Phase and Microstructure Evolution in Alkoxide‐Derived Mullite/MgO‐Partially‐Stabilized Zirconia

Abstract

The microstructure and phase evolution of a mullite/MgO‐partially‐stabilized‐zirconia alkoxide‐derived composite was studied by XRD and TEM/EDS in samples hot pressed at 850° to 1400°C for various periods of time. Upon hot press‐ing, Al‐Si spinel, with a composition close to that of (2;1) mullite, crystallized at a temperature as low as 850°C. It converted to (3;2) mullite at temperatures above 1200°C. Eine spherical metastable cubic‐ZrO2 particles, uniformly distributed in the amorphous matrix, precipitated at T ∼ 850°C. They coarsened at higher temperatures and trans‐formed to tetragonal or monoclinic ZrO2, depending upon the grain size. These primary precipitates grew to become intergranular zirconia in the final structure. The solid solu‐tion of zirconia in mullite decreased with increasing temperature, so that secondary zirconia particles precipitated at higher temperatures and remained as inclusions inside mullite grains. Irregularly shaped elongated zircon was found at T ∼ 1200°C. It decomposed at higher temperatures to form zirconia particles, which existed at the grain bound‐aries of the final mullite. Other phases found in hot‐pressed samples included MgAl2O4 spinel and sapphirine (2MgO‐2Al2O3‐SiO2). For powders heat treated in air, γ‐Al2O3(instead of Al‐Si spinel) was formed at temperatures above 900°C. The γ→α‐Al2O3 phase transformation occurred at T 1300°C, followed by a rapid grain growth. In contrast with the hot‐pressed samples, mullite in heat‐treated powders crystallized only at temperatures above 1400°C. Previous studies on phase evolution during pressureless heat treatment of powders were generally unindicative of phase and microstructural evolution upon hot pressing.