Research On The Modification Of NiTi Alloy By Micro-Arc Oxidation

The low Ni content Al₂O₃ ceramic coatings on biomedical NiTi alloy were prepared by micro-arc oxidation (MAO) pulsed bipolar power supply in order to improve the corrosion and wear resistance, suppress the release of Ni ions and enhance the use security and biocompatibility of biomedical NiTi alloy. 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, wear resistance, in vitro biological performance of the coatings were studied by pull-off test, ball-disk friction and wear test, electrochemical test and in intro test, respectively.In the electrolytes of NaAlO₂ and NaAlO₂-NaH₂PO₂, the coatings are both composed of onlyγ-Al₂O₃ crystal phase, and its crystallinity is increased with the increase of the electrolyte concentrations, treatment voltages and treatment times. In the electrolytes of NaAlO₂, the Ni content of the ceramic coatings is about 0.7³ at.%. The surface morphologies and surface roughness of the coatings are greatly influenced by NaAlO₂ concentrations, and the bonding strength of the coatings is enhanced with the increase of the NaAlO₂ concentrations. The maximal value is 28 MPa. In the electrolyte of NaAlO₂-NaH₂PO₂, the surface of the coatings show the typical micro-arc oxidation porous structure, and Ni content is about 3⁷ at.%. The number and size of micro-pores on the surface and the surface roughness of the coatings are varied with the treatment times, NaH₂PO₂ concentrations and treatment voltages. When the electrolyte is 0.15 mol/L NaAlO₂-0.01 mol/L NaH₂PO₂, anodic voltage is 400 V, and treatment times are 2⁹0 min, the bonding strength of the coatings can exceed 60 MPa. More NaH₂PO₂, higher anodic voltages and cathodic voltages addition have negative relation with the bonding strength of the coatings.At the initial stage of MAO process, the aluminate ions in the electrolyte are deposited on the surface of anodic NiTi sample, which promotes the formation of Al₂O₃ insulation film and creates the condition for the subsequent electrical breakdown and plasma discharge. According to combined analysis, the ceramic coatings prepared in NaAlO₂-NaH₂PO₂ electrolyte can be divided into three layers: transition layer, dense inner layer and porous outer layer. The transition layer contains amorphous Ni₂O₃, TiO₂, Al₂O₃ and phosphate. The dense inner layer is mainly composed of Al₂O₃ and a little Ni₂O₃, TiO₂ and phosphate. The porous outer layer also mainly contains Al₂O₃ and a less Ni₂O₃ and TiO₂. The three layers structure is beneficial to provide excellent bonding strength, corrosion and wear resistance for the ceramic coatings on NiTi alloy.Micro-arc oxidation coatings obviously improve the wear resistance of NiTi alloy. The friction coefficient and the wear rate of NiTi alloy against GCr15 steel ball is 0.75⁰.85 and 3.5μg/Nm, respectively, and the wear mechanism is mainly abrasive wear. The friction coefficient of MAO coatings against GCr15 steel ball varies from 0.7 to 0.9 with the different coating preparation process. The friction coefficient of the coatings is more stable than that of NiTi alloy, and the wear mechanism of the coatings is adhesion wear. When the treatment voltage is 400 V and the treatment is 90 min, the wear rate of this coating is 0.4μg/Nm, which indicates that the wear resistance of the coating is 9 times higher than that of uncoated NiTi alloy. Micro-arc oxidation coatings significantly improve the corrosion resistance of NiTi alloy and suppress the release of Ni ions. When the treatment voltage is 400 V and the treatment is 90 min, the corrosion resistance of this coating is about two orders of magnitude higher than that of uncoated NiTi alloy, and the account of Ni ions release is one order of magnitude lower than that of uncoated NiTi alloy.After micro-arc oxidation treatment, the coated NiTi alloy can induce calcium deficiency HA, which indicates that the coated NiTi alloy presents bioactivity. According the results of hemolysis ratio, kinetic clotting time and platelet adherence, it can be concluded that the hemocompatibility of NiTi alloy can be markedly improved after MAO treatment. The in vitro osteoblast culture tests indicate that the coated NiTi alloy has a better cell propagation ability compared to uncoated NiTi alloy, which illustrates that the rough and porous Al₂O₃ coatings can effectively improve the biocompatibility of NiTi alloy.