Study Of Hydroxyapatite Coatings On Pure Titanium Substrate By Electrophoretic Deposition Method

Hydroxyapatite (HA or HAp) which widely exists in human bone and teeth has excellent biocompatibility and bioactivity. It has gained much attention of many biomaterials researchers. As the intensity and toughness of man-made HA are low, it can't be employed as load-bearing applications. In order to get a metallic biomaterial which has bioactivity, the HA is coated on the surface of Ti and its alloy used coating techniques. The composite material has good biocompatibility and bioactivity, it also meet the requirements under loading conditions. Nevertheless, the differences between the expansion coefficients of HA and Ti are bigger, the thermal stress will make the coating flake away from the substrate.Bioglass (BG) is a kind of bioactivity materials. Its expansion coefficients changes follow the changes of its component. It can be adjusted close to the expansion coefficients of Ti and its alloy. Then BG coatings can be used as transitions between HA coatings and Ti and its alloy. The combine strength can be improved as well as the bioactivity can be kept. At present, the techniques used in the preparation of bioactivity coatings include plasma spray, electrodeposition, sol-gel, ion-beam sputter, and so on. These conventional methods have got excellent achievements in the preparation of HA coatings, but there also are many defects: the coating is not very uniform, crystallinity is low, and the combining strength is low, and so on. Then finding a coating preparation method which can accurately control the coating component and crystallinity has great meanings. This paper used electrophoretic deposition method (EPD) in the preparation of HA coatings and BG/HA composite coatings. The microstructures of coatings were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), electron probe microanalysis (EPMA), Fourier transform infrared spectroscopy (FT-IR).The results show that, there are many factors on the preparation of EPD coatings, such as voltage, deposition times, aging times of solutions, and so on. When the voltage gets 200V, the coatings are very uniform and compact. After the heattreatment, the crystallinity of HA is high. The substrate surface is oxidated and a TiO₂ layer coated it. This layer maybe can reduce the thermal stress between coatings and substrates. The bonding strength can be improved through this way. When the voltage is higher, there are many big size particles deposited on the coating. This will affect the uniformity of the coating. These big size particles maybe flake away from the coating and leave holes. These defects will invalidate the coating. Although it can be improved through prolong the deposition time, the coating will be much thicker as the deposition time is longer. The dissolution of thicker coating in physiological environment is low. It will delay the combination of the substrate and human being's tissue. When the voltage is lower, the dissolution of the coating is much quick. It is also a disadvantage to the combination of the substrate and human being's tissue. In the deposition process, the deposited coating is crisp and easy to flake away. Although it can get a thicker coating in the low voltage deposition through prolong the deposition time, these coatings are not uniform, the sinter after heattreatment is not very good, the bioactivity of the coating also is not very good.The aging time of the solution also has effects on the coating. When the aging time is 1 day, there are many big particles in the solution, the deposited coating will chap and flake away from the substrate just after dried. When the aging times are 3 and 7 days, there is no chap on the deposited coatings. The sizes of the particles on the deposited coating, which is deposited in the 7 day aging solution, are very uniform, and the sinter is good.Adding BG a transition interlayer can solve the problem that the coating flake away as result of the thermal stress. The expansion coefficient of BG is similar with the Ti and its alloy. The morphologies of the composite coatings are similar with the single HA coatings. XRD patterns and FT-IR spectra show that the crystallinity of HA and BG component is very high. The results of the scratch test show that the critical load of composite coating is higher than that of single HA coating. It is shown that the bonding strength is higher than the single HA coating, and the HA coatings which is added BG transition interlayer have plastic deformation ability which is good in improving the coating mechanical property.