PIRAC nitriding of a NiTi shape memory alloy : augmentation of degradation resistance

Abstract

By virtue of their unique superelastic and shape memory phenomena, NiTi alloys have lately been providing safer solutions to lingering problems in the biomedical field. Their application is especially important considering the ever increasing popularity towards the approach of minimally invasive surgery. Unfortunately, despite numerous successful clinical trials, the biocompatibility of Nitinol remains controversial as a result of its high nickel content (~50at.%). In this regard, the main motivation of this study was the use of surface engineering to enhance the corrosion response of a NiTi alloy. In view of this, a TiN based coating was produced on Nitinol coupons using powder immersion reaction assisted coating (PIRAC), an innovative surface engineering technique based on the annealing of samples in powders of an unstable nitride enclosed in chromium-rich stainless steel bags. Modifications to the PIRAC procedure, including the introduction of specific oxygen getters, were carried out to enhance the performance of the process itself. The response of PIRAC coated Nitinol in comparison to the bare metal was then investigated via the use of potentiodynamic polarisation, ASTM F746 crevice-jig potentiostatic testing and electrochemical impedance spectroscopy in Hank’s solution maintained at a temperature of 37℃. Samples of the widely-established biomedical alloy, 316LVM stainless steel, were used as control. The tested PIRAC coupons were characterised by a uniform TiN coating followed by a Ti2Ni under-layer with a combined thickness of around 1 µm. Results obtained from the electrochemical techniques indicated that PIRAC is capable of significantly improving the corrosion response of Nitinol in the test conditions, especially in situations where crevice corrosion is favoured. Despite this improvement, 316LVM displayed slightly superior characteristics to PIRAC coated Nitinol during potentiodynamic polarisation and impedance spectroscopy. The effects, if any, on the structural integrity of the TiN coatings obtained on Nitinol substrates via PIRAC after exposing the latter to high strain levels were also studied. Tensile testing of PIRAC coated Nitinol coupons showed that the produced TiN coatings are not able to retain their structural integrity after being subjected to strains of up to 3% and 6% as a result of their inability to behave superelastically. On the other hand, it was made evident that PIRAC processing does not compromise the superelastic behaviour of the Nitinol substrate.