Preparation Of Bioceramic Coating On Titanium Substrate

Ti-based bioactive coating materials as high application prospect materials for medical implants are great in material science. In this thesis, in order to the preparation of bioceramic coating on the surface of pure titanium, the following research contents are dealt with. 1) micro-arc oxidation process used to form the coating on pure titanium that possesses microporous surface structure, contains high content and appropriate proportion of calcium and phosphorus elements, and mainly composes of anatase type titanium dioxide; 2) the hydrothermal process to synthesize calcium and phosphate in the ceramic coating to hydroxyapatite; 3) analysis of effect factors on micro-arc oxidation and hydrothermal synthesis process; 4) measurement on the wear resistance and corrosion resistance of the coating. Accordingly, the preparation technology of Ti-based bioactive coating material by micro-arc oxidation and hydrothermal process is basically formed.The electrolyte formula and the corresponding combination of electrical parameters suitable for micro-arc oxidation of titanium is determined tyrough experiment. The electrolyte formula is: calcium acetate 20g / L, sodium polyphosphate 9.3g / L, Sodium 12g / L, EDTA disodium salt, 2.0g / L. Besides, the corresponding electrical parameters are as follows: positive voltage 460V, negative voltage 160V, pulse frequency 800Hz. The processing time is 20 minutes, and the electrolyte temperature is between 15～30℃. Under these conditions, the prepared ceramic coating has a uniform porous distribution on the surface, and the pore diameter is 3～5μm; besides, calcium and phosphorus are relatively high in ceramic coating, moreover, the ratio of calcium and phosphorus are about 1.69, which is similar with the hydroxyapatite; in addition, the ceramic coating is mainly composed of anatase titanium dioxide and rutile titanium dioxide, and mostly of anatase titanium dioxide.The experimental results show that the concentration of electrolyte components is the main effect factor on the surface morphology, chemical composition and phase composition of the micro-arc oxidation ceramic coating. While for the determined electrolyte, the electrical parameters also have a significant effect on characters of the micro-arc oxidation ceramic coating such as surface morphology and so on. Consequently, in electrolyte, the concentration of calcium acetate and sodium polyphosphate as well as proportion of calcium and phosphorus should be limited to some range, and additionally the positive voltage, negative voltage and frequency are required to match each other. The calcium and phosphorus in the electrolyte can enter the ceramic coating through micro-arc oxidation process, and are mainly distributed on the outer surface of ceramic coating. With deionized water as media and by high-temperature hydrothermal treatment, the calcium and phosphate in the ceramic coating can synthesize to hydroxyapatite. The temperature and time of the hydrothermal process are the important factors that influence nucleation and growth of the hydroxyapatite, while temperature is the major factor in this formation. In the experimental range of temperature and time, at 150℃/2-hour the cystiform nucleation of hydroxyapatite is observed, which indicates the hydroxyapatite can nucleate under this condition; keeping the temperature for 4 hours at 175℃, more hydroxyapatite are obtained on the surface of ceramic coating. Hence, the formation of hydroxyapatite is a process of nucleation and growth of nucleation; the temperature and time are the thermodynamics and kinetics conditions affecting the nucleation and growth.The comparison of wear resistance shows that the wear resistance of titanium samples after micro-arc oxidation process increase by 3 times; the measurement of the corrosion by electrochemical corrosion method shows that the corrosion potential increases from -0.12237 to -0.11444 V, the corrosion current reduces from 2.4692E-5 to 8.8864E-6, and the polarization resistance increases from 1056.5 to 2935.6, which all reveal that the corrosion resistance of the ceramic coating are better than pure titanium.