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

Abstract

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°C under a (N2+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 (β, ε, and δ′) phases as the minor constituents. The formation of hexagonal phases is nitrogen-dependent: (β, ε) for N2/(N2+Ar)<20%, and (δ′) for ratios >40%. The cubic and hexagonal phases simultaneously grow during the nucleation and early stage of growth up to a thickness of approximately 0.6μm, 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 δ-Nb1−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.