Surface characteristics and corrosion behavior of laser surface nitrided NiTi shape memory alloy for biomedical applications

Laser gas nitriding (LGN) has been applied to fabricate a TiN coating on the surface of NiTi alloy for the purpose of improving the surface properties. The microstructure, surface characteristics, and corrosion properties of the laser gas nitrided layer on NiTi alloy surface were investigated. The nickel release rate of the LGN specimen in Hanks’ solution was also investigated. The results of x-ray diffraction, x-ray photoelectron spectroscopy, and scanning electron microscopy confirmed that a continuous TiN layer which is nearly free of nickel has formed on the outermost surface of the substrate. Good metallurgical bonding between the nitride layer and the substrate has been achieved. The corrosion resistance of the LGN alloy in Hanks’ solution at 37 °C showed significant improvement. The nickel release rate in Hanks’ solution at 37 °C of the surface nitrided alloy as determined by atomic absorption spectrometry has been decreased significantly. The surface TiN layer was very inert, and no obvious changes in chemical composition and surface appearance were observed after immersion test in Hanks’ solution at 37 °C for 15 days.