Hydrothermal synthesis of Al 2 O 3 -SiO 2 -ZrO 2 (Y 2 O 3 ) powder and their application for high-temperature ceramics

transformation of ZrO2 (monoclinic) into ZrO2 (tetragonal). The mechanism and hydrothermal reactions are discussed. The influence of the obtained nanosize powders on the sintering rate, phase composition and properties of mullite zirconia ceramics was studied and compared with the composition of traditionally obtained ceramics. The pressed samples were sintered at temperatures from 1100 to 1500 °C. The effect of hydrothermal preparing process is emphasized. It is shown that the densification of ceramic samples obtained from hydrothermally and traditionally prepared powders is greatly affected by the preparation methods used. XRD and SEM examinations showed the differences in mullite and ZrO2 (tetr.) phase formation temperatures, as well as in the morphology and grain size of sintered ceramic samples. The development of crystalline phases – mullite, ZrO2 (tetragonal or monoclinic), as well as corundum (α-Al2O3) – in ceramic samples from conventionally obtained powders starts at 1300 °C, whereas from hydrothermal (in particular mullite phase) at1100 °C. The values of compressive strength showed that they are greatly influenced by the preparation methods of starting powders – the compressive strength of samples prepared by the conventional method and sintered at 1400 °C reaches 160– 170 MPa, whereas that of samples prepared by the hydrothermal method and sintered at 1500 °C reaches 100–105 MPa.