Effect of Heat Treatment on the Physicochemical Properties of Ultrafine ZrO2–Y2O3–CeO2–Al2O3–CoO Powders

Abstract

Variations in the phase composition, specific surface area, and morphology of structural components in the ultrafine powder of composition (wt.%) 70 (90 ZrO2 (3 Y2O3, 2 CeO2)–10 Al2O3)–30 CoAl2O4 (70ZA30CoA), produced by hydrothermal synthesis combined with mechanical mixing, were studied in the heat treatment process up to 1300°C. The study employed Xray diffraction, scanning and transmission electron microscopy, petrography, and BET. The formation of CoAl2O4 in the 70ZA30CoA powder in the heat treatment process was accompanied by reversible phase transformations: T-ZrO2 → M-ZrO2 → T-ZrO2. The M-ZrO2 content increased from 15% to 46% in the temperature range 850–1000°C and decreased to 13% after heat treatment to 1150°C. The process involved slight coarsening of the primary T-ZrO2 particles, while the size of the primary M-ZrO2 particles remained practically unchanged. The phase transformation was due to a decrease in the free energy of the ultrafine 70ZA30CoA powder, representing a thermodynamically nonequilibrium system. The phase composition changed color of the 70ZA30CoA powder in the following sequence: gray → gray blue → dark cyan → bright blue. Morphological analysis of the structural components showed that the CoAl2O4 formation and reversible T-ZrO2 → M-ZrO2 phase transformation were accompanied by shape change, loosening, and subsequent sintering of the agglomerates. The chain-like agglomerates of various shapes and sizes indicate that the 70ZA30CoA powder sinters actively at 1300°C. The decrease in the specific surface area from 46 to 1 m2/g depending on the heat treatment temperature was determined by the development of three structural transformation processes: formation of CoAl2O4, phase transition of the ZrO2 solid solution, and sintering of the 70ZA30CoA powder. The established regularities are of fundamental importance for the microstructural design of ZrO2 composites such as ZrO2–Y2O3–CeO2–Al2O3–CoO materials of blue and other colors for various applied purposes.