Study On Preparation And Performance Of Bioactive Phosphate Coatings On Mg Alloys

As promising biodegradable materials,magnesium?Mg?based alloys have attracted the attention of numerous researchers due to their excellent cell biocompatibility and good biomechanical compatibility.Mg-based biomaterial,as a promising candidate for degradable biomaterial,has a density similar to that of human bone.The elastic modulus of Mg alloys is much closer to that of human bone compared with conventional surgical orthopedic implant alloys such as stainless steels,cobalt-chromium alloys and titanium alloys.This can effectively reduce the occurrence of stress shielding and stimulate the injured bony tissues in the healing responses.Compared with conventional alloys for bone fixation in fracture surgery,Mg-based alloys are degradable in a biological environment and can be excreted through urine,thus avoiding a second surgery to remove the implant or device when bone healing is completed.As a human essential element,Mg promotes bone tissue healing,via binding with phosphates,promoting the mineralization process of bony tissue and forming hydroxyapatite or calcium phosphate.Despite the desirable mechanical and biological compatibility of Mg alloy as an orthopedic implant material,it is susceptible to rapid corrosion in the physiological environment and largely limits its application as implants.The rapid corrosion of Mg-based implants in the physiological system can cause accumulation of subcutaneous hydrogen gas?H₂?,the formation of subcutaneous gas cavities and separation of the interface of implants/tissue,which can directly result in surgery failure.Alkalization,which always accompanies the hydrogen evolution reaction,may cause alkaline poisoning if the p H value of the tissue microenvironment exceeds 7.8.Moreover,the rapid corrosion of Mg alloys not only results in hydrogen gas and alkalization but also rapidly decreases the mechanical strength of the implants and might untimely result in premature failure.As the common surface modification technology,phosphate conversion coating technology has been extensively studied.Because of their excellent mechanical and biological compatibility,many kinds of phosphate conversion coatings have been researched on biomedical Mg alloys,such as calcium phosphate?Ca P?,zinc phosphate?Zn P?,manganese phosphate?Mn P?,magnesium phosphate?Mg P?and so on.The main contents and results of this paper are as follows:?1?Magnesium phosphate conversion coating was prepared on the AZ31 magnesium?Mg?alloy to improve its corrosion resistance.The effect of p H value?2.5,3.0,3.5 and 4.0?and preparation temperature?40°C,60°C and 80°C?on the formation of conversion coatings was investigated in this study.The morphologies,compositions and cross-section morphologies of coated samples were analyzed by scanning electron microscope?SEM?,energy dispersive spectrometer?EDS?,X-ray photoelectron spectroscopy?XPS?and X-ray diffraction?XRD?.Electrochemical impedance spectroscopy?EIS?and potentiodynamic polarization?PDP?measurements were conducted to evaluate the effect of p H value and temperature on the short-term corrosion resistance of coated samples in Hanks'solution at 37°C and p H 7.4.Besides,a5-day immersion test was carried out to evaluate the long-term corrosion resistance of conversion coatings.During the immersion test,the volume of hydrogen evolved from each sample was recorded.The formation mechanism of magnesium phosphate conversion coating was elucidated according to the p H value and Mg²⁺ion concentrations at different regions in the conversion solution.The microstructure of conversion coating was investigated and its relationship with corrosion resistance was also discussed.Overall,it is found that the coating formed at 80°C and p H of 3.0 offers the best corrosion resistance in Hanks'solution at 37°C and p H 7.4.?2?Seven different conversion coatings?which can be classified into three types:magnesium phosphate?Mg P?type,zinc phosphate?Zn P?type and calcium phosphate?Ca P?type?were prepared on Mg alloy substrates to compare the corrosion resistance and biocompatibility of these coatings.The surface morphologies and compositions of different phosphate conversion coatings were investigated by SEM,EDS and XRD.Cytotoxicity and biocompatibility of different coated samples and bare Mg alloy were studied using the CCK-8 test.The corrosion resistance of different conversion coatings was comparatively studied by electrochemical tests?OCP,EIS and PDP?and long-term immersion test in Hanks'solution.During the long-term immersion test,bare Mg alloy,Mg P type and Ca P type coatings suffered from both filiform and pitting corrosion,while Zn P type coatings suffered mainly from pitting corrosion.Based on the experimental results,the corrosion mechanism of different types of conversion coatings and Mg alloy substrate in Hanks'solution was proposed.A“time constant shielding?TCS?”effect was proposed to explain the interaction of conductive loops and inductive loops in the Nyquist plot in EIS.?3?By virtue of its high corrosion resistance and desirable mechanical properties,Mg-based bulk metallic glass?BMG?is a promising candidate material for the biodegradable implants.To investigate the impact of microalloying element Ga on the glass-forming ability?GFA?and its effect on the mechanical properties and corrosion behaviors of Mg-Zn-Ca BMG,a series alloys of(Mg₆₆Zn₃₀Ca₄)_100-xGa_x?x=0,0.25,0.5,0.75,1.0 and 1.25?were synthesized and investigated in this study.According to the XRD results and SEM?backscattered electron?images,the critical diameter?D_c?of Mg₆₆Zn₃₀Ca₄ BMG is about 3.5 mm,and 1.0 at.%Ga addition improves the D_c to about 5 mm.Meanwhile,the 1.0 at.%Ga addition improves the fracture strength of Mg Zn Ca BMG from 651 MPa to 752 MPa.According to the results of electrochemical tests,the addition of Ga can also help to form the passive film on the metallic glass substrate and therefore improves the corrosion resistance.?4?For the seek of improving the corrosion resistance and bioactivity of Mg Zn Ca metallic glass,electrodeposition calcium phosphate?Ca P?coating was prepared on the Mg Zn Ca metallic glass ribbon using different electrodeposition parameters.The effect of electrodeposition voltage on the surface morphology and anti-corrosion performance of Ca P coating was investigated.And it is found that electrodeposition Ca P coating can effectively facilitate the precipitation of calcium phosphates,which indicating the excellent performance of mineralization.?5?By using the thermodynamically stable phase diagram of magnesium phosphates,the formation mechanism of magnesium phosphate conversion coating was discussed.By the comparison of the thermodynamically stable phase diagrams of magnesium phosphates,zinc phosphates and calcium phosphates,their formation mechanism and discrepancy were illustrated.According to the p H universal thermodynamically stable phase diagram of calcium phosphates,the mechanism of electrodeposition Ca P coating was discussed,and a new theory of Ca P coating was proposed and explored in depth.