Evaluation of nanostructured carbonated hydroxyapatite coatings formed by a hybrid process of plasma spraying and hydrothermal synthesis

Carbonated hydroxyapatite (CHA) coatings on a titanium alloy were prepared by hydrothermal synthesis of precursors plasma-sprayed with brushite as a raw powder. The structures, residual stresses, and bond strengths of the precursors and CHA coatings were investigated. The results showed that the sprayed precursors consisted of β-Ca₂P₂O₇, α-Ca₃(PO₄)₂, and CaHPO₄, whereas the CHA coatings exhibited a unique phase construction, nanostructured and needlelike crystals, and a fairly low tensile residual stress. The bond strength of a CHA coating 200 μm thick was 15 MPa, equivalent to that of a plasma-sprayed hydroxyapatite (HA) coating. The evaluation of the CHA coatings was performed together with that of plasma-sprayed HA coatings immersed in distilled water. The dissolution and bond-strength degradation of the CHA coatings were much lower than those of the plasma-sprayed HA coatings.