Structure And Composition Control Of Porous Coatings In Medical Titanium Alloys During Micro-arc Oxidation

Ti6Al4V alloy is a widely used bone implant material in the clinic.Micro-arc oxidation(MAO)can prepare porous bioactive coating on Ti6Al4 V surface,and the composition of the coating can be regulated by the interaction between alloy elements and electrolytes.However,the directional design of the structure and composition of the coating is still faced with a great challenge,the key of which is that the interaction mechanism between alloy elements and electrolytes is not precise.In this paper,Ti-x Al binary alloys with different Al contents were designed to explore the effect of Al on the coating's structure during micro-arc oxidation.Meanwhile,Ti6Al4 V ternary alloy widely used in the clinic was selected to study the effect of electrolyte composition on coating's structure and composition in the process of Ti6Al4 V micro-arc oxidation.The interaction mechanism between alloy elements and electrolyte composition was explored,and its biocompatibility was evaluated.It is found that the content of ejected oxides around the discharge channel increased with the increase of Al content in Ti-x Al alloys.Thereby,the “cortex-like” structure on the surface of pure Ti gradually evolved into a “crater-like” structure on Ti-20 Al.Moreover,with raising Al content,the critical breakdown voltage of the coating and the working voltage in the stable stage increased linearly.And the plasma discharge breakdown became difficult.Finally,a dissolution mechanism model of oxide with Al element was proposed.Under the guidance of the dissolution mechanism model,a highly connected micro-nano hierarchical coating was prepared on Ti6Al4V(the values of Al and V in the coatings decreased to as low as 0.06 at.% and 0.3 at.%,respectively).Compared with Ti6Al4 V substrate,Al/Ti and V/Ti of coatings were reduced by 98.3% and 73.8%,respectively,which means selective removal of Al and V in MAO coatings happened.Simultaneously,the “escape”mechanism of Al and V elements was discussed.It pointed out that the synergistic dissolution of borate mixed electrolytes with strong bases on molten oxides is the main factor to reduce Al and V significantly in the coatings.Cytocompatibilty tests confirmed that the hierarchical oxide coatings with lower Al and V were beneficial to the adhesion,spreading significantly,proliferation,differentiation,and calcification of the osteoblasts.