Calcium phosphate apatites with variable Ca/P atomic ratio II. Calcination and sintering

The calcination and natural sintering of calcium deficient hydroxyapatite powders Ca10−x(PO4)6−x(HPO4)x(OH)2−x (with 0 x 1) were studied. For temperature below 700°C, particle coalescence occurs without densification. The particle coalescence is associated with a reduction of the specific surface area. This surface decreases all the more as the Ca/P molar ratio of the powder is small. The mechanism agrees with a transfer of matter occurring by superficial diffusion, which is enhanced by the augmentation of vacancies in the apatitic structure (i.e. by a decrease of the Ca/P ratio). The sintering of compacted powders begins at 700°C. At the same temperature, the calcium deficient hydroxyapatites dissociate into biphasic mixtures of hydroxyapatite and tricalcium phosphate. The sintering is slowed down when the content of TCP in the biphasic mixture increases. In parallel, the grain size increases. This result relates to the augmentation of the coalescence of particles at low temperature.