EFFECT OF HEAT TREATMENT ON THE STRUCTURE AND PHASE COMPOSITION OF THE NANOSIZED POWDER BASED ON THE ZrO₂ SOLID SOLUTION

Abstract

The nanosized powder of composition (mol.%) 90 ZrO₂–2 Y₂O₃–8 СеO₂ was produced by hydrothermal synthesis in an alkaline environment and heat treated in the range 400–1300 °С. 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, Т-ZrO₂ began to form already in the hydrothermal synthesis process. The F-ZrO₂ → Т-ZrO₂ phase transformation was completed in the range 700‒850 °С. Some Т-ZrO₂ particles were characterized by a twin substructure. The volume of the Т-ZrO₂ unit cell 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. The formation of M-ZrO₂ was not found. 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 °С. The Vickers hardness of the ceramics produced from the powder treated at 850 °С was 3.1 GPa and critical fracture toughness factor KIс was 8.4 MPa • m^1/2. The preservation of the tetragonal structure (T-ZrO₂), which is characterized by the ability to martensitic T-ZrO₂ → M-ZrO₂ phase transformation and the determined strength characteristics open the way for microstructural design of smart materials, including those with shape memory effect, in the ZrO₂‒Y₂O₃‒CeO₂ system.