Structure and properties of Ni1-xTixN thin films processed by reactive magnetron co-sputtering

A comparative study has been carried out on microstructure evolution and residual stress along with nanomechanical, nano-scratch, and corrosion behavior of thin films of Ni1-xTixN (x = 0.35, 0.42) nanocomposite and pure Ni deposited on mild steel substrates. The nanocomposite thin films were deposited in a reactive atmosphere having Ar:N-2 = 1:2 by magnetron co-sputtering of high purity Ni (DC) and Ti (RF) targets. Rietveld analysis of grazing incidence X-ray diffraction (GIXRD) peaks has shown formation of 39 or 54 vol% TiN in the nanocomposite films deposited using RF target power of 250 or 300 W, respectively, at a constant DC power of 150 W. Further, with increase in Ti target power, the grain size of both matrix and TiN have decreased, as confirmed by analysis of GIXRD peak broadening and dark-field transmission electron microscopy. The biaxial residual stress measured using the Sin(2)psi technique has revealed that the stress is tensile in pure Ni thin film, whereas it becomes compressive in presence of TiN in Ni1-xTixN films. Furthermore, with increasing Ti target power, nanoindentation hardness, elastic modulus, and scratch-resistance are increased, whereas surface roughness and coefficient of friction are decreased. Potentiodynamic polarization experiments in 3.5 wt% NaCl solution has shown the pitting corrosion resistance to scale with TiN content due to formation of the TiO2-rich passivating layer, as confirmed by X-ray photoelectron spectroscopy. Further, the breakdown potential for passivation determined from Tafel plots scales with TiN content. The present study provides a novel method for altering the volume fraction of TiN and tailoring the microstructure of the investigated Ni1-xTixN nanocomposite films by varying the Ti (RF) target power to obtain a desirable combination of surface properties required in protective coatings, with fresh insight into the mechanisms governing structure-property relations.