Thermal and chemical stability of fluorohydroxyapatite ceramics with different fluorine contents

Hydroxyapatite (HA) plays an important role in orthopedics and dentistry due to its excellent bioactivity. However, the thermal decomposition and the poor corrosion resistance in an acid environment have restricted the applications of HA. In this study, several fluorine-substituted hydroxyapatite (FHA) ceramics with the general chemical formula Ca10(PO4)6(OH)2−2xF2x, where x=0.0,0.2,0.4,0.6,0.8,1.0, were prepared. Thermogravimetric analysis in the temperature range from 25°C to 1400°C showed that the FHA ceramics with x>0.4 had remarkably improved thermal stability as compared to pure HA. X-ray diffraction of the FHA ceramics sintered at 1300°C for 1 h further confirmed the thermal stability. Dilatometer analysis showed that the fluorine addition substantially increased the onset sintering temperature of the FHA ceramics. Density measurements showed that the fluorine addition into the HA matrices slightly retarded the densification of the FHA ceramics. Corrosion testing on the polished surfaces of the FHA ceramics using a 2.5 wt% citric acid solution indicated that the FHA ceramics with x⩾0.4 had substantially improved corrosion resistance.