Oxygen diffusion hardening of tantalum coatings on cp-titanium for biomedical applications

Protective tantalum coatings on titanium substrates were produced using a two step process. At first, substrates were coated with Ta layers of 5μm thickness by physical vapor deposition (PVD). In a second step, the coated samples were hardened by oxygen diffusion for up to 3h. During this process the samples were exposed to oxygen for 1-2h at a pressure of 6.7·10^-3mbar at 350-450°C, followed by 1-2h annealing in oxygen-free atmosphere at the same temperature. X-ray diffraction (XRD) analysis demonstrated a shift of peaks for oxygen diffusion treated samples, which was attributed to the diffusion of atomic oxygen into the Ta-layer. The hereby caused mechanical stress in the crystal lattice led to an increase in Vickers hardness of the Ta layers from 570HV to over 900HV. In order to compare the adhesion of untreated samples with oxygen diffusion treated samples, the coatings were investigated using Rockwell measurements. These tests demonstrated an increase of critical force for coating delamination from 12N for untreated samples up to 25N for diffusion treated samples.