Study On Process, Structure And Properties Of Hydroxyapatite Coating Prepared By Hydrothermal Electrochemical Method

Hydroxyapatite coatings were formed on titanium substrate using hydrothermalelectrochemical method in an autoclave. The purpose of this work is to improve thebonding strength between the coating and the substrate and to carry on previous technicalprocess research and theory analysis for its application.Firstly, the effect of working conditions on the microstructure, the surfacemorphology, the quality of deposition and the bonding strength of the coatings werestudied. In order to improve the bonding strength, rougher surfaces were formed afteretching titanium substrate in the NH₃/H₂O₂ solution. Next, the hydroxyapatite compositecoatings were prepared by hydrothermal electrochemical codeposition of hydroxyapatiteand Ti particles or TiO2 particles or ZrO₂ particles in the electrolyte containing particles.The effect of working conditions on the content of particle in the coatings and thedeposition mechanism were studied. The effect of particles addition on the thermalstability and the bonding strength were discussed. Finally, the bioactivity andbiocompatibility of the composite coatings were evaluated.A pH microprobe technique was developed to measure in situ the pH value on theelectrode/solution interface. Various testing instrument including XRD, FTIR, SEM, EDSand DSC were employed to characterize the coatings. The mechanism of electrochemicaldeposition for the composite coating was discussed based on the experiment result andthermodynamic calculation. The morphology of hydroxyapatite crystal was discussedaccording to theoretical model of anionic coordination of polyhedron growth units. Thecorrosion resistance of the coatings were tested through anodic polarization curve. TheA.C. impedance technique was employed to study the relation among the workingconditions and the microstructure and the bonding strength of the coatings. Thebiocompatibility and bioactivity of the coatings were evaluated using the cell cultureexperiments in vitro and simulated body fluid immersion.The experiment results show that the composition of deposits is calcium deficienthydroxyapatite. The crystallinity of deposits increases continuously with the electrolytetemperature increasing and the composition becomes close to stoichiometric HA. Thedeposits prepared at 200°C are identified as stoichiometric hydroxyapatite crystal rods.The edge of the hydroxyapatite crystal has a flat hexagonal plane. The indention surfacesare formed on titanium substrate after etching in the 1:1 NH3/H2O2 solution. The processof hydrothermal electrochemical codeposition hydroxyapatite coating is consistent withGuglielmi's model, the strong adsorption is rate controlling step. The codeposition ofparticles has not significantly affected the deposition mechanism, but has hindered thegrowth of hydroxyapatite. This means that hydroxyapatite crystal has same (002)preferred orientation with smaller size. During heat treating, hydroxyapatitedecomposes by the promotion of particles and reacts with particles, which reduce thethermal stability of hydroxyapatite, but enhance the cohesive strength of the coatings. Theaddition of particles improve the bonding strength of the coatings, that is explained interms of the reduction of the mismatch of thermal expansion coefficients betweencoatings and substrates. The strengthening mechanism of the composite coatings is alsorelate to the dispersion strengthening by homogeneous distribution of particles. Thecoating surfaces are covered by carbonate apatite layer after being immersed in SBF for 7days. The cells grow well on the surface of coatings and tightly contact with the coatingssurface. The proliferation behavior of the cells in the composite coatings is the same asthat of the hydroxyapatite coatings. Though the hydroxyapatite content in the coatingsreduces due to the addition of the particles, the biocompatibility and bioactivity ofcomposite coatings have not reduced. The HA/TiO2 and HA/ZrO2 composite coatingshave the widest passivation potential range and the least passivation current indicatingwell corrosion and dissolving resistance. A.C. impedance technique can characterize therelation between the density and the bonding strength of the coatings. It is suggested thatA.C. impedance technique is a very promising research method and can be used forinvestigation of microstructure of hydroxyapatite composition.