Densification behaviour and mechanisms of synthetic hydroxyapatites

Starting from Ca(OH)2 and H3PO4 hydroxyapatite powders with three different crystallinity degrees have been prepared and characterized. Densification extent and mechanisms were studied through dilatometric measurements in isothermal regime in the range of temperature 750–1250°C: the influence of different powder features (including the effect of calcination treatment) have been evaluated. Powder characterized by the lowest crystallinity degree has the highest densification extent; overlapping phenomena occurring during the sintering treatments are responsible for unexpectedly low values found for the shrinkage rate, which can easily lead to a misidentification of the rate controlling mechanism. An interpretation of the densification mechanism, consistent with all experimental findings, is proposed.