Study On Prepareation, Formation Mechanism And Corrosion Properties Of Surface Biological Coating And Self-nanocrystalline Layer On ? Ti Alloy

P-type titanium alloy has become a research hot topic of biomaterials because of its excellent biocompatibility, high strength and low Young's modulus. However, as a bio-inert material, the surface of Ti alloy is frequently encapsulated by fibrous tissue without producing any osseous junctions with the surrounding tissues after implantations. Moreover, the implants are easily subjected to corrosion in the physiological environment. The surface modification of (3 titanium alloys becomes the key point for developing the implant materials. Up to date, many modification methods have been developed by the researchers, but some problems still need to be solved.For example, the weak bonding between coating and matrix causes the exfoliation or cracking of the coatings. The corrosion resistance of the surface layer needs to be further improved. In the present thesis, the biological activity and corrosion properties of the surface layer produced by new methods were studied. The results will provide theoretical and experimental basis for the application of ?-type titanium alloy.In this thesis, a biomedical P-type Ti-30Nb-8Zr-0.8Fe alloy (TNZF) was selected to be experimental alloy. The porous coatings were prepared by anodic oxidation (AO) plus micro-arc oxidation (MAO), in order to increase the density and thickness of inner layer. Surface self-nanostructured layer was produced by surface mechanical attrition treatment (SMAT). XRD, OM, SEM, and TEM were used to characterize the microstructures of the surface layer. The electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves were measured to clarify the corrosion behavior of surface modification layers.The effects of MAO voltage on the structures of Ca-P containing porous coatings were studied. The results show that the average diameter of the pores and the Ca/P ratio gradually increased with the increase of MAO voltage.?-TCP phase appeared in the coatings when the voltage was higher than 350 V. The amount of a-TCP phase also increased with the voltage. When MAO voltage was higher than 450 V, the ablated coating surface was observed. The surface roughness decreased. The optimal MAO voltage was 400 V. The formation process of MAO coatings can be described as follows. Firstly, the dense passive film formed at the first stage of anodic oxidation. Then, the passive film was broken down and many small discharge channels formed. At the same time, the Ca2+?PO43-ions diffused through discharge channels, resulting in amorphous Ca-P compound. At the stage of micro-arc discharge, the extreme high energy and press power in discharge channels caused the molten oxides to spray outward, resulting in the outer porous layer. The un-molten matrix was oxidized at high temperature. The dense inner oxide film formed through the oxygen inward diffusion. The crystallized a-TCP phase formed under higher than 350 V.The electrochemical characterization of MAO coating in 0.9%NaCl solution shows that the corrosion potential moved to the positive direction and the impedance increased with increasing voltage. Hydroxyapatite (HA) started to form as coated specimen was immersed in simulated body fluid for 10h. HA covered the whole surface after the immersion of 1d, which was attributed to the induction role of a-TCP phase in MAO coating. The electrochemical impedance spectroscopy (EIS) shows that HA played a role of preventing the penetration of ions.AO film prepared before MAO was characterized of dense amorphous oxide with the thickness of about 0.5?m. The surface morphology and the composition of the AO+MAO coating were nearly the same as MAO coating. But the inner layer of AO+MAO coating were denser and thicker as compared with single MAO coating, which can be explained by the following reasons. The thicker oxide film formed at the primary stage effectively delayed the outward spraying of the molten oxide. Because of slower cooling rate the molten oxide flowed between oxide film and matrix, and finally freezed at the bottom of the discharge channels. On the other hand, oxygen diffusion occurred during the process. The denser inner layer formed near the matrix at high temperature through the oxygen inward diffusion. The inner oxide layer played an important role of increasing the corrosion resistance of the porous coating.A nanostructured surface layer was successfully prepared on TNZF alloy by surface mechanical attrition treatment (SMAT). The nanocrystallines with the size of 20-50 nm formed within the layer of 15 ?m in depth from the surface when the treating time was 10 min. The thickness of the nanocrystallines layer increased to 30 ?m at treating time of 60 min. The nanocrystalline clusters were observed. The self-nanocrystallization mechanism was analyzed as follows. Firstly, the multisystem slips under the heavy deformation increased dislocation density. High-density dislocations divide primary grains into many small domain areas. High strain energy accumulated on the interfaces among these areas drived the lattice rotation, resulting in a large number of nanocrystallines with large angle grain boundaries. The large size nanocrystallines continued to splite into the smaller size nanocrystallines owing to the sustained deformation.The electrochemical investigations of the SMATed samples in 0.9% NaCl and 0.2% NaF solutions indicate that the nanocrystallized surface behaved higher impedance and larger phase angle nearly 90° in the wide frequency region. Also, the corrosion potential increased and the corrosion current density lowered. The improvement of the corrosion resistance is attributed to the rapid formation of stable and dense passive film on the nanocrystallized surface with high activity. The pitting corrosion resistance of nanocrystallized surface significantly increased.