Study On Preraration And Properties Of Magnesium-based Calcium Phosphate Coating With Controllable Degradation

Magnesium （Mg） and Mg alloy have widely known as a new generation of hard tissuerepair alternative materials，due to their good biocompatibility, mechanical properties ofmatching human bone and degradation in vivo. In these aspects, they have surpassed thetraditional biomedical bone repair materials. However, the degradable rate of Mg and Mgalloys was so fast that they were restricted for used in clinical application. At present, how tocontrol the degradation rate of Mg alloy effectively has become a hot and difficult research athome and abroad. This research raised the ideas that prepare phase controlled calciumphosphate coating by plasma spraying on Mg alloy based on the modification of Mg alloybone fixation materials and bone surface of materials. Combining the technical characteristicsof HA and plasma spraying features, the research is to achieve controlled degradation of Mgalloy coating material. The plasma spraying ternary parameter map was built that can controlthe composition of calcium phosphate coating by studying on influence rules of sprayparameters to the morphology and composition of calcium phosphate coating. According tothe degradation experiments in the SBF and electrochemical corrosion experiments, it wasconfirmed that controlled of calcium phosphate coating prepared according to the ternaryparameter map could control the degradation of the coating. What’s more, the preparationprinciple and the controllable degradation mechanism of calcium phosphate coating werediscussed deeply. This study provides a new surface modification of ideas and theory andpractice reference for study on controlled the degradation of medical Mg-based materials tomeet the needs of the clinical application degradation.Result shown that within a certain range, the coating density increased, HA crystallinityand the relative percentage decreased with the increase of the spraying power. The coatingdensity decreased, HA crystallinity and the relative percentage rise with the extension of thespraying distance. The coating density was incremented, the coated HA crystallinity and thecontents of radiate rose along with the increase of powder particle size. Steam heating and canincrease the calcium phosphate coating morphology of HA crystallinity and the relativecontents of the plasma flame and the cold post-processing and vice versa. Finally thereasonable parameter range of for good microscopic morphology and phase composition ofcalcium phosphate coating process as follows: spray power20²5kW; spraying distance of90to110mm; particle size of30to90μm, and then deal with the choice of technologyaccording to actual needs.The performances of calcium phosphate coating after optimization were characterized. Calcium phosphate coating was mainly composed of three elements of Ca, P, O, the Ca/Pwas about1.787, and the elements distributed uniformly in the coating. The coatingcontaining the HA phase and HA decomposition （[Ca-P]） including Ca3（PO4）2, Ca₂P₂O₇, CaOand amorphous. The surface of the calcium phosphate coating appeared uneven and theroughness was about6.67μm. HA in Calcium phosphate coating got dehydroxylation and thethermal decomposition, because of the weakening of the OH^-raman peaks and passivation ofPO₄^3-raman peak in the coating. The average bond strength of calcium phosphate coatingapproximately was24.73MPa. The formation principle of calcium phosphate coatingincluded physical deformation process and chemical phase transition. Physical deformationwas spherical solidâ†'melt to liquidâ†'cooling and solidificationâ†'flatteningdeformation. Chemical phase transition was the dehydroxylation and thermal decompositionreaction at earlier stage, including nucleation into the crystallization process later.Calcium phosphate coatings prepared by plasma spraying effectively alleviated the rapiddegradation of the magnesium alloy in different simulated body fluids, and it is universal. Thedegradation rate order from small to large of calcium phosphate coating and Mg alloy in thefour simulated body fluids were: Water <Hanks <SBF <Saline. The degradation products ofmagnesium alloy and calcium phosphate coating in the SBF solution Mg（OH）2, at the sametime both will be the formation of calcium and phosphorus mineralization in sedimentarylayers.The plasma spraying ternary parameter map was built that can control the composition ofcalcium phosphate coating. According to the ternary parameter map successfully preparedcalcium phosphate coating with controlled relative content of HA/[Ca-P]. SBF degradationand electrochemical corrosion resistance results shown that the degradation rate of calciumphosphate coating and the pH were reduced in turn with the increasing of HA relativepercentage. Electrochemical corrosion resistance also gradual increased with the increasing ofHA relative percentage. when the relative percentage content of HA more than53.63%, thedegradation rate was controlled less than1.25×10^-5g/cm²h, and the pH was maintained asmall range of7.0⁸.0. It was confirmed that controlling the phase of calcium phosphatecoating could control degradation of the Mg alloy coating materials. Degradation mechanismwas analysised that the formation of the ion micro-environment, the content and thedistribution uniform of [Ca-P] in calcium phosphate coating determined together thecontrollable degradation of Mg alloy coating materials.Finally，the plasma spraying （PS）/micro-arc oxidation （MAO） and plasma spray/spincoating PHB composite process were studied preliminary to optimize the performance of calcium phosphate coating on Mg alloy. The electrochemical polarization curve result shownthat the PS-MAO and PS-PHB composite layer could both improve the corrosion resistanceof calcium phosphate coating by plasma sprayed.