Adhesion properties of plasma sprayed hydroxylapatite coatings for orthopaedic prostheses

Using Air Plasma Spraying (APS) and Vacuum Plasma Spraying (VPS) techniques, hydroxylapatite (HA) and mixtures of HA and titanium (Ti) were deposited on a Ti6A14V alloy (and on an AISI 316L steel) subjected to different surface treatments. The deposits were investigated for their crystallinity, thickness, and adhesion properties. Higher adhesion values were obtained with VPS rather than with APS. By utilising VPS, the deposition conditions were selected in order to achieve crystallinity values between 70 and 90%. The adhesion results depend on the crystallinity (increasing with its decrease), on the thickness (decreasing slightly with its increase) and especially on the surface finish of the metallic substrate. A porous Ti precoat was more effective than either chemical etching in HCl or sandblasting; sandblasting being the least effective. In particular, the double deposits consisting of a porous Ti precoat and a successive layer of HA proved to be most interesting for their higher adhesion properties and for their capability of providing primary stability due to the presence of the HA and secondary stability, in the case of its reabsorption, due to the porous metal.