Alloying effects on the structure and elastic properties of hard coatings based on ternary transition metal (M = Ti, Zr or Ta) nitrides

Multicomponent alloying is an effective design route to increase toughening in hard nitride-based coatings. We provide density functional theory calculations and experimental assessment of the elastic properties of a series of ternary transition metal nitride coatings. M11 − xM2xN films (M1, M2 = Ti, Zr, Ta) were elaborated by reactive magnetron co-sputtering. The structural (texture and microstructure) and growth morphology evolution as a function of film thickness and chemical composition were investigated by X-ray diffraction, X-ray reflectivity, electron probe microanalysis and scanning electron microscopy. The effective elastic constants, C33 and C44, were obtained from longitudinal and transverse sound velocity measurements, using picosecond ultrasonics (PU) and Brillouin light scattering (BLS), respectively. We employ Blackman diagram, as well as Pugh and Pettifor criteria, to derive trends in elastic anisotropy, bonding character and brittleness vs. ductility. The influence of microstructure and preferred orientation on the elastic response of polycrystalline films is discussed. For Ta-rich TiTaN and TaZrN systems, the formation of a nanocomposite system consisting of a mixture of cubic and hexagonal phases has a beneficial contribution to toughening.