Pulsed electrodeposition of compact, corrosion resistant, and bioactive HAp coatings by application of optimized magnetic field

NiTi shape memory alloy possess spectacular properties, so it has a great potential to use as bio-implants. However, releasing toxic Ni ions from its surface have restricted its successful use in human's body. The present research attempts to modify the surface conditions of NiTi through deposition of calcium phosphate (CaP) coatings to overcome the abovementioned challenge, besides improving the other critical characteristics required for successful use of NiTi in bio-related applications. CaP deposits were deposited on NiTi by means of pulsed electrodeposition under magnetic field. Since the hydroxyapatite (HAp) has the closest Ca/P ratio to that of human's bone among the other CaP types, and considering its superior bioactivity, the processing variables including pulse current density, deposition time, as well as magnetic field direction and magnitude were adjusted in such a way that the Ca/P ratio of HAp obtains. The aim behind application of magnetic field was to attain favorable morphological and structural properties. Then, the characteristics of coatings electrodeposited under optimized conditions were investigated via Fourier transform Infrared (FTIR) and X-ray diffraction (XRD). Electrochemical potentiodynamic polarization studies test in Ringer's solution was performed to assess the corrosion behavior of the coatings. A noticeable increment in polarization resistance of NiTi, about 10 times, is obtained with deposition of the optimized coatings. Ni ion release was found to decrease by 89% after deposition of HAp coatings on NiTi. In vitro bioactivity of the coatings were studied through immersion of HAp coatings in simulated body fluid (SBF). Nucleation of apatite particles all over the coating microstructure demonstrate the desirable bioactivity of the deposits. In general, compact, corrosion resistive, and bioactive HAp coating with minimal Ni ion release were deposited on NiTi by controlling the operating conditions.