Preparation And Evaluation Of Micro-arc Oxidation Coating On Titanium

Titanium has been widely used to manufacture clinical implants owing to its good biocompatibility, corrosion resistance, and mechanical processing performance. However, the interface integration between the implant and the bone is relatively weak due to the inertia of titanium. To endow the titanium implant with good bioactivity and other comprehensive performance, micro-arc oxidation (MAO) is usually adopted to modify the surface of titanium.Porous TiO₂ and apatite/TiO₂ composite coatings were prepared on the titanium surface by MAO, as discussed in this thesis. Scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transformation infrared spectroscopy (FT-IR) were employed to investigate the surface morphology and phase composition of the coatings. The hardness, roughness, and corrosion resistance of the coatings as well as the bonding force between the substrate and coating were also studied. The growing mechanism of the apatite layer on different MAO coatings was studied by immersion the MAO samples in a simulated body fluid (SBF) for the biomimetic deposition. The biocompatibility and bioactivity of the MAO coatings were also investigated by in vitro cell culture. The results are listed as follows:The surface morphology and phase composition of the MAO coating were affected by the composition of the electrolyte during MAO process. When the MAO was conducted in an electrolyte containing calcium acetate monohydrate and sodium phosphate monobasic dehydrates, the coating was porous below the voltage of 330 V. The average pore diameter and coating thickness increased with the applied voltage and oxidation time. With the increase of applied voltage, some anatase TiO₂ transformed to rutile TiO₂. Some apatite appeared on the MAO coating at 330 V. When the voltage was increased to 390V, petal-like apatite/TiO₂ composite coating formed on the pure titanium.The hardness, roughness and corrosion resistance of the coatings significantly increased after the MAO treatment. The hardness and roughness of the MAO coatings increased also with the applied voltage within current experimental scale. The bonding force between the substrate and coating was high. The bonding force increased firstly and then decreased with the applied voltage. The apatite of the composite coating as well as the anatase TiO₂ in the porous layer exhibited good apatite-inducing ability in SBF. The apatite coating was firstly dissolved into the SBF, resulting in the increase of Ca and P concentrations in the SBF. Then the Ca and P concentrations in the SBF decreased continuously due to the formation of a new apatite layer on the MAO coating. The formation of the apatite layer in SBF was suggested to a dissolution-precipitation mechanism. The electrostatic interaction was the most important factor in inducing apatite nucleation. The level of cytotoxicity was grade 0 for all the MAO samples. At earlier stage, the number of the MC3T3-E1 cells attached on MAO coatings significantly increased. After culturing for 7days, a cell layer formed on the porous TiO₂ coating because of cell proliferation. Large amount of cells were attached on the apatite/TiO₂ composite coating. Cell pseudopods exhibited a network structure. Whereas the number and dimension of the cells attached on the titanium were small. MTT tests further showed that porous TiO₂ and apatite/TiO₂ coating were beneficial to the cell multiplication and differentiation. Additionally, the apatite/TiO₂ composite coating was more obvious than the porous TiO₂ in this aspect. This indicated that the surface structure and chemical constituents had a very important influence on cell growth.