Sintering behavior and thermal stability of Na+, SiO44− and CO32− co-substituted hydroxyapatites

Sinterability and thermal stability of nanocrystalline hydroxyapatite powders with co-substituted sodium, silicate and carbonate ions have been investigated. The effect of powder characteristics on the thermal stability and physico-chemical properties of the produced ceramic bodies have been correlated. Thermal stability of hydroxyapatite depends on the amounts and nature of different substitutents. Substitution, of both sodium and carbonate ions, in hydroxyapatite increases the thermal stability and enhances the grain growth at relatively low sintering temperatures. In silicate containing hydroxyapatite the start of sintering and hence the sintering density and the mechanical properties depends on silicate contents. Substitution of silicate up to 2.23 wt% silicon on hydroxyapatite inhibited densification and decreased mechanical properties at temperatures within 900–1100 °C. As the level of silicate substitution was increased above 2.23 wt% Si of hydroxyapatite, the mechanical properties were improved regardless of the presence of a considerable amount of pores. This finding might be attributed to the formation of silicocarnotite (Ca 5 (PO 4 ) 2 SiO 4 ).