Preparation And Property Study Of Calcium Phosphate Coating On The Surface Of Medical Magnesium Alloy In Neutral Solutions

Magnesium?Mg?alloys have great potential to serve as degradable metallic biomaterials due to their distinct properties including similar density and Young's modulus to natural bone,superior physical and mechanical properties,and good biocompatibility of Mg ions with human body.However,a major drawback of Mg alloys is their rapid degradation in vivo,severely limiting their clinical applications.Micro-arc oxidation?MAO?,also known as plasma electrolytic oxidation?PEO?,is an effectively surface modification technology that can effectively improve the corrosion resistance and biocompatibility of magnesium alloys.Since calcium and phosphorus are the main components in bones,preparation of calcium phosphate?Ca-P?coatings on magnesium alloy surface have become a hot issue.In this paper,hexamethylenetetramine was selected as a corrosion inhibitor of magnesium alloys,while environmentally-friendly organic phytic acid?IP6?and inorganic phosphoric acid?PA?were used as phosphorus-containing electrolytes.Calcium phosphate?Ca-P?coatings were fabricated for the first time in a neutral solution.The influences of EDTA-Ca Na₂,IP6 and PA concentration,and treatment time on coating performance were investigated.The formation mechanism of MAO coatings in solutions containing hexamethylenetetramine was clarified.The effects of four organic calcium salts including EDTA-Ca Na₂,calcium glycerol phosphate,calcium lactate,and calcium gluconate on coating property were compared.The obtained results are following:1.The influences of EDTA-Ca Na₂,IP6 and PA concentration,and treatment time on coating properties were investigated by an orthogonal experiment.The results show that coating corrosion resistance is synergistically determined by coating characteristics with coating thickness playing a particularly important role.EDTA-Ca Na₂ can increase calcium content of anodic coatings but acts as a corrosive agent of magnesium alloys and decreases corrosion resistance of MAO coatings.As a strong chelating agent,IP6 can promote EDTA-Ca Na₂solubility and therefore increases the calcium content more effectively than PA does.Compared with PA,IP6 more effectively improves corrosion resistance mainly by increasing coating thickness and bonding strength between coating and substrate.During MAO,P and F compete with each other to enter into anodic coatings.MAO coatings developed in the solution composed of both IP6 and PA achieve low F content?3.49 at%?and good biocompatibility,while those fabricated in solutions containing only IP6 or PA achieve high F content?higher than 19.00 at%?and exhibit high toxicity.2.In a base solution composed of 6 g/L NH₄HF₂,6 g/L EDTA-Ca Na₂,12 g/L IP6 and 15 g/L PA,0 g/L,120 g/L,240 g/L,360 g/L,and 480 g/L hexamethylenetetramine were separately added and their effects onformation regularity and corrosion resistance of anodic coatings were studied.The results indicate that hexamethylenetetramine can increase the solubility of calcium salts.With the increase of hexamethylenetetramine concentration,the solution becomes clearer with the continually increased p H value and higher final voltage.Hexamethylenetetramine is a corrosion inhibitor of magnesium alloys and the mechanism may be due to its interaction with magnesium ions and adsorptioin on the surface of magnesium alloys.The best corrosion resistance of MAO coatings is achieved when hexamethylenetetramine is 360 g/L.3.The influences of EDTA-Ca Na₂,calcium glycerophosphate?G-Ca?,calcium lactate?R-Ca?and calcium gluconate?P-Ca?concentration on the calcium content and corrosion resistance of MAO coatings were compared using an orthogonal experiment with four factors and three levels.The sequence on the calcium content in anodic coatings is G-Ca>EDTA-Ca Na₂>R-Ca>P-Ca.With the increase of calcium salt concentration,the calcium content gradually increases.The order on the corrosion resistance of MAO treated samples is G-Ca>EDTACa Na₂>P-Ca>R-Ca.G-Ca is a film-forming agent during MAO treatment of magnesium alloys,G-Ca can significantly increase the calcium content and improve the corrosion resistance of MAO treted samples.EDTA-Ca Na₂ and R-Ca are corrosive agents of magnesium alloys,which is harmful for coating formation and corrosion resistance.P-Ca can increase the calcium content in MAO coating.Proper P-Ca concentration can further improve the corrosion resistance of MAO coatings.