Sintering, Phase Stability, and Properties of Calcium Phosphate-Mullite Composites

In this contribution, we report the results of a study to probe into the combined effect of the sintering conditions and mullite (3Al2O3·2SiO2) addition (upto 30 wt%), on the densification mechanism, phase assemblage, and microstructure development in calcium phosphate (CaP)‐mullite composites. The experimental results reveal that close to 95% theoretical density can be achieved by sintering the compositions in the temperature range of 1300°–1350°C. Finer scale microstructural analysis using transmission electron microscopy reveals the presence of both β‐ and α‐TCP (Ca3(PO4)2–tricalcium phosphate) and a crystalline residue of gehlenite at triple junctions. The shrinkage kinetics of the composites have been analyzed to qualitatively understand the sintering mechanisms. For the composites, solid‐state sintering in the initial stage, followed by liquid‐phase sintering at or near the sintering temperature are postulated to explain the effect of temperature and volume fraction of the second phase. Some important features of the liquid‐phase sintering have been discussed using the CaO–Al2O3–SiO2 ternary phase diagram. A comparison with the earlier published results reveals that a better combination of long‐crack fracture toughness, compressive strength, and flexural strength can be obtained with the newly developed composites.