Microstructure and microcrack formation at phase boundaries in Na3PO4-doped hydroxyapatite

Hydroxyapatite, Ca-10(PO4)(6)(OH)(2), doped with 0.3 to 5.0 mass% Na3PO4 as external sintering aid was studied by transmission electron microscopy. De nsification of the powder blend was performed by hot-pressing at 1200 degre esC for 30 min. The overall microstructure of the material was composed of homogenous, equiaxed hydroxyapatite grains with an average size of about 1. 5 mum. Apart from the matrix grains, a sodium-plus calcium-containing secon dary phase, beta -NaCaPO4, precipitated at triple pockets of few hundred na nometer scale. The occurrence of this precipitated phase was typically acco mpanied by microcrack formation. Microcrack opening typically occurred at o ne edge of the three or four visible phase boundaries. It should be emphasi zed that nearly all of the secondary phase pockets revealed such a microcra ck. The microcrack formation process is likely to be governed by two intera cting material characteristics the thermal expansion mismatch of the adjace nt phases and the interface bonding strength.