Mechanism of Reduced Sintering Temperature of Al(2)O(3)–ZrO(2) Nanocomposites Obtained by Microwave Hydrothermal Synthesis

A novel method to obtain Al(2)O(3)–ZrO(2) nanocomposites is presented. It consists of the co-precipitation step of boehmite (AlO(OH)) and ZrO(2), followed by microwave hydrothermal treatment at 270 °C and 60 MPa, and by calcination at 600 °C. Using this method, we obtained two nanocomposites: Al(2)O(3)–20 wt % ZrO(2) and Al(2)O(3)–40 wt % ZrO(2). Nanocomposites were characterized by Fourier transformed infrared spectroscopy, Raman spectroscopy, X-ray diffraction, and transmission electron microscopy. Sintering behavior and thermal expansion coefficients were investigated during dilatometric tests. The sintering temperatures of the nanocomposites were 1209 °C and 1231 °C, respectively—approximately 100 °C lower than reported for such composites. We attribute the decrease of the sintering temperature to the specific nanostructure obtained using microwave hydrothermal treatment instead of conventional calcination. Microwave hydrothermal treatment resulted in a fine distribution of intermixed highly crystalline nanoparticles of boehmite and zirconia. Such intermixing prevented particle growth, which is a factor reducing sintering temperature. Further, due to reduced grain growth, stability of the θ-Al(2)O(3) phase was extended up to 1200 °C, which enhances the sintering process as well. For the Al(2)O(3)–20 wt % ZrO(2) composition, we observed stability of the zirconia tetragonal phase up to 1400 °C. We associate this stability with the mutual separation of zirconia nanoparticles in the alumina matrix.