Plasma spraying of functionally graded hydroxyapatite/Ti–6Al–4V coatings

Functionally graded hydroxyapatite (HA)/Ti–6Al–4V coatings were produced by plasma spray process using specially developed HA-coated Ti–6Al–4V composite powders as feedstock. The microstructure, density, porosity, microhardness, and Youngʹs modulus (E) were found to change progressively through the three-layered functionally graded coating that composed of the layers 50 wt.% HA/50 wt.% Ti–6Al–4V; 80 wt.% HA/20 wt.% Ti–6Al–4V, and HA. No distinct interface between adjacent layers of different compositions was evident from scanning electron microscope observation. X-ray diffractometry showed that the coatings composed of HA and Ti phases. Microhardness, as measured through the indentation technique, and tensile adhesion strength decreased correspondingly with increasing HA content in the single-layered composite coatings. The application of HA/Ti–6Al–4V composite powders improved the tensile adhesion strength of the coatings significantly. The Youngʹs modulus and fracture toughness results showed highly anisotropic elastic behavior with relatively higher E and KIC (fracture toughness) values parallel to the coating surface due to the intrinsic lamellar structure of the plasma sprayed coatings.