Influence of powder characteristics on plasma sprayed hydroxyapatite coatings

Abstract

Phase transformations, particle breakdown, and partial decomposition occur in hydroxyapatite (HA) powder feedstock during plasma spraying. The biological responses of the coatings consequently change from the bioactive nature of the starting material to a less biocompatible one. This paper investigates the influence of powder characteristics on the phase composition and microstructure of plasma sprayed HA coatings. The raw HA was prepared by chemically reacting calcium hydroxide with orthophosphoric acid. Subsequently, HA was either calcined and crushed, flame spheroidized, or spray dried. These three types of HA powders were plasma sprayed on steel substrates to form coatings. A previous study showed that the calcined HA powder suffered from particle breakdown in the plasma. The plasma sprayed HA powders contained other calcium phosphate phases (amorphous and crystalline) apart from hydroxyapatite. The flow properties and stability of spheroidized HA were better than calcined HA and spraydried HA. Standard metallographic preparation of the cross sections of the coatings revealed different microstructural features among the coatings. The HA coatings prepared from calcined HA were highly porous and lacking in intimate lamellar contact. The spheroidized HA powders produced the coating with the lowest porosity. Characterization of the powders and coatings was carried out using x-ray diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy.