| Zinc alloy has been widely concerned as a biomedical material due to its good corrosion resistance,biocompatibility,mechanical property and degradability.However,as bone implants,zinc alloy shows the high corrosion rate that can cause it to lose their functionality prematurely during service.In addition,the resulting high zinc ions concentration of zinc alloy will also lead to poisoning risk.Consequently,fabricating a bioactive phosphate coating on the surface of the zinc alloys can solve such issues and effectively improve the bioactivity of the zinc alloy implants.Phosphate chemical conversion(PCC)method utilizes the chemical reaction between the metal material and the chemical conversion solution to prepare the chemical conversion film coating on the metal surface.The chemical conversion coating has the advantages of rapid formation,high bonding strength,convenience and economy.Therefore,the fabrication of PCC coating on zinc alloy is of great significance method for the development of surface modification of biodegradable biomaterials.In this paper,PCC coating on zinc alloy surface was fabricated by ultrasonic irradiation chemical conversion process under medium temperature condition.By adjusting the Zn2+concentration of conversion solution and PCC parameters,such as conversion time,transformation temperature,and the PH values,a procedure for preparation of PCC coatings on the surface of zinc alloy were determined.And the phase composition,microstructure,corrosion resistance,biological properties of PCC coatings on zinc alloy were researched and characterized by X-ray diffraction(XRD),field emission scanning electron microscopy(FE-SEM),electrochemical workstation,simulation of body fluid immersion and other means.The results show that the PCC coatings composed of Hopeite(HP,Zn3(PO3)2)and Scholzite(SCT,CaZn2(PO4)2·2H2O)as the main phases can be prepared on the surface of the zinc alloys by adjusting the composition of the conversion bath.After adjustment and optimization of the conversion parameters,a uniform and dense biologically active coating,with a thickness of about 2.5 μm,and micron-sized HP and SCT grains with good crystallinity,obtained on the zinc alloy substrates.Since the zinc alloys can participate in the chemical reaction during the conversion process,the coatings have a strong adhesion with the substrate.In addition,the coatings also have good wettability and corrosion resistance by optimizing their microstructure.The Zn2+ concentration of the conversion solution and the conversion process parameters have great effects on the microstructure and properties of the PCC coatings on the surface of zinc alloy.When the conversion solution without external Zn2+ was added to prepare the PCC coatings on zinc alloy,the zinc ion source could be provided by the matrix self-dissolving.The PCC coatings with compact,uniform,and fine grain size crystals was prepared as the conversion time is 10 min,the temperature is 50℃,and the pH value is 4.0.And their wettability and corrosion resistance are also good at that conditions.A long conversion time would cause excessive corrosion of the zinc alloys substrate,which could affect the crystallization process.The increase of conversion temperature would make the grain size of PCC coatings become coarser,and the increase of pH value in the conversion solution could ameliorate the crystallinity of the HP crystals of the conversion coatings on zinc alloy.Adding Zn2+source to the conversion solution was more conducive to the formation of a dense coating.When the conversion time was 5min,the conversion temperature was 25℃,and the pH of the conversion solution was 4.0,the HP coating on the surface of the zinc alloys had good crystallinity with fine and compact grain.In addition,after Ca2+was added to the conversion solution,HP and SCT PCC coatings could be obtained.In this case,when there is no external Zn2+ in the conversion solution,the conversion time was 20min,the conversion temperature was 50℃,and the pH of the conversion bath was 4.0,which could generate enough Zn2+ during the coating formation process to form HP and SCT PCC coatings.After the Zn2+source was added to the conversion solution,the microstructure and performance of the coatings could be further optimized by adjusting the ratio of Zn2+and Ca2+.With the increasing of Ca2+,the phases of SCT phase in the conversion coating increased,and the crystals of the coating became finer and denser.When the ratio of Zn2+ and Ca2+ in the conversion solution is 1.0,the prepared conversion coatings were compact and fine crystals with a grain size of about 2 μm.In addition,after immersion in the simulated body fluid,a large amount of calcium phosphate deposits appeared on the surface of PCC coatings,indicating that the obtained zinc-based phosphate coating had good potential of biological activity. |
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