoindentation, respectively. As nitrogen is substituted by oxygen, implications for the charge balance may be expected. A reduction in equilibrium volume with increasing O concentration is identified by X-ray diffraction and density functional theory calculations of Ti-Al-O-N supercells reveal the concomitant formation of metal vacancies. Hence, the oxygen incorporation-induced metal vacancy formation enables charge balancing. Furthermore, nanoindentation experiments reveal a decrease in elastic modulus with increasing O concentration. Based on ab initio data, two causes can be identified: Metal vacancy-induced reduction in elasticity and, second, the formation of, compared to the corresponding metal nitride bonds, relatively weak Ti-O and Al-O bonds. In the third and last part, consequences induced by reactive cathodic arc evaporation of Ti-Al-O-N in an industrial deposition system with two-fold substrate rotation are addressed experimentally. The formation of alternating O- and N-rich sublayers is identified by atom probe tomography and can be understood by considering the substrate rotation-induced variation in plasma density and fluxes of film-forming species. The effect of plasma density and fluxes on the incorporation of reactive species is studied in stationary deposition experiments and preferred N incorporation occurs, when the growing coating surface is facing the arc source. Thus, the growing surface is positioned in a region of high plasma density characterized by large fluxes of film forming-species. Preferred O incorporation takes place in a region of low plasma density where small fluxes are present, when the growing surface is blocked from the arc source by the substrate holder. Hence, compositional modulations are caused by substrate rotation as the growing coating surface is periodically exposed to regions of different plasma density and fluxes. In summary, the combination of theoretical predictions by ab initio calculations and experimental verification by industrial-scale growth experiments enables to identify physical and chemical mechanisms defining metastable cubic phase formation. The evolution of the local chemical composition is prerequisite for a meaningful model of the phase formation. Furthermore, understanding of the chemical composition-induced defect structure facilitates materials design of protective coatings with tunable mechanical properties. The selection of process parameters such as substrate rotation speed and reactive gas mixtures allows for developing chemically modulated architectures of functional protective coatings