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Preparation and Corrosion Behavior of Containing Hydroxyapatite Coatings on AZ31Magnesium Alloy

Author: TangHui
Tutor: WangFuPing
School: Harbin Institute of Technology
Course: Chemical Engineering and Technology
Keywords: AZ31magnesium alloy Hydroxyapatite Mico-arc Oxidation SealingTreatment Corrosion Behavior Biological Activity
CLC: TG178
Type: PhD thesis
Year: 2013
Downloads: 512
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Abstract


Magnesium and magnesium alloys have good biocompatibility, mechanicalproperties of matching human bone and degradation in vivo, therefore they arepotential medical implant materials. However, the degradation rate of magnesium istoo rapid at the physiological systems. Because the fast degradation rate ofmagnesium in physiological solution, the implant will lose its mechanical integritybefore the tissues sufficiently heal. As a metal material, the structure andcomposition of magnesium alloys have large difference with bones. Its biologicalactivity is poor. It is hard to form bone binding with bone tissue, and that isdelaying the time of tissue healing. Therefore how to control the degradation rate ofmagnesium alloys and improve the biological activity have become a hot researchtopic in recent years. This research based on the magnesium alloy as bone fixationmaterials and bone tissue engineering materials. Combining the biological activityof HA excellent characteristics, proposes using sol-gel method, micro-arc oxidationand liquid deposition technique to prepare hydroxyapatite coating on the surface ofAZ31magnesium alloy. In order to improve the corrosion resistance and delay thedegradation rate of magnesium in physiological solution, increase the biologicalactivity. The effect of the process parameters on the composition and properties ofthe coatings have been investigated systematically. And the coating whichpossessed the excellent performance was built. The corrosion failure mechanism,stress corrosion behavior and biological activity of the coating was study. In orderto further improve the corrosion resistance of the coating, a composition coatingwas prepared by sol-gel method or liquid deposition techniques. It was found thatthe biological activity of the coating was also improved. Prefabricated coating,emission spectroscopy and the original analysis of the performance semiconductorsolutions for micro-arc oxidation coating breakdown modes are discussed.A micro-arc oxidation coating was prepared in NaOH,CaCH2CH（OH）CH2（OH）PO4electrolyte. The composition of the coatings wereMgO and HA. Glycerol phosphate and phosphate ions moved to the anode byelectrophoresis, Calcium ions reached to the surface of the anode by diffusion andstirring. The class micelle particles also reached to the surface of the anode byelectrophoresis and diffusion effects. Complex chemistry, physics, plasma reactionwere occurred between the ions and the substrate. The micro-arc oxidation coatingwas finally formed, and the coating contained calcium, phosphorus, magnesium andoxygen. The composition, structure, surface roughness, bonding strength, wear, corrosion resistance and degradation rate of the coatings were influenced by thecomposition of electrolyte, and also influenced by the process parameters of thepower in micro-arc oxidation. The corrosion resistance, wear resistance of themagnesium alloy were improved significantly by micro-arc oxidation. Thedegradation rate of the magnesium alloy was decreased significantly by micro-arcoxidation. The optimal micro-arc oxidation treatment condition is that theelectrolyte solution is a mixture of0.1mol/L NaOH and0.03mol/LCaCH2CH（OH）CH2（OH）PO4, the applied voltage is400V, and the treatment time is40min. The average corrosion rate of hydrogen evolution of the sample is0.007ml/cm2/day when immersed in Hank’s solution for7days. And the bondingstrength between the coating and the sunstrate is61MPa.The corrosion process of the MAO coating in Hank’s solution is a diffusioncontrolled process. It include early immersion stage, medium immersion stage andlate immersion stage. The coating could hinder the corrosive medium and protectthe substrate. The corrosive medium can through the coating by diffusion, and thecorrosion product also can through the coating by diffusion. As increasing theimmersion time, the corrosion resistance of the coating is decreased. The cracks anddefects increases with increasing the immersion time. And the coating will lose theprotective function. The breakdown of gas film and solid film are taken place in themicro-arc oxidation discharge process. When the voltage of the power is low, thebreakdown of the solid film is difficult. Improving the voltage of the power, thebreakdown of the solid film become easy.It was found that the dissolution and the deposition of HA on the surface aresimultaneously exit. It is a a dynamic process when the micro-arc oxidation coatingsample immersed in the SBF solution. The concentration of calcium in SBF solutionincreased at the beginning of the incubation, that increased the surpersaturation ofcalcium ions. It was considered as the reason why HA deposited on the surface ofthe coating. The degradation rate of the substrate and the MAO coating wasaccelerated by tensile stress. The degradation rate of the substrate and the MAOcoating had little changes by compressive stress. The degradation rate of thesubstrate and the MAO coating was increased with increasing the tensile stress.However, the degradation rate of the MAO coating specimen always less than thatof the substrate. That is to say the MAO coating have protected the substratewhatever with of without the stress.The MAO coating was sealed by sol-gel vacuum deposition and liquiddeposition treatment. The fluid can get into the discharge channel exited in MAOcoating, deposit the discharge channel, plug discharge holes. That can improve the corrosion resistance of the MAO coating. It was found that the biological activity ofthe coating was also improved.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metal corrosion protection,metal surface treatment > The corrosion of various metals and alloys,corrosion and surface treatment
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