Plasma sprayed hydroxyapatite coatings on titanium substrates Part 2: optimisation of coating properties

Heat treatment and the introduction of a Ti bond coat have been applied to hydroxyapatite (HA) coatings sprayed using different plasma powers and gas mixtures. Attempts were made in this way to achieve optimal coating propert ies for orthopaedic implants. In particular, the effects on the degree of c rystallinity, the adhesion, the OH- ion content and the purity were evaluat ed. Heat treatment at 700 degrees C for Ih in air proved to be effective in increasing the crystallinity, regaining the OH- ion and removing other non -HA compounds, although it caused a significant decrease in the degree of a dhesion (interfacial fracture toughness) for those specimens sprayed at hig h powers. This heat treatment was found to induce significant transformatio n of amorphous HA to the crystalline form while not detrimentally changing the properties of the underlying Ti-6Al-4V substrates. Precoating with a 10 0 mu m Ti layer increased the adhesion of the HA coatings on Ti-6Al-4V subs trates, primarily by providing a rougher surface and promoting better mecha nical interlocking. Changes in coating properties during immersion in biolo gical fluids were also studied and were found to depend critically on the c hemical composition of the fluids. Small precipitates formed on the coating surfaces when immersed in Ringers solution. These might account for the ap parent drop in the degree of crystallinity when measured using X-ray diffra ction. A significant drop in the interfacial adhesion was found for those c oatings sprayed at high powers. This could be offset by prior precoating wi th a titanium bond coat and suitable heat treatment. In summary, the follow ing processing sequence is suggested in order to achieve optimum coating pr operties: precoating the substrate with a layer of Ti (similar to 100 mu m) , spraying HA at a sufficiently high-power level (depending on particle siz e and gas mixture) and heat treatment at 700 degrees C for 1 h in air. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.