Thermal spraying of hydroxyapatite (HA) coatings: Effects of powder feedstock

Hydroxyapatite(HA) has biocompatible properties that can promote osteogenesis or new bone formation. Thermal spray processes like plasma spraying have been found to be effective in coating the hydroxyapatite onto metal. However, plasma spraying changes the degree of crystallinity as well as the phase composition of HA. This paper reports the preparation and characterization of HA powders and coating by plasma spray process and suggests that the state of the starting powder adversely affects the coating characteristics. The raw HA powders are synthesized through a chemical reaction involving calcium hydroxide and orthophosphoric acid. Phase analysis by a X-ray diffractometer revealed that the synthesized powder consists of predominantly HA phase. Calcined and crushed HA powders of various size ranges were fed into a plasma jet to produce HA coatings on metallic substrates. In addition, some HA powders were sprayed into distilled water by plasma spraying and combustion flame spraying to study powder melting characteristics. The morphology of the rapidly solidified powders and thermal sprayed coatings were examined by scanning electron microscope (SEM). The X-ray sedimentation particle size analyzer and image analyzer performed the particle size analysis. The results indicate that particle cohesion, size range and thermal treatment during thermal spray processing affect the phase and structure of the as-sprayed coatings and post-spray treatments may be accessary to produce a dense and adherent coating having the desired biocompatible properties.