Hydroxyapatite Coating Prepared on Porous Titanium Substrate by Sol-gel
|Course||Condensed Matter Physics|
|Keywords||Sol-gel Method Porous Titanium Hydroxyapatite Corrosion Resistance Bioactivity|
Hydroxyapatite, the main inorganic composition of human skeleton, possesses good biocompatibility and bioactivity, and has been widely used in clinical applications. However, the use of hydroxyapatite （HA） is limited in loading and supporting application due to its brittleness, low tensile strength, and unsatisfactory mechanical properties. The composite materials of metal substrate with HA coating are considered the promising materials for bone replacement because the metal substrate can provide sufficient strength and toughness, and their surfaces also possess good biocompatibility and bioactivity. A powder metallurgical method is used for the fabrication of porous titanium substrate, and then a sol - gel technique was used for deposition of hydroxyapatite coatings throughout the porous titanium substrates. The influence of the sol pH value and sinter temperature on the microstructure and corrosion resistance of the HA coatings has been characterized by SEM, XRD and DSC/TG. The bioactivity of the porous titanium / HA composite has also been studied.The porous titanium substrates have been prepared by powder metallurgical method with titanium powder and ammonium bicarbonate （NH4HCO3） as raw materials. The result shows that the porous titanium substrates are composed ofα-phase with an open pore structure. The porosity of the titanium substrate ranges from 15.3 % to 57.0 %, and the porosity and the average pore size of the porous titanium substrate increase with the increase of the mass ratio between the ammonium bicarbonate and the titanium powder. For the case of the mass ratio being 7:3, most pore connectors are in the range of 100 um to 200um, and the porous titanium substrate is more suitable for implant materials.HA coating was prepared by sol-gel method throughout the porous titanium substrates and then heat treated at different temperatures. The results show that the formation temperature of hydroxyapatite is about 550℃, which is much lower than that of the HA coatings prepared by plasma spray method, the purity of hydroxyapatite increases with the heat treatment temperature increasing. The surface of the HA coating is dense and uneven,which is beneficial to increase the coating contact area with body fluids and improve the bioactivity of the coating.The influence of the sol pH value and the sinter temperature on the microstructure and the corrosion resistance of the HA coatings has been studied. It is found that that the pH value affects the porosity of the HA coatings. At same sintered temperature, more holes occur in the HA coating in the case of pH=0.5 than the case of pH=0.1, which can enlarge the interaction area between the HA coating and the outside environment in favor of inducing new bones formation, While sintered temperature affects the pore radius of the HA coatings. When pH value is same, the holes formed at 800℃is smaller than those at 600℃. The lower the pH value, the higher the sintered temperature, and the better corrosion resistance of the HA coating is in the hydrochloric acid.The bioactivity assessments in vitro were performed by soaking the porous titanium/HA coating specimens into the simulated body fluid （SBF）. It shows that bone-like apatite has been formed on the surface for the porous titanium/HA coating specimen soaked in SBF for a certain period of time, indicating that the HA coating has favorable bioactivity. With the extension of soaked time, the Ca/P atomic ratio of the porous titanium / HA coating surface decreases gradually, and finally maintains at around 1.40.