THERMAL-EXPANSION OF HYDROXYAPATITE BETA-TRICALCIUM PHOSPHATE CERAMICS

The thermal lattice expansion of calcined hydroxyapatite and β-tricalcium phosphate single-phase powder was investigated by high-temperature X-ray diffractometry. The thermal expansion of densely sintered hydroxyapatite-β-tricalcium phosphate composite ceramics was examined by thermomechanical analysis. Hydroxyapatite-β-tricalcium phosphate mixtures with a Ca/P atomic ratio varying from 1.50 to 1.67 were precipitated from Ca(OH)2 and H3PO4 solution at room temperature. The powder, calcined in air at 800°C for 3 h, was pressed in a mould at 98 MPa. The pressed powder pellets were sintered in air at 1150°C for 1 h. High-temperature X-ray powder diffraction data were measured in the temperature range from 25 °C to 1150 °C and calibrated by a platinum internal standard technique. Thermo- mechanical analysis was carried out with a 0.08 kPa compressive load in the range from 25 °C to 1250° C with an α-Al2O3 polycrystalline standard. The hydroxyapatite and β-tricalcium phosphate are characterised by non-linear thermal expansion between 25 ° C and 600° C because of HPO42- condensation and dehydration. In the upper temperature region, the hydroxyapatite c-axial dimension expanded sharply from 1000° C to 1150° C owing to the formation of oxyapatite. The c-axial dimension of β-tricalcium phosphate shrank from 850 to 950° C which was ascribed to the formation of an intermediate phase between the β- and α-tricalcium phosphate phases. The mean (25-1000 °C) thermal expansion coefficient of the densely sintered hydroxyapatite-β-tricalcium phosphate ceramics increased almost linearly with an increase in β-tricalcium phosphate content. © 1990.