Comparative study of apatite formation on CaSiO₃ ceramics prepared by glass crystallization and sintering methods in simulated body fluid

Hydroxyapatite (HAp) formation in simulated body fluid (SBF) was investigated in CaSiO₃ ceramics of slightly silica rich composition (Ca/Si=0.9) prepared by glass crystallization and sintering methods. The starting CaSiO₃ powders were prepared by coprecipitation using NH₄OH as a precipitant. The ground CaSiO₃ powders were sintered at 1400°C for 2h. Crystallized glass samples were prepared by melting at 1600°C, quenching in water, grinding the powder, densifying and crystallizing the pelletized ground glass powder at 900-1400°C. The resulting crystalline phases are wollastonite (Wo) in the samples fired at 900-1100°C, Wo and pseudowollastonite (Pw) at 1200°C, and Pw and cristobalite at 1300-1400°C. The bulk density of the sintered sample was 2.42 g/cm³ while those of the crystallized glass samples were 2.57-2.71 g/cm³. All the samples were soaked in SBF using a sample/solution ratio of 8mg/ml at 36.5°C for 1-25 days. The sintered sample and glass sample crystallized at 1400°C showed HAp formation after soaking for 3 days, with an HAp layer forming on the surfaces after longer soaking. On the other hand, the glass sample crystallized at 900°C showed no apatite formation even after prolonged soaking (25 days). Differences in the microstructures and phases formed in these two types of sample are thought to give rise to the difference in HAp formation.