The purpose of this investigation was to develop titanium nitride (TiN)/titanium (Ti) coating on orthodontic nickel-titanium (NiTi) wires and to study the stress corrosion of specimens in vitro, simulating the intra-oral environment in as realistic a manner as possible. TiN/Ti coatings were formed on orthodontic NiTi wires by physical vapor deposition (PVD). The characteristics of untreated and TiN/Ti-coated NiTi wires were evaluated by measurement of corrosion potential (E-corr), corrosion current densities breakdown potential (E-b), and surface morphology in artificial saliva with different pH and three-point bending conditions. From the potentiodynamic polarization and SEM results, the untreated NiTi wires showed localized corrosion compared with the uniform corrosion observed in the TiN/Ti-coated specimen under both unstressed and stressed conditions. The bending stress influenced the corrosion current density and breakdown potential of untreated specimens at both pH 2 and pH 5.3. Although the bending stress influenced the corrosion current of the TiN/Ti-coated specimens, stable and passive corrosion behavior of the stressed specimen was observed even at 2.0 V (Ag/AgCl). It should be noted that the surface properties of the NiTi alloy could determine clinical performance. For orthodontic application, the mechanical damage destroys the protective oxide film of NiTi; however, the self-repairing capacity of the passive film of NiTi alloys is inferior to Ti in chloride-containing solutions. In this study, the TiN coating was found able to provide protection against mechanical damage, while the Ti interlayer improved the corrosion properties in an aggressive environment. (C) 2014 Elsevier B.V. All rights reserved.
