Research Of Ceramic Coating On NiTi Alloy Prepared By Micro-arc Oxidation

The nearly equiatomic Ni Ti alloy has been widely used in numerous biomedical applications due to its distinctive shape memory effect, super-elastictiy, Wear resistance and corrosion resistance capacity. In spite of the intriguing properties, Ni Ti alloy cannot form chemical construction with bone tissue used as a medical alternative materials, and Ni Ti alloy provides the source for potential Ni ions release, which can trigger the risk of allergy and adverse reactions to human system. The micro-arc oxidation(MAO) technique were focused on the treatment of Ni Ti to improve the above problems. In this paper, the ceramic coatings on Ni Ti alloy were prepared in Na Al O2、Na2Si O3 Electrolyte by MAO Constant current power supply.Microstructure of the coatings was systematically observed by SEM, XRD, and XPS, and the corrosion exeperiment was tested by Electrochemical Workstation. A comprehensive study of the electrolyte composition and concentration, oxidation time impacting on the quality of the film. And the formation mechanism of the coatings was also analyzed. The results show that: he ceramic coatings prepared by MAO exhibit good mechanical properties and corrosion resistance.In the electrolytes of Na Al O2,the film are formed quickly by MAO constant current power supply, the thickness and roughness of film increse with the power-on time, and the film are composed of Al2O3 crystal phase.Ni and Ti are not found on the surface,the results show that the dielectric film on Ni Ti alloy surface is formed through electrochemical deposition in the electrolytes of Na Al O2, and throught repeated discharge, melting,injectin cooling and solidification, then the thickness of film increase. The quality of the coating formed Under these conditions is very poor, the film will be off at the room temperature for a certain time.In the electrolytes of Na2 Si O3, Ni Ti alloy are also treated by MAO constant current power supply, The voltage-time in the process contain two stages: “growth period”, “jumped period”. in early time the surface morphology of Ni Ti did not change significantly for a long time, and only occurred simply anodizing process, During the early process, Ni in Ni Ti is oxidized to Ni ion, and the Ni ions are dissociated in electrolyte. Ti was left in Ni Ti, which generate much Ti content appearing on the surface of the sample, and contribute to prerequisites for the reaction. After the applied voltage reaches a certain value, Na2 Si O3 electrolyte participate in the reaction and form insulating amorphous silicon oxide layer. Then the voltage increases rapidly which is considered as “jump zone”. In order to explore a relatively excellent process parameters, we study the relationship between the thickness, hardness of the coating with the concentration of Na2 Si O3, we find that the thickness is added With the increasing concentration of Na2 Si O3,the coating formed on the surface of Ni Ti is rough and porous,which can effectively improve the biocompatibility of material.Micro-arc oxidation coatings obviously improve the corrosion resistance of Ni Ti alloy, and suppress the release of Ni ions. The polarization curves shows that the the corrosion resistance of coating are not enhanced with the increasing concentration of Na2 Si O3. And the corrosion resistance of coating treated in 42g/L Na2 Si O3 is the best, compared to the base material, After modification, the corrosion current density of the sample reduced two orders of magnitude,and the corrosion potential increase 0.3V, the release of Ni ions greatly reduced.