The Influence of ZrO₂-Based Solid Solution Amount on the Physicochemical Properties of Al₂O₃–ZrO₂–Y₂O₃–CeO₂ Powders

Abstract

For the first time, Al₂O₃-based nanocrystalline powders with different ZrO₂ amounts were produced by hydrothermal synthesis in an alkaline medium for designing ZTA composites. In ZTA composites, ZrO₂-based solid solution particles codoped with ceria and yttria are dispersed in a rigid Al₂O₃-based matrix. To examine the variation in physicochemical properties, the 90 wt.% Al₂O₃–10 wt.% ZrO₂ (Y₂O₃, CeO₂) and 58.5 wt.% Al₂O₃–42.5 wt.% ZrO₂ (Y₂O₃, CeO₂) powders were used. The ZrO₂-based solid solution had composition 90 mol.% ZrO₂–2 mol.% Y₂O₃–8 mol.% CeO₂. The hydrothermal powders were heat-treated in the temperature range 400–1450 ^oC. The powder properties were studied by X-ray diffraction, differential thermal analysis, and electron microscopy. The powder specific surface area was determined by the BET method. The size of the primary particles was determined by the Scherrer equation. The AMIC software (Automatic Microstructure Analyzer) was used to process the results of morphology. Phase transformations and active sintering of the ZTA powders determined the dependences of the primary particle size and powder specific surface area on the heat treatment temperature. With increasing ZrO₂ content, the temperature of   F-ZrO₂ → T-ZrO₂ phase transformation decreased, the probability of M-ZrO₂ formation increased, and the sequence of Al₂O₃ phase transformations varied after the boehmite decomposition. The variation in morphology and dependence of the powder specific surface area in the heat treatment process indicated that the powders had increased sintering activity. The dependence of the shape factor characterizing the nanocrystalline 90AZG and 58.5AZG powders on the heat treatment temperature was studied. The starting nanosized 90AZG and 58.5AZG powders had similar distribution of agglomerates according to the shape factor. Round agglomerates and multifaceted regular agglomerates were predominant. The regularities of variation in the shape factor with temperature with the topochemical memory effect of the powders were established. The mechanical properties of the samples sintered from the hydrothermal nanocrystalline 90AZG and 58.5AZG powders. With increasing ZrO₂ content, the microhardness of the ZTA composites decreased from 195 to 160 MPa, fracture toughness (K_Iс) increased from 6 to 8 MPa · m^0.5, and Vickers hardness decreased from 8.3 to 5.6 GPa. The improvement in consolidation methods for ZTA composites will allow tool, structural, and functional ceramics with the necessary properties to be produced.