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.

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