Microstructure evolution of sol-gel HA films

Bioactive hydroxyapatite films were fabricated on titanium alloy by a sol-gel method. The samples were firstly precoated at a low drying temperature for many times and then calcined at a relative high temperature. The phase constitution, microstructure and composition analysis were studied by X-ray diffraction (XRD), electronic probe microanalysis (EPMA), and the microstructure evolution was explained by some schematic illustrations. When the samples were precoated only once, hexagonal structure is always observed. After the samples were precoated many times, the hexagonal structure can be preserved only when the drying temperature is 500 °C or above. When the drying temperature is below 500 °C, the dried layer easily redissolved into the next coated sol because of the existence of some water-soluble ions such as NO3−, consequently the hexagonal structure disappeared and a coarse surface with some small pores formed. High calcining temperature can accelerate the atomic diffusion which may integrate the adjacent grains, but excessively high temperature such as 900 °C or above would make the whole film be incinerated completely.