Phase transformation on hydroxyapatite decomposition

The collapse of sintered hydroxyapatite (HA) has been attributed to HA decomposition; however, the detailed variations in microstructure are still unclear. Two phase transformation routes of HA decomposition during sintering were identified by transmission electron microscopy in this study. In the first route, HA is transformed to tetracalcium phosphate and needle-like beta-tricalcium phosphate which is subsequently converted to alpha-tricalcium phosphate (alpha-TCP) above 1100 degrees C. In the second route, HA is transformed directly to alpha-TCP and calcium oxide at 1400 degrees C, accompanied by nanopore formation. In the second route, the alpha-TCP grew with a preferred orientation to form stripe-like grains. Further holding at 1400 degrees C for 4 h resulted in recrystallization; i.e., equi-axial grains formed within a stripe-like grain. Nanopore defects dispersed in the alpha-TCP grains are the main factor for the low density and decreased mechanical strength of the sintered bulk. (C) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.