| Titanium and titanium alloy have the excellent properties of high strength, biocompatibility and corrosion resistance. So far, they have become the best substituting materials of hard organizations, such as bone and tooth. However, because of their poor binding capacity with bone and wear resistance, it has aroused people's attention day by day that the surface modification of titanium alloys is carried out in order to improve their biological properties. Micro-arc oxidation of titanium and titanium alloy can synthesize a porous oxide layer on their surfaces, which bonds strongly with the substrate and has ceramic properties. Compared with the substrate, the wear resistance and corrosion resistance of this oxide layer have been obviously improved. Bioceramic coating by micro-arc oxidation regards the coating containing Ca and P as a typical representative, especially, the hydroxyapatite coating has more excellent biocompatibility. Therefore, the synthesis of bioceramic coating on titanium alloy by micro-arc oxidation has theoretical significance and practical value.In this paper, the bioceramic coatings were prepared using micro-arc oxidation method with two different ways, i.e., the bioceramic coating containing Ca and P was synthesized by changing the order of micro-arc oxidation, and the bioceramic coating containing hydroxyapatite was directly synthesized by micro-arc oxidation. The following aspects were systematically studied:First of all, it was found that the order of micro-arc oxidation can influence the surface morphologies of bioceramic coating on titanium alloy for the first time. The bioceramic coating containing Ca and P element were obtained by changing the order of micro-arc oxidation for the first time. The coating containingα-Ca(PO3)2 and CaTiO3 phase which possess better biocompatibility were successfully synthesized. The influence of the order of micro-arc oxidation on bioceramic coating on titanium alloy was studied. The influences of the parameters such as the order of micro-arc oxidation, the composition of electrolyte, time, and current density on the morphology, phase structure, and elemental composition of the coating surface were synthetically analysized. The synthesis mechanism that the bioceramic coating on titanium alloy was prepared by changing the order of micro-arc oxidation was studied. The results showed:1. In 4 groups of different electrolytic solutions of C4H6O4Ca, Ca(NO3)2, (NaPO3)6, Na3PO4, 8 groups of micro-arc oxidation coating with different morphologies were obtained by changing the order of micro-arc oxidation. Through analyzing the uniformity degree, pore size and pore connectivity of the coating, it can be concluded that the coating surface quality on titanium alloy prepared by the sequence of sodium hexametaphosphate-calcium acetate was better than other coatings.2. 8 groups of Ca(NO3)2-(NaPO3)6, (NaPO3)6-Ca(NO3)2, Ca(NO3)2-Na3PO4, Na3PO4-Ca(NO3)2, C4H6O4Ca-(NaPO3)6, (NaPO3)6-C4H6O4Ca, C4H6O4Ca-Na3PO4, and Na3PO4-C4H6O4Ca of coatings contained Ca and P element, and contained electrolyte elements (such as W, Al and Ti, etc) and matrix elements at the same time. But the difference between every element contents was relatively large. The coating contained mainly TiO2 phase. Among them, the coating prepared by the sequence of sodium hexametaphosphate-calcium acetate containedα-Ca(PO3)2 and CaTiO3 phases, and their biological activities were better.3. With the increase of the micro-arc oxidation time and current density, the microporous quantity of the coating surface became more, and then became less again; the pore size changed from small to big, and the porosity became great. P content in the coating reduced gradually, Ca content increased gradually, and Ca/P increased. The largest thickness of the coating reached 70-80μm in this study.4. The microhardness of the coating containing Ca and P was relatively large, and its biggest value reached 800HV200g, which was well more than the microhardness (200-300HV200g) of matrix. With the increase of the current density, the microhardness of the coating increased constantly, and then reduced gradually.5. The wear resistance of the coating containing Ca and P by hexametaphosphate-calcium acetate technics was better than that of matrix alloy. With the increase of the current density, the wear resistance of the coating was strengthened gradually.6. The corrosion resistance of the coating containing Ca and P in the simulated body fluid and simulated saliva was superior to that of matrix. With the increase of the current density, the corrosion potential of the coating increased gradually, and the corrosion current of the coating reduced firstly, and then increased. The complete dipping test revealed that the corrosion rate of the coating was wholly lower than that of titanium alloy.Secondly, the new breakthrough on the technology utilizing micro-arc oxidation to directly synthesize bioceramic coating containing hydroxyapatite was obtained. The stabilization of pH value of micro-arc oxidation electrolyte was successfully achived utilizing additive. the technique for directly preparing HA utilizing micro-arc oxidation was developed, and the influences of the adding sequence of solution on the bioceramic coating was also discussed. Through studying the influences of electrolyte composition, pH value of the solution, additives, electric parameters, and oxidation time on the coating surface morphology, phase structure and elemental composition, the mechanism utilizing micro-arc oxidation to directly synthesize bioceramic coating containing hydroxyapatite was revealed. The friction and wear properties of the coating containing hydroxyapatite were studied. The results showed that:1. In the course of the micro-arc oxidation, the pH value, additive, electric parameters, oxidation time of the solution have important influence on the synthesis of hydroxyapatite. The pH value and additive of the solution determined the stability of the electrolyte; the electric parameters and oxidation time determined the ratio of Ca and P of the coating. Only when the ratio of Ca and P reached 2, is there the emergence of hydroxyapatite, and simultaneously Ca3(PO4)2 generated. With the increases of the oxidation time and current density, Ca3(PO4)2 became gradually amorphous phase.2. The friction coefficient of coating containing hydroxyapatite was first increased and then decreased with friction time; and the friction coefficient was first decreased and then increased when electricity density.Finally, the biological properties of the biological coatings prepared using two different methods were studied. The acute toxicity test, hemolysis experiment, oral mucosa stimulation experiment, cell toxicity test, and coating dissolution experiment were carried out, respectively. The results showed:1. The bioceramic coatings prepared by the two different methods had no acute toxicity reaction, the hemolysis rate was all less than one, the cell toxicity was all 0, and the coatings had no stimulating effects. From the point of view of the biocompatibility, the bioceramic coatings prepared by the two different methods were ideal biological materials.2. When the coating containing Ca and P prepared by hexametaphosphate -calcium acetate technics and the coating containing HA were dissolved in distilled water, The overall changing trend of Ca2+ concentration in the distilled water was first rising and then descending. When the dissolved time in the distilled water was beyond 110 hours, the Ca2+ concentration of the coating containing HA was obviously higher than that containing Ca and P. The coating containing Ca and P in distilled water grew mainly in sheet materials, while the coating containing hydroxyapatite grew mainly in granular materials.
|
PDF Full Text Request