Abstract
In this paper, the surface modification of the NiTi alloy was accomplished by anodizing in the ethylene glycol (EG)-based electrolyte and subsequent heat treatment at 823 K (550 °C) for 30 minutes. The field emission scanning electron microscopy images revealed that anodizing at 30 V led to the formation of a 3-D interconnected nanoporous TiO2 layer on the surface of NiTi. The existence of this oxide layer did not have a negative effect on the superelastic behavior of NiTi. In the next stage, this modified surface of samples was coated with calcium phosphate (Ca-P) ceramic using the pulsed electrodeposition method. Based on the results, anodizing of the NiTi substrate before electrodeposition promoted the quality of the applied coating. Moreover, electrodeposition at the higher current densities of 15 and 20 mA cm−2 increased the possibility of the hydroxyapatite phase formation in the coating rather than the other less stable calcium phosphate phases. Additionally, both the TiO2 layer and the Ca-P coating significantly improved the corrosion resistance of the NiTi alloy and suppressed the release of Ni ions from its surface. At last, in comparison to the bare NiTi, the mechanical locks between the nanoporous structure of the modified sample and the Ca-P coating increased the bonding strength.
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