A thermoanalysis of phase transformations and linear shrinkage kinetics of ceramics made from ultrafine plasmochemical ZrO_2(Y)-Al_2O_3 powders

摘要

The methods of X-ray diffraction analysis, thermogravimetric analysis, differential scanning calorimetry, and dilatometry are used to study special features of the structural-phase state of the 80 mass% ZrO_2(Y)- 20 mass% Al_2O_3 plasmochemical powders (PCPs) and their effects on the sintering of composite ceramics. It is revealed that the ZrO_2(Y)-Al_2O_3 powder composite represents a mechanical mixture containing crystalline tetragonal zirconium dioxide and aluminum oxide nanoparticles, the latter found in an amorphous state and partially included into the ZrO_2(Y) lattice, thus forming metastable solid solutions of variable composition. Heating of the composite powder within the temperature range 740-1,000 °C reveals an exothermal effect associated with decomposition of metastable states of aluminum oxide. This is accompanied by the formation of the corundumphase nuclei having subcritical dimensions. They achieve the critical sizes at higher temperatures T>1200 °C, when α-Al_2O_3 is finally crystallized. The shrinkage response of the powder compacts during non-isothermal sintering is measured in a sensitive dilatometer. It is shown that the shrinkage curve consists of several stages that closely correlate with the concurrent structural-phase transformation in the composite ZrO_2(Y)-Al_2O_3 powder mixture. The decisive contribution into shrinkage during non-isothermal sintering of composite comes from the high-temperature stages with the maximum shrinkage rate at the temperatures 1,250 and 1,550 °C. It is found out that the regime of sintering the ultrafine PCPs (T = 1,600 °C, t = 1 h) allows producing composite ceramic materials with a porosity of Q ≈ (5-7) %, microhardness H_v = 12.3 GPa, and crack resistance K_(1c) = (10-11) MPa m~(0.5).