Structure, phase stability and elastic properties in the Ti1-xZrxN thin-film system: Experimental and computational studies

The composition-dependence of the structure and elastic properties of ternary Ti1-xZrxN alloys is systematically investigated by combining thin film growth and ab initio calculations. Single-phase Ti1-xZrxN thin films (0 ⩽ x ⩽ 1) with a rocksalt structure have been deposited using dc reactive magnetron sputtering at Ts = 300 °C in Ar/N2 plasma discharges. The structure, stress state and phase stability upon thermal annealing were studied by X-ray diffraction (XRD), while the acoustic and elastic properties were measured using Brillouin light spectroscopy, picosecond ultrasonics and nanoindentation. First-principles pseudopotential calculations of the total energy, lattice constants, bulk modulus, and single-crystal elastic constants Cij for several cubic ordered structures of Ti1-xZrxN alloys were also carried out. The positive values of the computed formation energies indicate that the homogeneous Ti1-xZrxN alloys can be only stabilized at high temperatures. However, the magnetron-sputtered thin films were found to retain their as-grown single-phase cubic structure during post-deposition annealing at 850 °C for 3 h. The calculated equilibrium lattice parameters are in good agreement with the stress-free lattice parameters a0 determined experimentally from XRD using the sin2ψ method: they both exhibit a positive deviation from Vegard-like linear interpolation. The calculated bulk modulus, elastic constants and Poisson's ratio gradually decrease from TiN to ZrN. These computed values were used to interpret the experimentally derived elastic constants and Young's modulus as functions of composition.