Plasma spraying of combustion flame spheroidized hydroxyapatite(HA) powders

摘要

Tailoring powder characteristics to suit the plasma spray process can alleviate difficulties associatedwith the preparation of hydroxyapatite (HA) coatings. Commercial HA feedstock normally exhibit anangular morphology and a wide particle size range that present difficulties in powder transport fromthe powder hopper to the plasma spray gun and in nonuniform melting of the powders in the plasmaflame. Hence, combustion flame spheroidized hydroxyapatite (SHA) was used as the feedstock forplasma spraying. Spherical particles within a narrow particle size range are found to be more effectivefor the plasma spray processes. Results show coatings generated from spheroidized HA powdershave unique surface and microstructure characteristics. Scanning electron microscope (SEM)observation of the coating surface revealed well-formed splats that spread and flatten into discconfigurations with no disintegration, reflecting adequate melting of the HA in the plasma andsubsequent deposition consistency. The surface topography is generally flat with good overlapping ofsubsequent spreading droplets. Porosity in the form of macropores is substantially reduced. Thecross-section microstructure reveals a dense coating comprised of randomly stacked lamellae. Thetensile bond strengths of the SHA coatings, phase composition, and characteristics of the coatingsgenerated with different particle sizes (125 to 75μm, 45 to 75μm, 20 to 45μm, and 5 to 20μm) showed that a high bond strength of~16 MPa can be obtained with SHA in the size range from 20to 45μm. This can be improved further by a postspray treatment by hot isostatic pressing (HIP).However, larger particle size ranges exhibited higher degrees of crystallinity and relatively higher HAcontent among the various calcium phosphate phases found in the coatings.