Dissolution and mineralization of plasma-sprayed wollastonite coatings with different crystallinity

Wollastonite coatings were prepared by plasma spraying and their behaviour during heat treatment was investigated. The phase compositions and the surface morphologies of these coatings were examined by XRD and SEM. While the as-sprayed coating contained plentiful amorphous phase, the heat-treated coating showed high crystallinity. The dissolution behavior of coatings was evaluated by immersion in TRIS-buffer solution. The surface and cross-section morphologies of the coatings after immersion were observed. The results indicated that the preferential dissolution of the amorphous phase led to the disintegration of the coating. In contrast, the heat-treated coating exhibited lower solubility and remained integer. After immersion, pH changes and the concentrations of Ca and Si in the solutions were also measured. It was found that the wollastonite dissolved incongruently, whereby Ca was released at a higher rate than Si, resulting in the presence of a reactive layer enriched in silica at the coating surface. The difference in bioconductivity of wollastonite coatings with different crystallinity was investigated by immersion in simulated body fluid (SBF) solution. While apatite granules appeared on the as-sprayed coating surface after 1 day of immersion, they could not be found at the surface of the heat-treated coating.