Calcium phosphate invert glass-ceramic coatings joined by self-development of compositionally gradient layers on a titanium alloy

A glass-ceramic layer containing β-Ca₃(PO₄)₂ crystals could be joined easily with a new type of titanium alloy (Ti-29Nb-13Ta-4.6Zr) consisting of a β-titanium phase by heating the metal, on which glass powders with a composition of 60CaO·30P₂O₅·7Na₂O·3TiO₂ were placed, at 800°C in air. Measurement of tensile bonding strength revealed that the joining between the coating layer and the substrate is very strong. Even after the large deformation (e.g., ≈90° in bending angle) of the titanium alloy, the coating layer was not peeled off from the substrate. A compositionally gradient layer in the TiO₂-P₂O₅-Na₂O-CaO system is developed automatically on the titanium alloy during the heating, resulting in the formation of the strong joining. By soaking in simulated body fluid at 37°C, hydroxyapatite phase was formed newly on the surface of the coating layer.