On the electrocrystallization of pure hydroxyapatite nanowalls on Nitinol alloy using a bipolar pulsed current

In this study a bipolar pulsed current was used for the electrochemical deposition of calcium phosphate coatings on the Nitinol superelastic alloy. The coatings were characterized using FE-SEM, XRD, FTIR, HRTEM, and gas adsorption techniques. Results show that the electrodeposited coating is a crystalline film which is composed of pure hydroxyapatite (HA) nanowalls. According to the FTIR data, the coating is free from other metastable calcium phosphate phases like octacalcium phosphate. It was shown that both monoclinic and hexagonal phases can be detected in HRTEM images and related SAED patterns. The coating, which is deposited at direct and reverse current densities of −3.0 and 0.1 mA/cm2 respectively, revealed a surface area of 38.5 m2/g and ultra-fine pores were detected in the surface of the coating plates. Study on the effect of the reverse pulse current revealed that the volume of porosity is increased by increasing in the current density of the reverse pulse. Also in this condition the resulted film was composed of nanosized HA crystals. It seems that the reverse current step can influence the porosity and the structure of the coatings by dissolution of the unstable phases which are formed during direct current step of the bipolar pulse deposition.