Hydrated precursor-assisted densification of hydroxyapatite and its composites by cold sintering

We achieved the densification of hydroxyapatite (HA) via cold sintering using CaHPO4·2H2O as an additive at temperatures lower than 200 °C and investigated the influences of CaHPO4·2H2O content and temperature on the relative density of cold-sintered HA and its composites. The elastic modulus, compressive strength and flexural strength of the cold-sintered specimens were comparable yet slightly inferior to the level of the dense HA prepared by conventional sintering at temperatures higher than 1000 °C. A possible densification mechanism was proposed for the formation of hydrogen bonds between the CaHPO4 fragments generated by the thermal decomposition of CaHPO4·2H2O and the phosphate groups in HA by water molecules, thus linking the HA particles together. This strategy of achieving dense target product from its hydrated precursors provides an important avenue for the development of cold sintering process.