Several sintering additives for hydroxyapatite (HA) have been tested in order to enhance its sinterability without decomposing the HA and/or decreasing bioactivity and biocompatibility, additionally providing a weak interface for HA ceramics. The ion species of sintering additives were selected from those in the mineral constituents of hard tissues and bioactive glasses. After investigation of phase diagrams in the CaO-P2O5-additive systems, and analysis of physicochemical properties of the additives, several sintering aids for HA have been chosen. Subsequently, densification, phase composition, grain growth and fracture behaviour of HA containing 5 wt% of each additive, sintered at 1000-1100°C, have been studied. H3BO3, CaCl2, KCl, KH2PO4, (KPO3)(n) and Na2Si2O5 did not enhance densification of HA. K2CO3, Na2CO3, KF and sodium phosphates improved the densification significantly. Except for KCl and some sodium phosphate, all the additives caused formation of large quantities of undesired β-tricalcium phosphate or CaO; therefore, they ark not appropriate for HA. In the case of sodium phosphate additives, it was possible to avoid formation of CaO or β-tricalcium phosphate by control of the additive quantity and chemical composition. β-NaCaPO4, has been found to be an effective sintering agent which causes neither decomposition of HA nor formation of other undesired phases.
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