Preparation And Research On Properties Of Al₂O₃ Coatings On NiTi Alloy Surface By Micro-arc Oxidation

NiTi alloy is widely used in biological devices due to its significant shape memory effect and super elasticity. It will release harmful Ni ions, so the body will suffer from corrosion after implantation, and the elastic modulus of NiTi alloys is higher than the human bone, it does not match human bone on the mechanical properties. To solve the above two disadvantages, this paper prepared Al₂O₃ ceramic coating on the NiTi alloy surface by micro-arc oxidation method. Microstructure of the coatings was systematically observed by SEM、XRD、EDS and XPS, and the formation mechanism of the Al₂O₃ coatings was also analyzed. The bonding strength、corrosion resistance、hardness and elastic modulus were studied by pull-off test、wear test and nanoindentation respectively.NiTi alloy surface layer ceramic coating was prepared in a mixed solution NaAlO₂ and NaH₂PO₂ by micro-arc oxidation treatment. The film prepared by micro-arc oxidized is porous morphology. MAO coating pores and poresize varies by process parameters. After the micro-arc oxidation, Ni、Ti content decreased, Al、O content were significantly improved, and film thickness changes as the anodic voltage、NaH₂PO₂ concentration、duty cycle and pulse frequency. The film composition is mainly γ-Al₂O₃, intensities of the diffraction peaks increases as the duty cycle increases,as the anodic voltage increases, with the increasing concentration of NaH₂PO₂ and with the increasing frequency。The corrosion resistance of NiTi alloys after micro-arc oxidation treatment can generally superior NiTi alloys that has not been micro-arc oxidation treatment, and with the electrolyte concentration、forward voltage、power pulse frequency and duty cycle changing, the corrosion resistance of NiTi alloys changes.When the anodic voltage was 420V、duty cycle was 45%、the electrolyte concentration was 0.03 M and the power frequency was 100 Hz, the corrosion resistance of this coating is about two orders of magnitude higher than that of uncoated NiTi alloy.The concentration of NaAlO₂ is 0.15 M, the concentration of NaH₂PO₂ is 0.03 M, anodic voltage is 330-420 V, power frequency is 100-700 Hz, duty cycle is 15%-45%, the bonding strength of the coating and the substrate is greater than 45 Mpa. Higher power pulses frequency, oversized anodic voltage and NaH₂PO₂ concentration will reduced the bonding strength of the coating and the substrate.The hardness and elastic modulus of Micro-arc oxidation coating layer was lower than the NiTi alloy substrate.