Investigation For The Electrophoretic Deposition Of Hydroxyapatite And Its Composite Coating On Titanium

Ca₁₀(PO₄)₆(OH)₂ (Hydroxyapatite, HA), a major inorganic component of human bone and other hard tissues, has been used extensively for biomedical implant applications and bone regeneration due to its bioactive and osteoconductive properties. However, the poor mechanical properties of pure HA ceramic, such as its brittleness, have limited its clinical applications. For this reason, a great deal of research concentrates on the development of HA coatings and/or composites. Deposition of HA coatings onto the surface of metal implants is a relatively recent development in clinical orthopaedics and has been achieved by a number of methods. Among these methods, electrophoretic deposition (EPD) has recently gained great interest due to its flexibility, low-cost and many other characteristics. Focusing on the problems of the sintering of the coating and coating-substrate interfacial bonding associated with the preparation of HA coating on titanium substrate by EPD technique, researches were done as summarized below.First of all, with the absolute alcohol based suspension, HA coatings were deposited on titanium substrate by EPD under varying conditions, such as deposition voltage, heat treatment temperature and suspension powder content. Scanning electronic microscope (SEM) and X-ray diffraction (XRD) were employed to characterize the morphologies and phase composition of the as-prepared coatings. Bonding strength between the coating and substrate was tested by shear strength testing experiment. The results show that, EPD under 30V makes suitable deposition efficiency and produces coating with good properties as well. Heat treatment under 850℃avoids the thermal decomposition of HA. The increase in the suspension powder contents is beneficial in increasing the deposition density of the HA coating, improves its sintering ability and increases its decification degree; On the other hands, the increase in density of the HA coating effectively restrains the surface oxidation of the titanium substrate during heat treatment and improved the interface between the coating and substrate; As a result, bonding strength between the HA coating and titanium substrate was significantly enhanced, from 4.54MPa when the suspension powder content is as low as 5 g/L to 19.92MPa when the suspension powder content is as high as 20 g/L.To be next, reaction bonding process was applied to the preparation of HA coating by EPD. HA/Al composite coating was deposited in the HA, Al mixed suspension. After a suitable heat treatment, HA/Al₂O₃ composite coating was formed. At the same time, monolithic HA coating was also prepared in the same conditions as comparison. SEM was employed to characterize the surface and cross-sectional morphologies of the as-prepared coatings. Chemical compositions of the composite coating were analyzed through EDS. Phase composition and thermal stability of the composite coating were characterized by XRD and FTIR. Bonding strengths between the coating and substrate were tested by shear strength testing experiment. Preliminary characterization of the biological property was also done by MTT test. The results show that the composite coating can be sintered with no decomposition at 850℃heat treatment. The reaction bonding process enhances the coating's densification degree and lowers the substrate's oxidation degree during the heat treatment. In comparison with the HA monolithic coating (19.92 MPa in average), the HA/Al₂O₃ composite coating exhibits much higher bonding strength (30.83 MPa in average). Results of MTT test show that, although the biocompatibility of HA/Al₂O₃ composite coating is relatively lower than the monolithic HA coating; preparation of these two kinds coatings both improved the biocompatibility of the titanium substrate.At last, with a stepwise deposition mode, HA/Al₂O₃ functionally gradient coating (FGC) was sequentially deposited in suspensions with different HA and Al content followed by a suitable heat treatment. XRD, SEM and EDS were employed to characterize the phase composition, morphology, microstructure and chemical composition of the FGC. Bonding strengths between the FGC and substrate were tested by shear strength testing experiment and compared with the monolithic HA coating. Results show that, the 850℃sintered FGC was composed of highly crystallized HA andγ-Al₂O₃ with no decomposition was found. Chemical composition in the FGC exhibits obvious gradually variation. The composition of the FGC gradually changes from the oxide layer, interface diffusion layer, HA/Al₂O₃ gradient layer to the top pure HA layer. The inner layer the HA/Al₂O₃ gradient coating, which contains the highest content of element Al, is the densest. From the inner to the outer layer, content of element Al gradually decreases while the porosity increases. The top layer of the FGC is a porous pure HA coating. The average bonding strength of the as-prepared FGC was about 23.54MPa, much higher than that of monlithic HA coating with the same thickness.