陽極氧化與水熱法於Ti-30Nb-1Fe-1Hf合金之氫氧基磷酸鈣披覆

Abstract

This study employed anodic oxidation and hydrothermal treatment in a low Young’s modulus (62GPa) Ti-30Nb-1Fe-1Hf alloy (Ti-Nb) to evaluate the bioactivity of alloy surface. The purposes of this study are threefold: firstly, the Ti-Nb was anodized in the calcium acetate monohydrate (CA) and β-glycerophosphate disodium pentahydrate (β-GP) to form a anodic oxide film (AOF) that contains Ca and P. The mechanism of the anodic oxidation of Ti-Nb was studied by evaluating the microstructures and compositions of the AOF. Secondly, after hydrothermal treatment, the phenomena of the precipitation of hydroxyapatite (HA) on the AOF were studied. In addition, the anodic behaviors and HA formation ability of AOF on Ti-Nb were compared with those of commercial pure Ti (c. p. Ti). Finally, the biocompatibility of Ti-Nb alloy treated with anodic oxidation and hydrothermal treatment was tsted. The AOF formed at two different stages, before and after dielectric breakdown. Before dielectric breakdown, a thin oxide film comprising a nearly amorphous structure with localized bubbles formed. The AOF is consisted of high-valent oxides (TiO2 and Nb2O5) in the outer layer and low-valent oxides (Ti2O3, TiO, NbO2 and NbO) in the inner layer. The non-uniform distribution of composition is due to inter-diffusion of anions and cations from electrolyte and metal. The species from the electrolyte (Ca and P) could only be incorporated in the outer part of the AOF and their contents decrease with the depth of the AOF. These electrolyte-derived species can suppress crystallization. Although sparking does not really influence the composition, it reduces the number of the hydroxyl groups on the surface. Compared to c. p. Ti, AOF on Ti-Nb has less rupture of AOF and higher dielectric breakdown voltage. It can be attributed to that adding Nb can inhibit bubbles forming inside the AOF. After dielectric breakdown, AOF grows by plasma electrolytic reaction, causing the film thickness to increase rapidly, which is accompanied with the formation of numerous craters in the AOF. Increasing anodizing potential increases the contents of Ca and P and also the Ca/P ratio, but reduces the adhesion strength between the AOF and the substrate. When anodizing to 300V, the AOF has a glassy amorphous structure. After 6 h of hydrothermal treatment at 250oC, a great number of crystalline HA precipitated on the surface of AOF anodized to 300V. The morphology and population density of HA crystals can be changed by controlling the anodizing potential and the solution pH in the hydrothermal treatment. Increasing the pH of the solution in hydrothermal treatment enhanced the precipitation of HA crystals. Numerous thin columnar HA crystals that almost covered the surface of AOF were obtained when hydrothermally treated in the pH13 solution. In contrast, the same hydrothermal treatment resulted in fewer and coarser columnar HA crystals on c. p. Ti. Accordingly, Nb in Ti-Nb alloy promotes the formation of the amorphous phases in AOF which, in turn, enhances the nucleation of HA crystals during hydrothermal treatment. Although high voltage (300V) can promote HA precipitation, it also deteriorates the adhesion property between the AOF and the substrate. A new electrolyte comprising CA, β-GP and HA powders has been developed, which allows the Ti-Nb to be anodized in this HA-containing electrolyte to at a relatively low voltage (230V). Experimental Results show that the AOF anodized in the HA-containing electrolyte exhibits an improved HA forming ability during hydrothermal treatment, it is attributing to the presence of HA powders in the electrolyte that enhances substantially the Ca content and Ca/P ratio in the AOF. On the other hand, the adhesive strength is little affected due to the decrease in the size of craters residing in the AOF. Regarding the biological responses, not much difference in biocompatibility of the treated and untreated Ti-Nb surfaces was obtained. However, the anodized and hydrothermally treated surface has been found to promote the of cell attachment.