Subsolidus Equilibria and Stabilization of Tetragonal ZrO2 in the System ZrO2‐Al2O3‐SiO2

TLDR: In this paper, the irreversible reaction between zircon and alumina to form mullite and Zirconia was observed to proceed very slowly at 1480°C, confirming the existence of a stable join between the two components.

Abstract: The irreversible reaction between zircon and alumina to form mullite and zirconia was observed to proceed very slowly at 1480°C, confirming the existence of a stable join between mullite and zirconia in addition to the join between mullite and zircon. Attempts to determine equilibrium assemblages in powder samples of several combinations of the constituents at 1300° and 1400°C were unsuccessful. Metastable tetragonal ZrO2 was observed in fired powder samples containing kaolinite, in fired zircon-alumina samples, and in calcined gels prepared from zirconyl nitrate, aluminum nitrate, and colloidal silica in water. Tetragonal ZrO2 was the dominant crystalline phase in gels fired for 24 h at 800°, 950°, and 1100°C; the thermal stability of the phase was enhanced by Al2O3 and by SiO2, but most effectively by mixtures of the two. Line broadening measurements and electron microscope observation indicate that the tetragonal ZrO2, with apparent crystallite sizes between 0.009 and 0.1 μm, develops as an intergrowth with, and is stabilized by, the amorphous matrix of Al2O3 and SiO2.