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Change in Microstructure and Solubility Improvement of HAp Ceramics by Heat-Treatment in a Vacuum

HAp and the related calcium phosphates have been of great interest as biological materials for regenerating hard tissues, but their solubility should be improved to fit the remodeling cycle of hard tissues in vivo before their practical use for bone grafts. Control of the grain size of hydroxyapatite (HAp) matrix and change in constituent phase of the surface layer were examined to improve the dissolution rate in HAp ceramics. Surface and boundary layers composed of calcium phosphate phases with high solubility were formed by annealing at 1350 ◦ Ci n a vacuum, and the apparent mass transfer coefficient in the initial stage of the solubility experiment remarkably increased in an acetate buffer solution at pH of 4.0. The soluble surface and boundary layers are composed of tetracalcium diphosphate monoxide (TTCP), α-tricalcium phosphates (α-TCP) and amorphous calcium oxide (CaO) with high solubility, and their formation mechanism was discussed. Refinement of the grain size of HAp matrix also contributed to a slight increase in the apparent solubility.