Study Of Fabrication And Performance Of Ceramic Coating On The Surface Of NiTi Alloy By Micro-Arc Oxidation

NiTi alloys have unique shape memory effects,superelasticity,low elasticity modulus,fatigue resistance and corrosion resistance and are used in biomedical fields such as self-expanding vascular stents,vena cava filters and orthodontic wires.However NiTi alloy as the biomedical material,when it is implanted into the human body,it will release toxic Nickel ions.In addition,NiTi alloy will be corroded and attrited in the human environment,and corrosion and wear will aggravate the release of Nickel ions.Therefore,it is essential to prepare a coating on the surface of the NiTi alloy,and this coating should have those properties:low Ni content,wear and corrosion resistance.In this paper,a ceramic coating was prepared on the surface of NiTi alloy by micro-arc oxidation technique.The effects of electrolyte additives and micro-arc oxidation electrical parameters on surface morphology and composition of coating were investigated by SEM and EDS.The corrosion resistance and wear resistance of the ceramic coating were studied respectively by the electrochemical workstation and the friction and wear tester to obtain the optimal electrolyte formulation and electrical parameters.On this basis,nano-ZrO₂ was added into electrolyte to fabricate the composite coating,and the structure and corrosion performance of the composite coating were studied,which will lays a theoretical foundation for the application of NiTi alloy in engineering and medical fileds.The ceramic coatings prepared under the additives of NaH₂PO₂ and Na₃PO₄exhibited a rough porous structure.The EDS analysis showed that the Ni content of the ceramic coating was much lower than that of the NiTi substrate,and the coating was well bonded to the NiTi matrix.However,the film prepared under the NaOH additive has a poor bonding state with the substrate,and may exist in the form of chemical adsorption on the surface of the substrate.Compared with three electrolyte systems,the surface morphology of the coating fabricated in NaH₂PO₂ were more uniform and corrosion resistance was better.In optimal electrolyte system NaAlO₂-NaH₂PO₂,the effects of NaH₂PO₂concentration and micro-arc oxidation electrical parameters on the surface morphology?composition?bonding strength of coating and substrate and corrosion wear properties of the ceramic coatings were systematically studied.The result shows that the coating was mainly composed of Al₂O₃.At 0.03mol/L NaH₂PO₂,voltage of 360 V,frequency of 500Hz and processing time of30 min,the surface of the ceramic coating was more uniform,and the coating bonded well with substrate.The electrochemical polarization curve and electrochemical impedance spectrum shows that the corrosion current density of the micro-arc oxidation sample reduced by two orders of magnitude than that of the substrate,the impedance modulus was higher than the substrate,and the corrosion resistance was significantly improved compared with the substrate.The friction and wear results showed that the main wear mechanism was abrasive wear.The wear volume of the coating was 22.21%of the substrate.The wear resistance of NiTi alloy was obviously improved.The number of micropores on the surface of the ceramic coating prepared in containing nano-ZrO₂ particles electrolyte was small and a large amount of particles were accumulated on the coating surface.The thickness of the composite coating was 5.71?m.It was composed of ZrO₂ and Al₂O₃.The EDS showed that the Ni element content of the composite coating only 0.2%much smaller than 50.8%of the substrate,so the addition of ZrO₂ was beneficial for decreasing the Ni content of surface.The electrochemical polarization curve shows that the corrosion potential of the composite film layer was increased by27.63%,and the corrosion current density was decreased by 13.8%,indicating that the addition of ZrO₂ nanoparticles contributed to the formation of a corrosion-resistant coating.