Structural Modification And Kinetics For The Apatite Formation Of The Microarc Oxidized Biocoatings On Ti6Al4V

TiO₂-based coatings containing P (PW), as well as containing Ca and P (CPW), were prepared on the surface of Ti6Al4V by microarc oxidation (MAO). Alkali- and heat-treatment was proposed to modify the surfaces of the MAO coatings for improving their apatite-forming ability. Biomimetic apatite was obtained by immersing the modified MAO coatings in a simulated body fluid (SBF). The surface structures of the non- and modified MAO coatings before and after SBF immersion were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), auger electron spectroscopy (AES), Fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM) etc.. The formation mechanism of alkali- and heat-treated MAO coatings and their inducation mechanism for biomimetic apatite formation were disscused thoroughly. In addition, the preliminary investigations regarding to the MG63 cell proliferations on the surfaces of the non- and modified MAO coatings were conducted in this work.An electrolyte containing (NaPO₃)6 and NaOH was used to prepare the PW coatings. And the CPW coatings were prepared in an electrolyte containing Ca(CH₃COO)₂·H₂O, Ca(H₂PO₄)₂·H₂O, EDTA-2Na and NaOH. Under certain applied voltage, the initial passivating film on the surface of Ti6Al4V was broken, and the surface of Ti₆Al₄V was oxidized. Under the applied electric field, the negatively charged PO₄^3-, CaY^2- and OH^- ions etc. could be injected into the MAO coatings. Under the setting voltage, the formation of the MAO coatings shows two stages of quick and slow growth. The MAO coatings formed at 300V show uniform surfaces with porous structure. With increasing the applied voltage, the rutile was formed in the MAO coatings, and the surfaces of the MAO coatings became rougher. At the same time, the thickness of the MAO coatings and the size and number of the MAO pores increased. Moreover, the contents of the Ca and P and the Ca/P ratio in the CPW coatings also increased. In the MAO coatings Ti, O, Ca and P atoms show a graded distribution. In addition, the PW and CPW coatings both show good adhesion with the Ti6Al4V substrate.During the alkali treatment process, the Ca, P and Al of the PW and CPW coatings were dissolved into the alkali solution. TiO₂ phase of the PW and CPW coating was attacked by OH^- ions to form HTiO₃^- ions. In the case of alkali-treated PW coating (PA), the HTiO₃^- ions could absorb Na⁺ ions from the alkali solution to form sodium titanate hydrates. And a network structure was formed on the surface of the PA coating by the interlacement of the sodium titanate hydrates. After heat-treatment of the PA coating at 400⁷00℃, the surface morphology of the PA was changed and the sodium titanate hydrates were dehydrated and then crystallized forming Na2Ti9O19 phase. In the case of alkali-treated CPW coating (CPA), the negatively charged HTiO₃- ions could be combined with released Ca2+ ions in alkali solution, forming calcium titanate hydrates with flake-like morphology. And the surface of the CPA coating shows a honeycombed structure. Increasing the alkali concentration facilitates the formation of calcium titanate hydrates. After heat-treatment of the CPA coating at 400⁸00℃, the surface morphology of the CPA was altered and the calcium titanate hydrates was dehydrated and then crystallized forming CaTi21O₃8 and CaTiO₃ phases.The CPW coating shows higher apatite-forming ability compared with the PW coating due to the introducation of Ca element. In the case of the CPW coatings formed at low applied voltage (200 and 250V), the contents of the Ca and P released from these coatings were little and then the SBF supersaturation decreased relatively, which could futher lower the apatite-forming ability of these coatings. High applied voltages (350⁴50V) would lead to the formation of rutile in the MAO coatings, which does not facilitate the apatite formation. Thus the CPW coating formed at moderate applied voltage of 300V shows good apatite-forming ability. After only heat-treatment of the CPW coatings at 400⁸00℃, the amorphous calcium phosphate was crystallized to form Ca₃(PO₄)₂ phase. Moreover, the ability to release Ca and P of the CPW coating decreased, resulting in a low apatite-forming ability.After alkali-treatment, the apatite-forming ability of the PW and CPW coatings were improved. The reseaon for this is that abundant Ti-OH groups were formed on the surfaces of the PA and CPA coatings during the SBF immersion process via an ionic exchange between Ca²⁺ and Na⁺ ions of the titanates and H3O+ ions of the SBF. The subsequent heat-treatment of the PA and CPA coatings decreased their ability to release Ca2+ and Na+ ions, thus decreasing the formation ability of Ti-OH group, which will further result in a lower apatite-forming ability. However the CPA coating after heat-treatment at 800oC shows good apatite-forming ability due to the formation of perovskite CaTiO₃ phase. The perovskite CaTiO₃ ( 0 22) plane has good crystallographic matching with HA (0001) plane probably providing good sites for apatite nucleation by the epitaxial deposition process, probably forming a small contacting angle between CaTiO₃ and apatite. Biomimetic apatite induced by all the coatings show important characters including: 1) containing CO₃^2-, HPO₄^2- and Mg²⁺ ions etc., 2) pore networks on the nanometer scale and 3) controllable crystallinity. And the apatite is oriented to the (0001) crystal plane. The Ca₁₀(PO₄)₆(CO₃)_0.5(OH) and Ca₉(HPO₄)_0.5(PO₄)₅(CO₃)_0.5(OH) were formed easily relatively according to the analysis of thermodynamics and kinetics.The roughness and wetting ability of the MAO coatings were improved by alkali-treatment, alkali- and heat-treatment and heat-treatment. The PW, CPW, PA and CPA coatings can provide surfaces suitable for the proliferation of the MG63 cells.In short, the applied voltage and electrolyte composition have a significant effect on the structures of the MAO coatings. The alkali- and heat-treatment of the MAO coatings result in special surface structures, facilitating the apatite formation. In this work, apatite formation and growth were mainly affected by the heterogeneous nucleation energy and the surface morphology of the coatings. The SBF supersaturation and the contacting angle between apatite nucleus and substrate have effects on the heterogeneous nucleation energy. In this work, the main factors to affect the SBF supersaturation could involove: 1) the absorption of Ti-O band to Ca2+ ions and phosphate radicals etc., 2) the formation of Ti-OH groups and 3) the contents of Ca and P released from the coating surfaces. In addition, the crystallographic matching relation of apatite and substrate could affect the contacting angle. Furthermore, the coatings with moderate roughness and wetting ability facilitate the cell proliferation.