Tantalum oxide films on NiTi plates

Equiatomic NiTi alloy exhibits unique properties such as shape memory effect and superelasticity, for which this alloy is used as biomaterials in medical devices such as wire guides, self-expandable endovascular and urinary stents and catheters. The use of NiTi in biomedical applications was possible due to the formation of a spontaneous titanium dioxide (TiO2) layer at the surface of the alloy. This TiO2 layer provides biocompatibility and prevents the release of Ni2+ ions, which is likely to create adverse reaction in the body. One of the concerns with NiTi implants is the possibility of the breakdown of this TiO2 layer in body fluids as such a breakdown may potentially result in the release of undesirable Ni2+ ions from the Ni-rich subsurface. Many surface treatments such as electropolishing, anodization, oxidation/thermal oxidation and coating with a biocompatible film have been used to improve the biocompatibility and corrosion resistance of NiTi alloy. Coating NiTi with a biocompatible corrosion resistant material is expected to further improve the alloys biocompatibility, corrosion resistance, and to supress any potential nickel ion release. Tantalum can be a candidate material for this as it is known to be both biocompatible and highly corrosion resistant. There is, however, inadequate information on sputter deposition and reactive sputter deposition of tantalum and tantalum oxide films on NiTi.

In this work, sputter deposition and reactive sputter deposition of tantalum and tantalum oxide films onto a NiTi alloy have been investigated. Various thin film properties such as morphology, structure, chemistry, topography and adhesion has been compared between thermally oxidised conventionally sputtered film and reactively sputtered tantalum oxide films to establish an optimised method to produce the best quality film. Parameters such as deposition time, argon to oxygen ratio, sputtering power, and application of an anode bias were varied to examine the effect on the tantalum oxide films which were extensively characterised using techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDX), x-ray photoelectron spectroscopy (XPS), X-ray Diffraction (XRD), atomic force microscopy (AFM) and scratch testing to determine the best deposition method. Such an investigation helped to establish the ideal sputtering parameters for deposition of tantalum oxide films onto NiTi.

This study compared deposition of tantalum oxide film by two methods: sputter deposition followed by thermal oxidation and reactive oxygen sputtering followed by thermal annealing and found that reactive sputter deposition of a tantalum oxide film onto a NiTi substrate provided better quality films and better adhesion to the underlying NiTi substrate. This study established a way of benchmarking binding energies of the various different oxidation states for x-ray photoelectron spectroscopy (XPS) analysis. Previously there was no way of identifying the various oxidation states of tantalum apart from Ta+5 and Ta0 . It is known from literature that tantalum pentoxide (Ta2O5) is biocompatible. However it was not known if the lower oxidation states of tantalum are biocompatible. Cytotoxicity tests showed that tantalum present in a lower oxidation state of TaO2 and also the sub-oxides of tantalum (Ta2O3, Ta2O and TaO) are biocompatible. The out diffusion of the interfacial Ti to the final overlayer surface has been found. Such outmigration of Ti has been found to be dependent on the time and temperature of post deposition heat treatment. A beneficial role of applying a negative electrical bias to the NiTi substrate, found in this investigation, indicates that it may be possible to obtain a Ta oxide film that can suppress Ti depletion from NiTi surface and the consequent Ni-enrichment. This can be suitably utilised to further improve the biocompatibility of NiTi alloys.