TiO2 coating on NiTi by low-temperature methods for enhancing corrosion resistance

Abstract

Nickel titanium (NiTi) is the most popular shape memory alloy in industrial and in medical applications, the attractiveness being attributable to its superior shape memory effect and superelasticity. Among the many applications of NiTi in biomedicine, one common use is to employ the material as bone plates for internal fixation in bone fracture surgery. When metallic materials are used as implants, corrosion attack by body fluids could be a problem, mainly because of the potential hazards induced by the metallic ions released into the human body. This is particularly so in the case of NiTi, which contains 50 atomic % Ni, because Ni is allergenic and toxic when its concentration in the human body exceeds a certain threshold. In view of this concern, improvement of the corrosion resistance of NiTi is of vital importance for its safe use as orthopaedic implants. NiTi derives its shape memory effect and superelasticity from a reversible phase transformation between the martensitic phase and the austenitic phase. The transformation characteristics and hence the thermomechanical properties of NiTi are very sensitive to thermomechanical treatment. Thus NiTi implants are subjected to thermomechanical treatments following a prescribed procedure to impart the desired characteristics before implantation. It is important that subsequent surface treatment to improve corrosion resistance should not disturb the properties already built in. This means that low-temperature methods (those involving a bulk temperature of less than 300 oC for the substrate) have to be employed in the surface treatment of NiTi. The present study is an attempt to improve the corrosion resistance of NiTi implants by TiO2 coating using various low-temperature surface treatment processes. A number of low-temperature methods for synthesizing a TiO2 coating on NiTi have been attempted in the present project, including: 1. Hydrothermal oxidation in water, 2. Sol-gel coating with steam crystallization, 3. Low-voltage DC anodization in acetic acid, 4. AC anodization and treatment in Ca-P containing solution, 5. Deposition of TiO2 coating by cathodic synthesis, and 6. Laser oxidation in ambient air, with the bulk temperature of substrate remaining low. The TiO2 coatings formed were characterized using scanning-electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffractometry (XRD), energy-dispersive X-ray spectrometry (EDS), and X-ray hotoelectron spectroscopy (XPS), etc., to determine the coating morphology and thickness, composition and phase, etc. The corrosion behaviors of the treated NiTi samples in Hanks' solution (a simulated body fluid) were studied using polarization techniques and electrochemical impedance spectroscopy (EIS), and compared with that of bare NiTi. The apatite-forming ability of the surface-treated samples was studied using immersion test in Kokubo's simulated body fluid. All these low-temperature methods were proven to be effective in achieving two important improvements in the surface properties of NiTi, the first being the reduction of Ni content in the surface layer, and the second being the increase in corrosion resistance, ranging from 1 to 2 orders of magnitude. Each of these methods has its own merits and limitations, and leads to a different degree of improvement. In all these methods, the improvements may be attributed to the formation of a thicker TiO2 layer on NiTi, either by the preferential oxidation of Ti (methods 1, 3, 4 and 6) or by deposition of an adherent TiO2 coating (methods 2 and 5). Moreover, in all these methods, the highest temperatures reached in the bulk of the substrate are below 200 oC. The coatings formed in methods (2), (4) and (5) were capable of inducing the formation of apatite as evidenced by the results of immersion tests in simulated body fluids of the Kokubo type.