Fibrous growth of tricalcium phosphate ceramics

Structural transformation and sintering processes of tricalcium phosphate (TCP) ceramics prepared from defective hydroxyapatite (Ca₉HPO₄(PO₄)₅OH) were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). Starting powders with Ca/P ratio ~ 1.5 were obtained by adding 0.5 l of 0.3 M H₃PO₄ solution to an equal volume of 0.45 M Ca(OH)₂. In the prepared ceramics, the onset temperature for transformation of defective hydroxyapatite into TCP (witlokite) agrees with the onset temperature for sintering (800°C). Sintering occurs through the formation of a fibrous structure, which resembles biological hard tissue. In the 1000-1200°C range, these fibres coalesce into grains of up to 0.6 μm in size with a fibrous-laminar morphology. At the end of this sintering stage witlokite transforms into αTCP. At about 1450°C, partial decomposition of αTCP into Ca₂P₂O₇ + Ca₄P₂O₉ is observed. AFM observations suggest that Ca₂P₂O₇ is Segregated in the liquid state and increases the velocity of grain growth (up to 12 μm).