Electronic origin of structure and mechanical properties in Y and Nb alloyed Ti–Al–N thin films

Ti1–xAlxN thin films are industrially well established protective coatings, whose beneficial mechanical properties are mainly based on the formation of a metastable microstructure and local composition during film synthesis. Alloying of a transition metal (TM) to Ti1–xAlxN is a promising approach to reach yet higher oxidation and corrosion resistance in high-temperature environments, while maintaining a high intrinsic hardness and elasticity, being essential for a good wear performance. In order to study the effect of alloying with Y and Nb on the structure and mechanical properties of the industrially preferred cubic (c) Ti1–xAlxN system, quaternary Ti1–x–zAlxYzN and Ti1–x–zAlxNbzN films were deposited by means of plasma-assisted reactive magnetron sputtering and investigated using X-ray diffraction, transmission electron microscopy and nanoindentation. It is shown that Y addition to c-Ti0.42Al0.58N changes its structure towards single phase wurtzite (w) Ti0.36Al0.55Y0.09N, with deteriorated mechanical properties. In contrast, by the addition of Nb the structure remains cubic up to the studied composition of c-Ti0.35Al0.57Nb0.08N and the film hardness increases from 30.1 to 39.5 GPa. Ab initio studies show that the effect of Y and Nb alloying on structure and mechanical properties of quaternary Ti1–x–zAlxTMzN is not only correlated with strain increase due to lattice mismatch, but rather the changed electronic configuration.