The microstructure and mechanical properties of thin film Ni(Ti) nanocomposite coatings containing both oxygen and nitrogen

Thin film Ni(Ti) nanocomposite coatings, containing both oxygen and nitrogen, were deposited onto tool steel substrates using a reactive DC magnetron sputtering system at room temperature. A NiTi alloy target (Ni/Ti = 80/20 at.%) was used during deposition. The evolution of the microstructure of these coatings under varying N2 gas flow rate was studied by focused ion beam microscopy and transmission electron microscopy. A clear structural change, from columnar grains to very fine equiaxed grains with a considerable level of porosity, was evidenced. X-ray diffraction, together with X-ray photoelectron spectroscopy, was employed to determine the crystal structure and composition of these coatings. Analysis of these data revealed that the nanocomposite coatings were composed of a Ni matrix with nanocrystalline TiN, embedded together with amorphous TiO2. Nanoindentation results demonstrated that Ni dissolved with ~ 15 at.% Ti solid solution possesses higher values of elastic modulus, hardness and better resistance to both elastic and plastic deformation than the Ni(Ti) nanocomposite coatings containing higher concentrations of both nitrogen and oxygen. The presence of porosity in the nanocomposite coatings was expected to worsen the mechanical properties of these coatings, while the amorphous TiO2 phase may help facilitate grain boundary motion.