Relationships among behaviour of in-flight particles, microstructure of splats and phases of coatings during plasma and HVOF spraying of hydroxyapatite

Previous in vitro and in vivo tests have revealed that a biological performance of thermal sprayed hydroxyapatite (HA) coatings was significantly related to their phase composition. It is clear that a predominant presence of HA, hence less HA decomposition, in the coatings is essentially required. In the present study, the relationship among in-flight HA particles, microstructure of HA splats, and the phases of resultant coatings was studied aiming to reveal the key factors responsible for the final microstructure of the coatings. High velocity oxy-fuel (HVOF) and direct current (dc) plasma spray techniques were utilised for the spraying of HA. Temperature and velocity of the in-flight HA particles were detected using a Spray Watch system. The microstructure of HA splats and coatings was characterised using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The microstructural features of individual HA splats were revealed through TEM observing both as-sprayed and ion-milled splats. The phases present in the HA coatings were characterised through X-ray diffraction (XRD). Results showed that plasma spraying brought about a full-melt state of HA particles, whilst HVOF achieved a partial molten state of the powders. It was found that the HA powders attained a temperature up to 2300°C during the plasma spraying, and the temperature is significantly influenced by spray parameters. For the plasma sprayed HA splats, amorphous calcium phosphate (ACP) and tricalcium phosphate (TCP) phases were observed at their fringes, which indicate significant phase transformation of HA at those locations. A nanostructure (∼30 nm grains) within the surrounding parts of the HA splats was revealed, while calcium phosphate grains with a size up to 5 mm depending on flattening state are present at the centre of the splats. Within the HVOF deposited HA splats, however, less decomposition of HA was indicated. Further phase analysis on the HA coatings suggested accumulated contribution of individual splats to the overall phases of the resultant coating. It revealed that overheating of HA powders is responsible mainly for the remarkable phase transformation of the sprayed materials. The microstructure of both the plasma sprayed and HVOF sprayed HA coatings was also characterised.