Effect of titanium nitride/titanium coatings on the stress corrosion of nickel–titanium orthodontic archwires in artificial saliva

Abstract 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 ( I corr ), 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.