Plasma sprayed hydroxyapatite coatings on titanium substrates Part 2: optimisation of coating properties
Y.C. Tsui, C. Doyle, T.W. Clyne
Biomaterials
Abstract
Heat treatment and the introduction of a Ti bond coat have been applied to hydroxyapatite (HA) coatings sprayed using dierent plasma powers and gas mixtures. Attempts were made in this way to achieve optimal coating properties for orthopaedic implants. In particular, the eects on the degree of crystallinity, the adhesion, the OH ion content and the purity were evaluated. Heat treatment at 700 C for 1 h in air proved to be eective in increasing the crystallinity, regaining the OH ion and removi...Moreng other non-HA compounds, although it caused a signicant decrease in the degree of adhesion (interfacial fracture toughness) for those specimens sprayed at high powers. This heat treatment was found to induce signicant transformation of amorphous HA to the crystalline form, while not detrimentally changing the properties of the underlying Ti 6Al 4V substrates. Precoating with a 100 m Ti layer increased the adhesion of the HA coatings on Ti 6Al 4V substrates, primarily by providing a rougher surface and promoting better mechanical interlocking. Changes in coating properties during immersion in biological uids were also studied and were found to depend critically on the chemical composition of the uids. Small precipitates formed on the coating surfaces when immersed in Ringers solution. These might account for the apparent drop in the degree of crystallinity when measured using X-ray diraction. A signicant drop in the interfacial adhesion was found for those coatings sprayed at high powers. This could be oset by prior precoating with a titanium bond coat and suitable heat treatment. In summary, the following processing sequence is suggested in order to achieve optimum coating properties: precoating the substrate with a layer of Ti (&100 m), spraying HA at a suciently high-power level (depending on particle size and gas mixture) and heat treatment at 700 C for 1 h in air.
keywords:Hydroxyapatite; Adhesion; Heat treatment; Pre-coating; Biological fluid immersion
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