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.