Research On Formation And Corrosion Resistance Of Hydroxyapatite Coating On Magnesium Alloys AZ91D

Magnesium alloys AZ91D have been the choice of biomaterials for implants of bone, tooth and hard tissue because of their excellent mechanical properties, bioactivity, biodegradability and bone inductivity. Unfortunately, it has a fatal drawback. That is, they have low corrosion resistance against body solution. To improve their corrosion resistance, in this paper we paint hydroxyapatite coating with good bone inductivity on magnesium alloys AZ91D by liquid phase precipitation/electrophoretic deposition, pulsed electrochemical deposition/hydrothermal treatment and micro-arc oxidation/sol-gel technologies. The micro morphology, component, structure, electrochemical properties and hydroxyapatite inductivity of coating we prepared are also studied.The hydroxyapatite coating on magnesium alloys AZ91D has been successfully prepared by liquid phase precipitation combined with electrophoretic deposition method. Under the optimum applied voltage of 40 V, the hydroxyapatite coating surface is covered by HA grains distribution homogeneously and compact with less cracks which Ca/P molar ratio is 1.583. The bonding strength is 18.28 MPa. Electrochemical impedance results show that this coating presented single capacitive arc and characterization of one time constant, higher impedance module value and negative phase angle. It can be conclude that corrosive medium was not easy to penetrate to the interface between substrate and hydroxyapatite coating, indicating smaller corrosion tendency and better corrosion resistance. The immersion test results show that this coating has induced new apatite coating with Ca/P molar ratio 1.245 in Hank’s solution for 7 d.The above coating prepared by electrophoretic deposition combined with substrate mechanically without transient phase, which resulted in poor bonding strength. To improve the bonding strength, the hydroxyapatite coating prepared by pulsed electrochemical deposition coupled with hydrothermal treatment has been studied, and the optimum applied voltage is 110 V. The hydroxyapatite coating prepared in electrolyte with NaNO3 and H2O2 addition is covered by plate and rod particles with mean size less than 100 nm, and hydroxyapatite particles preferred orientation tends to be diversified, the Ca/P molar ratio reaches to 1.687, the bonding strength increases to 25.03 MPa, corrosion current density decreases by two orders compared with coating prepared in no additives, and the corrosion resistance increases to 4.900×106 Ω, corrosion rate decreases to 1.700×10-5 mm·a’1. This coating can induce new apatite forming with plate and particles shape in Hank’s solution which Ca/P molar ratio is 1.326.Based on above research results, hydroxyapatite/chitosan/RuCl3 composite coating has prepared. The obtained hydroxyapatite grain with mean size 180 nm and 50 nm is distributed homogeneously and compact which Ca/P molar ratio is 1.678. Ru element absorption rate on composite coating reaches to 51.62%. The bonding strength increases from 27.32 MPa to 28.22 MPa. This composite coating has higher corrosion resistance, impedance module value and negative phase angle, lower corrosion current density and better ability of HA inductivity.During electrochemical deposition, magnesium alloys AZ91D participate in this reaction, the coating surface was covered by passivation coating, while this passivation has not achieve the desired effect as a transient phase. Micro-arc oxidation (MAO) process is used for modifying the surface of magnesium alloys AZ91D. Then bioactive hydroxyapatite coating is deposited on MAO coating by sol-gel technology. It can be found that the hydroxyapatite coating surface prepared by MAO treatment at 0.5 A·cm-2 current density appears to be much denser and smoother, and the hydroxyapatite particles has better crystallization, the Ca/P molar ratio is 1.711. The bonding strength reaches to 41.24 MPa. Compared with substrate, open circuit potential of this hydroxyapatite coating increased to-0.454 V, corrosion current density decreased by three orders, impedance module value at low frequency reached to 398.107 kΩ·cm2 and negative phase angle increased to 85°, indicating the MAO coupled with sol-gel treatment for AZ91D restrained the active dissolution of coating. This hydroxyapatite coating also can induce new apatite coating forming during immersed in Hank’s solution for 7 d. The above research for the development of hydroxyapatite coating provides theoretical basis.