Synthesis and sintering of hydroxyapatite derived from eggshells as a calcium precursor

In the present work, phase pure hydroxyapatite (HA) was successfully prepared using calcined eggshells as a calcium precursor via the wet chemical precipitation method. The sintering of eggshell-derived HA (EHA) compacts was carried out in air over a temperature range of 800–1400 °C. The sintered HA samples were evaluated in terms of phase stability, relative density, grain size, Vickers hardness and fracture toughness. The results showed that phase pure HA was obtained and remained stable after sintering at 1250 °C. However, secondary phases such as α-TCP and TTCP were obtained at 1300–1350 °C and melted upon further sintering at 1400 °C. A relatively high density of 97.4% was achieved in pure HA at 1250 °C whilst a maximum fracture toughness of 1.14 MPa m1/2 was attained at 1000 °C due to the small grain size of 0.33 µm obtained at this temperature. The study found that a combination of relative density, the reverse Hall–Petch relationship and grain growth affected the hardness of the HA samples, where the highest value of 4.96 GPa was achieved at a sintering temperature of 1250 °C.