Dissolution and mineralization behaviors of HA coatings.

The dissolution and mineralization behavior of HA coatings are two of the main factors governing the bioactivity of coatings. After different post treatment operators, the plasma-sprayed HA coatings have different characteristics, including different chemical composition, crystallinity, crystallite size and dissolution behavior. In this study, HA coatings were characterized by X-ray diffraction, scanning electron microscope, and Fourier transform infrared spectra before and after immersion in simulated body fluid (SBF). When immersed in SBF, both dissolution and precipitation occurred at the same time, but the kinetics of dissolution was quite different from that of precipitation. The former was dominated by ion exchange, while the latter was controlled by the ion concentration product and the solubility of the particles. Therefore, the dissolution behaviors of phosphate ions partly depended on the dissolution behaviors of calcium ions. With the increase of ions concentration in solution by dissolution, more nucleation sites appeared on the surface of coatings. Crystalline grains gradually grew up on the nucleation sites and developed into biomineral layers. The biomineral layers were the results of the precipitation of the ions in the solution; and the carbonates partially substituted phosphates to form bone-like apatite. The different dissolution characters resulted in quite different morphology of the biomineral layers: the coatings with low solubility induced biomineral layers of large grains; on the contrary, the biomineral layers of network structure were observed on the more soluble coatings.