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

• Published in: Applied surface science 2021-01, Vol.535, p.147621, Article 147621
• Main Authors: McNamara, K., Beloshapkin, S., Hossain, K.M., Dhoubhadel, M.S., Tofail, S.A.M
• Format: Article
• Language: English
• Subjects: Biomedical alloy; Minimally invasive surgical devices; Ni-leaching; NiTi; RBS; Shape memory alloy; Tantalum oxide coating; Ti migration; XPS
• ISSN: 0169-4332
• DOI: 10.1016/j.apsusc.2020.147621

[Display omitted] •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 (TiOx). 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 TiOx 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.