| Magnesium (Mg) and its alloys have been considered as highly potential metallicmaterials for biodegradable bone implants due to their superior mechanical propertiesthat are similar to natural bone, and the biocompatibility and biodegradation in thephysiological environment. However, the rapid degradation of Mg alloy implants inchloride containing physiological environment, especially at the initial phase, resultsin a fast reduction of mechanical support for the broken bone before the damagedbone healing. In addition, the rapid corrosion leads to the excess of Mg2+ions, H2release and abrupt pH value increase, all of which may damage the neighboringtissues. Therefore, it is vital to slow down the degradation rate of Mg alloy to tailorthe tissues’ growth rate.In this work, CaO-P2O5-SrO-Na2O (CaP) bioactive glass-ceramic coatings havebeen successfully fabricated on AZ31Mg alloys via sol-gel dip-coating method. Andpolyethylene glycol (PEG) was added to the sol to promote the crystallization of thecoatings (m-CaP). The thermal behaviors of the gels were analyzed by DSC/TG. Inaddition, the influences of sol concentration, heat-treatment temperature and PEGaddition on the microstructure and composition of the coatings were investigatedthrough SEM, XRD and FTIR. Surface wettability, electrochemical and immersiontests were also conducted to study the corrosion resistance of the coatings.The results showed that homogeneous and crack-free CaP bioactive glass-ceramiccoatings with the thickness of1.4μm were successfully fabricated on AZ31Mg alloyby adjusting the sol concentration to0.5mol/L and the heat-treatment temperatures to400~500°C. CaP coatings prepared at400°C mainly consist of glass withmetaphosphate chain structure and a small amount of Ca2P2O7crystalline phases. Asthe temperature increased from400to450°C, besides main phase Ca2P2O7,β-Ca(PO3)2and Ca4P6O19were identified as minor crystal phases in the glass-ceramic.No new phase was detected with the temperature increasing to500°C except for thefurther crystallization. The coatings improved the surface wettability of Mg alloys.However, due to the increased crystallization, the surface roughness were promoted,which led to the decrease of water contact angles, and subsequently better contactwith the corrosive media, finally resulted in the inferior corrosion resistance. Thecoatings heat-treated at400°C contained more amorphous phase that benefited therelease of residual stress in the coatings. Thus, the coatings with more amorphous phase would not crack easily during the electrochemical and immersion test,providing effective protection for Mg alloys. On the other hand, coatings that consistof more crystalline phases had larger in-plane residual stress, which would lead tocrack and thus decreased corrosion resistance.The glass transition temperature of CaP system decreased to~226°C due to theaddition of PEG, which promoted crystallization at a relatively low temperature.Meanwhile, the addition of PEG that can disperse the sol improved the homogeneityof the coatings, decreased the residual stress and ensured the surface integrity. As thedegradation rate of amorphous glass is much larger than that of the crystalline phases,the coatings that contained larger amount of crystalline phases would provide betterprotection on the basis of the surface integrity. Moreover, protection effect of PEGassisted synthesis of calcium phosphate coating gradually declined after the initialsoaking time, making it to be a potential biodegradable material. |
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