Thermal phase stability and properties of hydroxyapatite derived from bio-waste eggshells

In the present work, hydroxyapatite (HA) derived from calcined eggshell via a wet chemical precipitation method was investigated. The as-received eggshells was calcined at 900 degrees C to produce crystalline calcium oxide which reacted with orthophosphoric acid under controlled conditions. The final solution was filtered, dried and sieved to obtain ready to press, phase pure hydroxyapatite powder as confirmed by X-ray diffraction (XRD) analysis. Green samples were prepared and sintered in air at various temperatures ranging from 900 degrees C to 1300 degrees C at a ramp rate of 10 degrees C per minute and 1 hr holding time. The sintered samples were evaluated in terms of HA phase stability, relative density, grain size, Vickers hardness and fracture toughness. The results showed that the HA phase remained stable even after sintering at 1300 degrees C. In addition, the relative density, Vickers hardness and fracture toughness were found to increase rapidly between 900 to 1100 degrees C and reached a plateau thereafter. A relatively density of 98.5%, high Vickers hardness of 5.9 GPa and fracture toughness of 1.09 MPam(1/2) was achieved at 1250 degrees C despite having a large grain size of 2.5 mu m at this temperature.