Preparation And Properties Of Calcium Phosphate Glass-ceramic Coatings Coated Magnesium Alloy

Magnesium (Mg) and its alloys have been considered as a promising metallicmaterial for biodegradable bone implants due to their desirable mechanical properties,biocompatibilities, biodegradabilities and bone induction ability. However, the highcorrosion rate of magnesium alloys in chloride containing environment, leaded to thefast loss of mechanical integrity and the release of hydrogen, which limited theirbiomedical applications. Therefore, it is important to develop magnesium alloys withcontrollable degradabilities. Surface modification with appropriate coatings onMg–based alloys is regarded as an effective method to reduce the degradation rate ofthese Mg alloys.In this work, CaO-P2O5-Na2O-ZnO glass–ceramic coatings have beensuccessfully fabricated on AZ31magnesium alloys through sol–gel dip–coatingmethod. The influence of preparation parameters on the surface morphology andcomposition of the coating was studied by SEM, XRD and FTIR. The corrosionproperties and the apatite formation of coated AZ31magnesium alloys were alsoinvestigated by electrochemical test and immersion test. The results showed thathomogeneous and crack–free CaO-P2O5-Na2O-ZnO bioglass-ceramic coatings withthe thickness of~1.00μm were successfully fabricated on AZ31magnesium alloysunder the proper condition of sol concentration and drying. The heat treatmenttemperatures have a great influence on the composition and properties of coatings. Asthe heat treatment temperature increased from400°C to500°C, the portion of glassmatrix decreased and Ca2P2O7and Ca4P6O19phases crystallized from the glass matrix.All the coatings fabricated at different temperatures can increase the corrosionpotential and impedance of the substrate and decrease the corrosion density ofmagnesium alloy, improving the corrosion resistance of substrates. Particularly,Sample C400has the lowest residual stress, displaying the slightest cracking in theimmersion test, thus exhibited the lower corrosion rate and pH value during the7dayimmersion, suggesting the favorable protectiveness to the magnesium alloy substratesin a certain period. Moreover, it presented a better improved bioactivity than those of samples heat treated at450°C and500°C by the formation of the precipitations withhigher Ca/P atomic ratio on the coating surfaces.To further improve the protection period of coatings on magnesium alloy insimulated body fuid (SBF), a composite coating of CaO-P2O5-Na2O-ZnOglass/fluoride conversion layer was fabricated. On the one hand, the fluorideconversion layer synthesized on AZ31substrate via a pre-treatment of hydrofluoricacid was dense and has good adhesion strength with the substrate, possessing greatcorrosion protection ability. On the other hand, the formation of the middle fluorideconversion layer was also beneficial to the adhesion of the follow-up sol-gel coating.The results of electrochemical test and immersion test showed that compared with thesingle glass coating, composite coatings with the addition of fluoride conversionlayers exhibited improved corrosion resistance. After18days of immersion, thecomposite coating could still provide effective protectiveness to the magnesium alloysubstrate. Moreover, the results of bone-related proteins expression tests indicated thatthe composite coating coated samples have good bioactivity.