Preparation Of Bioceramic Film Containing Calcium Phosphate Compound On AZ91D Magnesium Alloy

Magnesium alloys have good application prospects as biodegradable materials due to their high specific strength,degradability and similarity to human bone skeletal modulus.However,the disadvantages of poor corrosion resistance and excessive degradation rate in the human environment limit its further application.In this paper,a ceramic film containing calcium and phosphorus compounds was prepared on the surface of AZ91D magnesium alloy by two-step micro-arc oxidation.Firstly,the silicon oxide and zirconia bottom layer with certain corrosion resistance were prepared in the silicate system electrolyte and the zirconate electrolyte by the first micro-arc oxidation,and then a ceramic film containing calcium-phosphorus compound was prepared in the calcium phosphate system electrolyte by the second micro-arc oxidation.By comparing the thickness and growth morphology of the calcium and phosphorus layers prepared on the two underlayers,it was found that the zirconia underlayer prepared in the zirconate electrolyte system is more conducive to the growth of the calcium phosphate layer.When the thickness of the silicon oxide bottom layer is 11?m,the calcium phosphate layer cannot continue to grow;but in the zirconate system,the calcium phosphate layer cannot continue to grow until the thickness of the bottom layer reaches 38?m.It is found from the cross-sectional microscopic morphology that after the second micro-arc oxidation,the thickness of the bottom layer is reduced,and the junction of the film base becomes uneven.The bottom layer is partially dissolved in the alkaline calcium phosphate system electrolyte and the calcium phosphorus layers grow alternately.The surface of the calcium phosphate layer prepared on the zirconia bottom layer is more continuous and complete than the one prepared on the silicon oxide underlayer.However,as the thickness of the underlayer increases,the difficulty of performing the second micro-arc oxidation gradually increases,the breakdown voltage required is higher and higher,the growth of the calcium-phosphorus layer becomes more and more difficult,and the surface morphology is more rough and porous.The presence of Ca₂P₂O₇ phase was detected from calcium and phosphorus layers,indicating that calcium and phosphorus were successfully introduced into the ceramic film from the electrolyte;the drop corrosion test found that in the zirconate system,When the thickness of the zirconia underlayer is 33?m and the total thickness of the ceramic membrane is 38?m,the drip corrosion time is the longest?about 730 s?,and the corrosion resistance is the best in the samples prepared by the two systems;The formation of HA was detected after immersion in the SBF solution for 14 days in the silica-based or zirconia-based ceramic film.Thus,the preparation of the calcium-phosphorus-containing bioceramic film was achieved.However,the corrosion of the calcium and phosphorus layers prepared on the underlayer of silicon oxide in the SBF solution for 21 days was severe.