Carbon-based nanostructured coatings on NiTi shape memory alloy for biomedical applications

Here, we propose carbon-based nanostructured coatings for nickel-titanium (NiTi) shape memory alloys (SMAs) for biomedical applications. NiTi SMAs are well-known biomedical materials; however, the elution of toxic Ni ions into the body has prevented SMAs from expanding their applications, particularly in the complete implantation of SMAs for human artificial muscles. One possibility for the suppression of leaching ions is to create a barrier coating, for which diamond-like carbon (DLC) may be a candidate. Due to the strong internal stress derived from the mismatch of the thermal expansion coefficient, however, the adhesive strength of DLCs is very low. In this study, we focused on the incorporation of tungsten into a DLC in order to reduce the internal stress of the coating. First, we present the definition of appropriate fabrication conditions. Uniaxial tensile tests were then performed, and we evaluated the adhesive strength of the coatings. We then determined the conditions that support the fabrication of a coating with the strongest adhesive strength. Thereafter, we deposited the coating onto the SMA with shape memory effects and conducted self-bending tests. We observed that the cracks initiated parallel to the longer direction of the specimens, most likely a result of a boundary slip between the parent phases of the SMA. The removed fraction area of the coating was estimated at less than 1 % after 105 cycles, indicating the potential usage of tungsten-containing DLC coatings for biomedical applications.