Solubility, Mechanical and Biological Properties of Fluoridated Hydroxyapatite/Calcium Silicate Gradient Coatings for Orthopedic and Dental Applications

Functionally graded fluoridated hydroxyapatite/calcium silicate (FHA/CS) bioceramic coatings (FGCs) were designed and prepared using suspension plasma spraying technology in order to improve the chemical stability and bonding strength of single HA coatings. Phase compositions, microstructures, solubility, mechanical and biological properties of the FGCs were investigated. The results showed that the coatings had a continuous compositional gradient along the cross section without a distinguishable interface. The amount of CS gradually decreased, and the amount of FHA gradually increased from the substrate to the FGC surface. The bonding strength of the FGCs was improved due to the design of gradient structure and reached 29.2 MPa, which was 20% higher than that of the pure FHA coating. Dissolution behavior of the FGCs was evaluated by immersing samples in citric acid-modified PBS solution (pH = 4.0), and the solubility resistance of the FGCs was improved due to the presence of the surface FHA layer resulting in a lower Ca2+ ion and Si4+ ion release. In addition, the FGCs showed a similar apatite-forming ability and cellular response in vitro to FHA coatings, suggesting a potential competitive use for coating bone implants.