Effect of Heat Treatment on the Structure and Phase Composition of the Nanosized Powder Based on a ZrO2 Solid Solution

Abstract

The nanosized 90 mol.% ZrO2–2 mol.% Y2O3–8 mol.% CeO2 powder was produced by hydrothermal synthesis in an alkaline environment and heat-treated in the range 400–1300°C. The powder properties were examined by X-ray diffraction (XRD), SEM and TEM, petrography, and BET. According to the XRD data, a low-temperature metastable cubic ZrO2 (F-ZrO2) solid solution formed after hydrothermal synthesis. According to the petrography and electron microscopy data, TZrO2 began to form already in the hydrothermal synthesis process. The F-ZrO2 → T-ZrO2 phase transformation was completed in the range 700–850°C. Some T-ZrO2 particles were characterized by a twin substructure. The T-ZrO2 unit cell volume monotonically increased from 133.58 · 10–3 nm3 to 137.09 · 10–3 nm3 and the degree of tetragonality from 1.0033 to 1.0140. No M-ZrO2 was found to form. The powder specific surface area decreased from 94 to 2 m2/g in the heat treatment process. The sizes of primary powder particles (5–10 nm) remained almost unchanged in heat treatment up to 1150°C. The Vickers hardness of the ceramics produced from the powder treated at 850°C was 3.1 GPa and critical fracture toughness factor KIc was 8.4 MPa · m1/2. The preservation of the tetragonal structure (T-ZrO2), which is capable of the martensitic T-ZrO2 → M-ZrO2 transformation, and the strength characteristics determined open ways for microstructural design of smart materials, including shape memory ones, in the ZrO2–Y2O3–CeO2 system.