Evolution of the microstructural and mechanical properties of hydroxyapatite bioceramics with varying sintering temperature

This study highlights the effects of sintering temperature on the microstructures, densification, grain sizes/boundaries, calcium/phosphorus (Ca/P) ion ratios, mechanical and bioactive properties of biocompatible hydroxyapatite (HA) ceramics prepared via cold isostatic pressing. X-ray diffraction refinement analysis revealed that the phase ratios of hexagonal HA and secondary phases were sensitive to the sintering temperature. Grain sizes, densities, and shrinkages of the as-sintered HA ceramics increased with increasing sintering temperature. The Ca/P ratios of the as-sintered HA specimens ranged from 1.63 to 1.71 for sintering temperatures of 900–1300 °C. The maximum mechanical hardness was achieved in the specimen sintered at 1200 °C due to the dense matrix formed with a smaller grain size and fewer flawed grain boundaries, as determined by high-resolution transmission electron microscopy. Ion release analysis (in a simulated body fluid solution) indicated that phosphorus ions were absorbed and rapid deposition of calcium ions occurred after immersion periods of above 4 days.