Process-phase-properties relationship in radio frequency (RF) plasma synthesized hydroxyapatite (HA)

Radio frequency suspension plasma spraying (RFSPS) was used to synthesize ultra-fine (20 nm-2 μm) hydroxyapatite (HA)-Ca₁₀(PO₄)₆(OH)₂ and ZrO₂-HA-Calcium Phosphate (CaP) nano-composite powders. The ultra-fine powders were consolidated using the spark plasma sintering (SPS) process and in-vitro investigation of the sintered compacts revealed bioactive behavior inherent in the original powder. The influence of various plasma processing parameters and HA suspension concentrations on the characteristics of the as-sprayed powders were studied. The powders were characterized using particle size measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). It was revealed from XRD and TEM analysis that a significant amount of amorphous material contributed to the overall phase composition of plasma processed HA. Consequently, quantitative XRD analysis was performed with the Rietveld technique, employing alumina (Al₂O₃) as the internal standard. As-sprayed powders produced with miscellaneous plate power levels and suspension concentrations, were analyzed. Rietveld analysis also determined structural changes to the HA lattice after the spray process. The compacts consolidated through SPS from ultra-fine HA powders exhibited Young's modulus and microhardness of approximately 103 GPa and 550 Hv, respectively, and fracture toughness ∼1.2 MPa m^1/2. In-vitro results indicated that the compacts were bioactive revealing a precipitated layer of apatite one week following immersion.