PHASE TRANSFORMATIONS IN PLASMA SPRAYED HYDROXYAPATITE COATINGS

Hydroxyapatite (HA: Ca 10(PO4)6(OH)2) is a bioactive material that has recently been receiving much attention mainly because of its ability to bond chemically with bone [1–3]. It is often applied clinically as a coating on an inert metallic implant such as Ti-6Al-4V. Plasma spraying is a widely used coating process to deposit HA powders onto the metallic implants. Amorphous calcium phosphate is normally produced in the coatings during the plasma spraying, not only due to the high cooling rate but also due to the relevant intrinsic properties of HA [4]. In addition, calcium phosphate phases other than crystalline HA and amorphous calcium phosphate are usually identified in the as-sprayed coatings. These phases include tri-calcium phosphate (TCP), tetra-calcium phosphate (TTCP) and/or CaO depending on the plasma conditions and the type of the HA material. A high crystallinity level is desirable in order for the materials to have good bioactive properties. Like other amorphous phases [5], the amorphous calcium phosphate in the as-sprayed coatings is thermodynamically metastable and an appropriate thermal treatment could induce a crystallization process to occur. That is why the as-sprayed coatings are usually subjected to a post heat-treating cycle. In this study, by means of differential scanning calorimetry (DSC) and X-ray diffraction (XRD), we aim to understand the phase transformations that could take place in an as-sprayed hydroxyapatite coating during a post heat-treatment process. We investigated the temperatures at which the phase transformations took place and elucidated fundamental understandings about the phase transformations (i.e. activation energy and enthalpy).