Fabrication and characterization of thin nano-hydroxyapatite coatings on titanium

Titanium is the most commonly used metallic material in the manufacture of orthopedic implants, and hydroxyapatite (HA) is bioactive and biocompatible when used as bone substitutes. To achieve better biocompatibility and excellent mechanical performance of prostheses, HA coating is often fabricated on titanium surfaces. The present study used dip-coating techniques to fabricate HA coating of organic sol–gel of Ca(NO3)2·4H2O and PO(CH3)3 and inorganic sol of Ca(NO3)2·4H2O and (NH4)2·HPO4. Scanning electron microscopy (SEM) and grazing-incidence X-ray diffraction (XRD) have been used to characterize the morphology and the distributions of crystallite size and micro-strains of the coatings. After firing at 400 °C, the apatite structure of coatings on titanium began to appear. Warren–Averbach Fourier transfer analysis of the diffuse reflections indicated that the mean crystallite size increased and micro-strain decreased significantly with the rise in firing temperature. In the range of 400–600 °C, the effect of firing temperatures on mean crystallite size and micro-strain of both coatings was obvious. Precursor types of HA coating significantly affected the aggregating size of particles of nano-HA coatings, which were 25–40 nm for organic sol–gel and approximately 100 nm for inorganic sol. The morphology of interfaces between coating and titanium indicated thin nano-HA coatings with thickness 2 μm for organic sol–gel and 5 μm for inorganic sol were intact and homogenous. The effect of firing temperatures on aggregating size of particles of both coatings was not obvious.