From freeze-dried precursors to microwave sintered Al2O3-ZrO2 composites

Homogeneous Al2O3-Zr0.91Y0.09O1.955 (65-35mol%) nanopowders have been prepared in a wide temperature range (from 1073 to 1573K) by thermal decomposition of amorphous precursors, made previously by freeze-drying of appropriate solutions in air. Electron microscopy images show that, whereas at low temperatures (973 K) pseudo-spherical particles constituted of Al2O3 and Zr0.91Y0.09O1.955 grains are observed (~38 nm), at high temperatures (1573 K) a homogeneous dispersion of nanocrystalline ZrO2-based grains (~186 nm) is dispersed in the sintered Al2O3 matrix. The comparison between phase and microstructure evolution in these samples clearly indicates that the disorder at the atomic scale in the precursor makes the attainment of higher temperatures necessary for nucleation and growth of both phases. Finally, a selected material was sintered in a mono-mode microwave device at 2.45GHz in air at 1573 and 1673K. This fast-microwave technology allows fabrication of composites with high densities (~99%TD) and excellent mechanical properties, such as hardness and Young?s modulus reaching 25.6GPa and 358GPa, respectively.