| Hopeite (Zn3(PO4)2·4H2O, HP for short) has been widely used for anticorrosion of metal surface, improving paint adhesion and surface lubrication due to its good corrosion resistance and protection. It has been found that HP has good biocompatibility, Recently. And under certain condition it can be transformed into hydroxyapatite (HA) which can be used as potential biomedical material. On the other hand, titanium (Ti) has been found wide application in the aerospace, shipbuilding and other industries because of its light-weight, high-strength and good corrosion resistance. Meanwhile, Ti owns the elastic modulus similar to that of bone, good biocompatibility and easy processing characteristics, which leads to widespread application in clinical of Ti. The preparation of HP coating on Ti can enhance the protection characteristics of metal surface used in industrial and military field. In addition, the preparation of HP coating on Ti can also be used as implant materials used in medical applications. Because of the presence of a passive oxide layer at its surface, Ti is difficult to get a phosphate coating by the PCC method, which has greatly limited its application.Based on the optimization of conversion solution which can be obtained by chemical conversion on titanium surface this research aimed to utilize different means of self-made auxiliary field with the combination of field effect and chemical conversion and achieved success in the preparation of zinc phosphate on the titanium substrate surface. According to the requirements of the test, four different auxiliary field methods are designed including ultrasonic field, magnetic field, electrostatic field and the cathode electrochemical deposition. At the same time, the mechanism of the four different auxiliary field effect are discussed and also the structure and properties of conversion film formed under the influence of were characterized. In addition, the properties such as phase composition, microstructure, surface composition, morphology, radicals, surface roughness, and the corrosion resistance of the conversion coating were evidenced by X-ray diffractometer (XRD), field emission scanning electron microscope (FE-SEM), infrared spectrometer (FTIR), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectrometer (XPS), scanning probe microscope (SPM) and three electrode electrochemical workstation system by way of adjusting parameters like the PH of conversion liquid, the conversion time and temperature. And on this basis, we choose conversion coating by ultrasonic processing at60℃and make cell culture experiment of human fibroblasts to discuss the biocompatibility of zinc phosphate conversion coating as biomaterial preliminarily.The results show that the conversion coating is mainly composed of Zn3(PO4)2·4H2O. Furthermore, ultrasonic action can significantly reduce the grain size to1/5～1/10of the grain size formed without ultrasonic. Besides, ultrasonic increase the degree of crystallinity on the premise of not changing the phase composition and grain shape of the conversion coating. Besides, ultrasound can increase the nucleation rate of the conversion film. The thickness, weight and roughness of that are reduced but the density increased. And the coating has more excellent corrosion resistant performance after ultrasonic processing. The conversion coating with ultrasonic processing under250W for60min has the much lower corrosion current of0.06μA/cm2than that of the pure titanium of17.5μA/cm2. The corresponding EIS also showed high impedance values. Moreover, ultrasonic power can influence the nucleation rate of conversion coating at the start of the chemical conversion and the nucleation number increases as the power increases from50W to250W.Magnetic field can increase the nucleation of zinc phosphate, the coatings is more uniform and compact, also the particle is smaller. The conversion coating particles’ surface present a layered structure after dissolution for a long time and recrystallization process with increasing time to60min. As a consequence, the surface area of the structure and roughness increases. Also we can conclude that the longer time chemical conversion takes, the better the corrosion resistance of membrane will be. The conversion film formed under the effect of magnetic field shows more excellent corrosion resistance than that without magnetic field at the same conversion time.The composition of conversion coating in cathodic electrochemical deposition process changes with treatment temperature, when the temperature is60℃, the conversion coatings are mainly zinc phosphate, however when the temperature is30℃, the main composition of conversion coating is pure metal Zinc. There are clear differences between the two on the corrosion resistance, the corrosion current of60℃zinc phosphate conversion coating is almost100times lower than that of zinc conversion coating30℃, as well as2to3orders of magnitude higher than that of the impedance values. In addition, after aging process of iron powder, the presence of Fe2+in the conversion of liquid can change the shape and properties of conversion coatings. For the solution without iron powder aging process, the conversion coating of electrodeposition at60℃is composed of mushroom-shaped HP crystal and zinc, and the coatings formed at30℃mostly consists of a very thin layer of simple substance zinc. The conversion coating formed in solution with iron powder aging at60℃is mainly composed of dense lath zinc crystals, while at30℃for simple substance zinc with hollow vesicular structures.Moreover, in the process of electrochemical deposition, current density play a key role in the formation of conversion coating, the greater the current density, the faster the transformation. In the solution with iron powder aging, current density can change the preferred orientation growth direction of conversion coating. With the growth of the conversion coating, high and uneven distribution of the local current density results in the submicron level of platelet crystal clusters into spherical distribution on the previous coating.In the process of field-assisted chemical conversion, the diffraction intensity of the peaks along the (020) and (040) planes obviously increases which implies the preferred epitaxial growth of HP along the planes. In the early stages of the transformation, the electric field can affect the degree of spreading of conversion coating on the substrate, without electric field, the conversion coatings spread widely and show criss-cross, with a larger strip thickness. As the electric field intensity increases, plate strip conversion coating appears dispersion phenomenon, the degree of spreading and interleaving is reduced. Meanwhile the newly formed conversion coating is characterized by irregular small block particles without electric field, while the particles formed with electric field are relatively smooth and rules. |
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