Structural and mechanical properties of Nb1 - xTixN thin films deposited by rf magnetron sputtering

Nb1-xTixNy thin films with different nitrogen contents (0.5 < y < 1.2) and Nb/Ti ratios (0 <= x <= 1) were investigated because of their potential to improve the lifetime of bearing components of orthopaedic implants. The films were deposited onto a medical-grade CoCrMo substrate by rf magnetron sputtering at 400 degrees C under a (N-2 + Ar) atmosphere. The samples were characterised using Rutherford backscattering spectroscopy (RBS), elastic recoil detection analysis (ERDA), X-ray diffraction (XRD) at grazing incidence and Bragg-Brentano geometry, scanning and transmission electron microscopy (SEM, TEM), and nanoindentation to investigate the dependence of the phase configuration, microstructure, and mechanical behaviour on the nitrogen content and Nb/Ti ratio. The results demonstrated that the coatings develop mixed structures composed of fcc-cubic as the major phase and hexagonal (beta, epsilon, and delta') phases as the minor constituents. The formation of hexagonal phases is nitrogen-dependent: (beta, epsilon) for N-2/(N-2+ Ar) < 20%, and (delta') for ratios >40%. The cubic and hexagonal phases simultaneously grow during the nucleation and early stage of growth up to a thickness of approximately 0.6 mu m, but the hexagonal phase gradually decreases through the film thickness, thereby giving rise to single-phase deposits at the upper part of the coatings. The addition of Ti improved the crystallinity and favoured the formation of the cubic phase, whereas a solid solution fcc delta-Nb-1 (-) xTixN was observed for the entire range of (x). The dominant orientation of the crystallites was (111), while a significant contribution of (311) was detected at higher Nb/Ti. The determination of stressed and stress-free lattice parameters allowed correlating the relative reduction of the compressive residual stress, hardness, and modulus in the nitrogen-rich films to the formation of vacancies in the metal sublattice. (C) 2016 Elsevier B.V. All rights reserved.