Preparation Of Anodic Coatings Containing Calcium,Magnesium And Copper Elements On Biomedical Titanium Alloys And Investigation On Entrance Mechanism

Titanium alloys are widely used for orthopedic implants due to their beneficial characteristics including low density,high strength,excellent corrosion resistance and biocompatibility.However,titanium alloys are bio-inert materials and do not possess any antibacterial ability.Therefore,it is important to search suitable surface treatment methods to improve biological property of titanium alloys.Micro-arc oxidation(MAO),a particularly appealing surface treatment method,can generate porous ceramic-like coatings on titanium alloys.The performance of an MAO coating is determined by the composition and concentration of the selected electrolyte as well as the applied electrical parameters.Calcium(Ca)is an essential element of the human body and an important component of bones and teeth.As the fourth most abundant cation in human body,magnesium(Mg)supports the mineralization of calcified tissues and indirectly influences mineral metabolism.In addition,Mg supplementation to bioceramic substrata may potentially improve integration of implants and their associated healing time in orthopaedic and dental surgery.Copper(Cu)is an essential trace element for mammalian life.Additionally,Cu ions exhibit the low cytotoxicity and can stimulate the activity of several enzymes and support the cross-linking of collagen and elastin in bones.Cu is also a very good antibacterial element.In a base solution containing environmentally friendly organic electrolyte phytic acid,EDTA-Ca Na₂(Na₂Ca Y),EDTA-Mg Na₂(Na₂Mg Y)and EDTA-Cu Na₂(Na₂Cu Y),which are stable with good solubility in water,were separately selected as Ca-,Mg-and Cu-containing electrolytes in this paper to prepare MAO coatings containing Ca-Mg,Cu and CA-Mg-Cu elements.This main work of the paper is followings:(1)MAO coatings containing both calcium(Ca)and magnesium(Mg)elements and possessing a proper coating porosity are desirable for surface treatment of medical titanium alloys.The influences of Na₂Ca Y,Na₂Mg Y,KOH,and final voltage on the Ca content,Mg content,and porosity of MAO coatings developed on titanium alloy were investigated by an orthogonal experiment.The results indicate that Ca and Mg separately enter into MAO coatings primarily via diffusion,and the competition between these two elements is relatively insignificant.The increasing KOH concentration can increase both Ca and Mg amounts in MAO coatings.High solution p H,which increases with increasing KOH,Na₂Ca Y or Na₂Mg Y concentrations,favors formation of MAO coatings.At the same processing condition,the Ca amount in MAO coatings is higher than the Mg amount,which may be mainly attributed to the smaller solubility product constant of Ca₃(PO₄)₂ than that of Mg₃(PO₄)₂.Final voltage is beneficial to coating formation and has a small impact on Ca or Mg contents but is the most important factor governing coating porosity.With the increasing final voltage,the coating porosity firstly increases from 300 to 350 V,but exhibits a decreasing trend after 350 V.(2)The effects of concentration of phytic acid,Na₂Cu Y and KOH in electrolyte and power control mode on surface morphology and composition of MAO coatings were studied.The results show that the used electrolytes determine the coating composition and surface morphology through influencing the solution p H and conductivity.During MAO,copper is simultaneously involved in the formation of the MAO coating on the anode and the precipitation of metallic Cu on the cathode.The copper content in MAO coatings can be improved slightly by Na₂Cu Y and significantly by KOH attributed to their influences on solution p H.Phytic acid is beneficial in developing MAO coatings,but at higher concentrations up to 4 g/L decreases the copper content.The copper in MAO coatings exists primarily in the form of Cu₂O,while metallic copper particles in the range of 150-600 nm are deposited on the cathode through the reduction reaction.Compared with the constant current density mode,the fabricated MAO samples under the constant voltage mode contain a lower content of Cu and more evident microcracks.(3)MAO coatings containing Ca-Mg-Cu were prepared by two-step MAO treatment.Anodic coating containing high Ca and proper Mg amount was firstly prepared on the anode.Then,the Ca-Mg-Cu coating was prepared by cathodic deposition method.The results show that the Cu particles with fine crystallization can be prepared by using a neutral or weak acid solution,and Cu particles grow outward from micro pores with slow growth rate and easily controllable adhesion between the deposited Cu particles and the substrate.The p H value and conductivity of electrolyte have a great influence on the quality of deposited Cu.The Cu content in MAO coatings is positively and negatively correlated with the electrolyte conductivity and the solution p H value.In the neutral solution,the large Cu particles are present in the solution with high conductivity.However,the Cu particles prepared in the solution with low conductivity are uniform and small in pore size.