ABSTRACT
Molten zircon, rapidly quenched by plasma spraying onto a cool substrate, solidifies to a metastable mixture of tetragonal zirconia and silica glass. Transmission electron microscopy reveals a fine dispersion of tetragonal zirconia, particle size <10 nm, in a glass matrix. The microstructure is consistent with spinodal decomposition of the supercooled liquid, within a wide metastable miscibility gap in the ZrO2-SiO2 system, and suppression of the equilibrium crystallisation of ZrO2 from the melt or phase separation by nucleation and growth of droplets at large undercooling. Tetragonal ZrO2 is retained because of a particle size effect related to a balance between the free energy difference between tetragonal and monoclinic ZrO2, the ZrO2-glass interfacial energy and strain energy arising from elastic constraint of thermal expansion mismatch. Heat treatment between 900 and 1100 °C results in coarsening of the tetragonal ZrO2. Above 1100 °C the ZrO2 coarsens to a particle size exceeding the critical dimensions for retention of the tetragonal phase on cooling, -20 nm, and monoclinic ZrO2 is observed. Reaction between monoclinic ZrO2 and SiO2 to form zircon takes place slowly between 1200 and 1500 °C but is catalysed in the presence of a liquid phase.
