Preparation And Properties Of Micro-arc Oxidation Coatings On Mg-2Sr Alloys In Different Electrolyte Systems

As the biodegradable implant materials,magnesium alloys take many advantages as follows:similar density and Young'smodulus to that of human bone,which can avoid stress shielding effect to accelerate the growth of human bone.Additionally,Magnesium in the skeletal system is beneficial to bone growth and the improvement of bone strength.And the dissolution of magnesium does not bring any side effects.However,the degradation rate of magnesium in the body is too fast to implement supportive function before new bone grow into mature ones.The hydrogen from degradation deposits around the implants,which prevents the healing of wound and improves the pH around implants.Even worse,alkalosis will happen.Purification,alloying and surface modification can be taken into consideration to enhance the corrosion resistance of magnesium alloys.The strontium?Sr?element is chosen in this experiment to prepare Mg-2Sr with high performance.Then micro-arc oxidation technology is used to prepare functional coatings on the surface of Mg-2Sr.The research theme of this article is preparation and properties of micro-arc oxidation coatings on Mg-2Sr alloys in different electrolyte systems.Firstly,the Mg-2Sr alloys were casted.Then the influence of polyethylene glycol?PEG1000?on the of coatings on Mg-2Sr alloys were observed.And the optimized content was clear.Next,the effect of the organic calcium sources?calcium gluconate,calcium lactate?and inorganic calcium sources?calcium carbonate,calcium hydroxide?on the structure,electrochemical behavior and degradation in vitro behavior of micro-arc coatings were studied.Lastly,the influence of different calcium phosphorus ratio on the structure and biological activity for micro-arc coatings were researched.It turned out that part of strontium element in Mg-2sr alloy produced the intermediate compounds with magnesium,others was present in the alloy in the form of solid solution.The actual composition of the alloy is Mg-2.24Sr.The thicknesses of micro-arc oxide coatings were reduced slightly with negligible difference after the addition of PEG1000.The adhesion strength of the coatings were improved with different degrees.Concretely,the largest one,that is 15.75 N,was observed in the coatings prepared in the electrolyte containing 8 g/L PEG1000,which had the strongest corrosion resistance and the lowest rate of degradation also.In addition,the phenomenon of weight gain happened in 2 days SBF immersion.New phases including Ca₂P₂O₇?CCP?,Mg?OH?₂,Sr-HA(Ca_10-xSr_x?PO₄?₆?OH?₂)and HA were found in the micro-arc oxide coatings prepared from organic calcium sources and inorganic calcium sources.Sr-HA is formed via the replacement of Ca element by Sr element in the hydroxyapatite.It is a kind of new biological compatibility of apatite beneficial to promote the growth,reproductive and repair of cells around bone implant.The granular corrosion products with higher calcium phosphate phase were observed in the micro-arc coatings prepared in the electrolyte containing calcium lactate after immersion in the SBF,which indicates excellent biocompatibility and biological activity.Additionally,nanorods corrosion products along the corrosion crack appeared in the coatings with the calcium phosphate of 1:4 and 1:5 in the electrolyte containing calcium carbonate.The weight loss rates of coatings prepared in the two kinds of electrolyte abovementioned were only around 13%after 21 days SBF immersion,which is beneficial to ensure the mechanical stability of the implants in vivo environment.