Sol-Gel Derived Hydroxyapatite/Titania Bioactive Coatings On Titanium Foam Substrate

Pure titanium and some of its alloys have long been used as implant materials in dental and orthopaedic applications due to their excellent mechanical properties and good corrosion resistance. However, solid metal implant materials suffer from the problem of biomechanical mismatch of the elastic moduli, because their moduli are higher than those of natural bones. Recently, there has been an increasing interest in fabricating porous scaffolds that mimic the architecture of human bone, and simultaneously exhibit moduli very close to those of the bone. In the present investigation, a powder metallurgical method (PM) was used for the fabrication of titanium foams. The elastic modulus and the porous structure of the titanium foams were tailored to be very close to those of natural bone. Moreover, a simple sol-gel technique was developed for the hydroxyapatite/titania (HA/TiO₂) coatings throughout the Ti foams successfully. Titanium foams after this HA/TiO₂ coating displays excellent bioactivity.Starting materials are commercially available Ti powders (particle size ≈ 10 μm) and ammonium hydrogen carbonate powders (NH₄HCO₃). At first, two kinds of powders were thoroughly mixed. After the ingredients homogeneously blended, the mixture was uniaxially pressed at a pressure of 80 MPa, followed by a two-step heat treatment, i.e. at 180℃ for 30 min and 1200 ℃ for 120 min, respectively.The scanning electron micrographs (SEM) of the open cell Ti foams show that the pore size is in the range of 30 - 250μm, and the porosities are between 7.3 - 35.4 %. The microstructure of the titanium foam matrix shows an alpha (α) phase with a grain size of 2-10 μm. The modulus of the Ti foam can be calculated from stress-strain curves. Results indicate that the elastic moduli of the Ti foams ranged from 9.92 to 7.20 GPa, which are much lower than that of the solid Ti-6Al-4V and any other Ti alloys. SEM observations on the fracture surface of the Ti foams revealed that it was a brittle fracture. Optical microstructure observations of the fracture surface indicated that the fracture travelled through the grain of the material, showing a transgranular fracture.A simple sol-gel method was successfully developed for a hydroxyapatite/titania (HA/TiO₂) double layer deposition on titanium foams. The sol-gel process started with...