Tantalum coating inhibits Ni-migration from titanium out-diffusion in NiTi shape memory biomedical alloy

Highlights: • Inward diffusion of oxygen and outward migration of titanium through tantalum film. • Outmigration of titanium depends on bias, film thickness and annealing temperature. • Applying of bias during deposition produces denser tantalum films. • Thin layer of reactively sputtered Ta oxide on NiTi restricts Ti outmigration. • Biocompatible Ti/Ta interdiffusion layer gives larger barrier against Ni leaching. • Potentially improving long-term biocompatibility of NiTi. Despite the presence of over 56% Ni by weight, equiatomic NiTi is generally considered biocompatible as it naturally oxidises to form a surface oxide mainly composed of biocompatible oxides of titanium (TiO_x). This layer is formed by an oxidation mechanism that promotes out-diffusion of Ti leaving a Ti-depleted, Ni rich subsurface. The long-term in vivo stability of the naturally grown TiO_x layer has been a concern as Ni can leach out through this thin, defective layer. The leaching of nickel (Ni) is thus a continuing threat to the alloy's otherwise outstanding biocompatibility. We have found that a layer of reactively sputtered tantalum (Ta) oxide on the bulk NiTi restricts Ti-out-migration through a biocompatible Ti/Ta inter-diffusion layer that provides a larger barrier against Ni leaching. We have investigated this inter-diffusion as a function of sputtering process parameters and post processing treatments. Surface and interface analytical techniques such as X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, cross sectional transmission electron microscopy and non-destructive ion beam analysis techniques such as Rutherford backscattering spectrometry and particle induced X-ray emission were used to evaluate the nature of this inter-diffusion layer which can improve long-term biocompatibility of NiTi.