Introduction High-entropy zirconate ceramics (HEZCs) have been studied extensively in recent years for the potential applications in thermal barrier coatings and high-level nuclear waste immobilization. While these HEZCs have the issue of poor toughness, which impedes their applications. The toughness of HEZCs can be improved by decreasing its grain size. While preparation of ultrafine-grained or nanocrystalline HEZCs is a challenge because the rapid grain growth inevitably occurred at high temperature densification process (typically above 1 500 ℃). In this work, the challenge of preparing dense ultrafine-grained high-entropy ceramics through conventional pressureless sintering process was addressed via a simple two-step sintering method. Ultrafine-grained (Ce0.2Nd0.2Sm0.2Gd0.2Y0.2)2Zr2O7 high-entropy zirconate with 99.0% theoretical density and 162 nm grain size was fabricated. Compared to the conventional method, two-step sintering provided the high-entropy zirconate with finer grain size and better microstructural uniformity, and excellent comprehensive mechanical properties including high hardness of 12.5 GPa and high fracture toughness of 2.4 MPa·m1/2. This work could help to understand the sintering kinetics of HEZCs, and also supplied a guidance to prepare the ultrafine-grained or nanocrystalline HEZCs by pressureless sintering method. Methods The starting high-entropy (Ce0.2Nd0.2Sm0.2Gd0.2Y0.2)2Zr2O7 ceramic powders were prepared via a polyacrylamide gel method. Analytical grade rare earth nitrate hexahydrates RE(NO3)3․6H2O (RE=Ce, Nd, Sm, Gd, Y) with a purity of 99.9%, zirconium oxychloride (ZrOCl2․8H2O, 99.0% purity), acrylamide (AM, C3H5NO, 99.0% purity), N-N’-methylene-bis-acrylamide (MBAM, C7H10N2O2, 99.0% purity) and ammonium persulfate ((NH4)2S2O8, 99.0% purity) were used as starting materials, which were purchased from Macklin Chemical (Shanghai, China). First, RE(NO3)3․6H2O and ZrOCl2․8H2O were mixed in distilled water according to the stoichiometric ratio of (Ce0.2Nd0.2Sm0.2Gd0.2Y0.2)2Zr2O7. After a vigorous stirring for 30 min, a clear solution was obtained and the concentration of Zr4+ was 0.02 mol/L. Then the AM, MBAM and (NH4)2S2O8 with a mole ratio of 24/2/1 were added in the above solution and dissolved completely by vigorous stirring. The mole ratio of AM/Zr4+ (abbreviated as A/Zr) in mixed solution was 80/1. After that, the mixed solution was heated in a water bath at 80 ℃ until it converted into wet gel. Subsequently, the wet gel was dried in an oven at 80 ℃ for 72 h to obtain the xerogel. Finally, (Ce0.2Nd0.2Sm0.2Gd0.2Y0.2)2Zr2O7 powders were prepared after a calcination of the xerogel at 1 000 ℃ for 2 h in air. These (Ce0.2Nd0.2Sm0.2Gd0.2Y0.2)2Zr2O7 powders were uniaxially pressed into green pellets with a diameter of 8 mm and a thickness of (2±0.1) mm under 300 MPa, then it was sintered in air through conventional sintering and two-step sintering. For the conventional sintering, the green pellets were heated to 1 200–1 500 ℃ and held for 2 h directly; For the two-step sintering, the green pellets were firstly heated to a higher temperature of T1 and held for time of t1, then immediately cooled down to a lower temperature of T2 and held for time of t2. After that, the samples were naturally cooled down to room temperature in the furnace. It should be noted that in the heating process, the heating rate was 10 ℃/min before 1 000 ℃. After the furnace temperature reached to 1 000 ℃, the heating rate was down to 5 ℃/min. The crystalline phases of samples were detected by X-ray diffractometer (XRD, Bruker D8 Davinci with Cu Kα radiation, Germany). The microstructure and element distribution of HEZC powders were characterized by transmission electron microscopy (TEM, JEOL-2100 F, Japan) equipped with energy dispersive X-ray spectroscopy (EDS). The morphology of sintered sample was observed by field emission scanning electron microscopy (SEM, Hitachi S-4800, Japan). The average grain size of sintered sample was determined by Nano Measurer software. At least 200 grains were randomly selected in measurement. The density of sintered sample was measured by Archimedes method. The relative density was calculated according to the theoretical density derived from the Rietveld refinement. The Vickers hardness and fracture toughness were evaluated by a micro-Vickers hardness tester (HV-1000A, Laizhou Huayin, China) under a load of 9.8 N with dwelling time of 15 s. The nanoindentation tests were carried on a Nano Indenter G200 (U9820A, Agilent Technologies), using a constant strain rate of 0.05/s. The Young’s modulus was calculated according to the Oliver-Pharr method from the loading-unloading curve. Results and discussion All diffraction peaks of the starting powders (Ce0.2Nd0.2Sm0.2Gd0.2Y0.2)2Zr2O7 prepared via polyacrylamide gel method matched well to that of the defective fluorite phase, indicating that the starting powder was single-phased HEZC with defective fluorite structure. The TEM image of HEZC powders presented that the average particle size was about 25 nm. The EDS mapping of HEZC revealed the homogeneous distribution of elements without segregation. Samples sintered at 1 200 ℃ and 1 300 ℃ exhibited a porous structure; When sintered at 1 400 ℃, it exhibited a relatively dense structure, and some closed pores were still remained in the sample; When sintered at 1 500 ℃, a dense and pore-free microstructure can be observed. It should be noted that the dense HEZC bulks with relative density of 99.4% can be prepared when sintered at 1 500 ℃ for 2 h, implying a high sintering activity. The grain size increased slowly as the sintered temperatures increased from 1 200 ℃ to 1 400 ℃, whereas grain size increased rapidly beyond 1 400 ℃. Therefore, the rapid grain growth could be avoided if sintering temperatures below 1 400 ℃ in the conventional sintering. However, the densification process could not be accomplished under such sintering conditions. The pore pinning effect was of great significance for preparing the ultrafine-grained or nanocrystalline HEZCs via pressureless sintering method. And based on the XRD analysis, it can be concluded that the structure of (Ce0.2Nd0.2Sm0.2Gd0.2Y0.2)2Zr2O7 evolved from the defective fluorite toward to the pyrochlore at temperatures above 1 400 ℃. With the understanding of the sintering behavior of HEZC, two-step sintering (TSS) could be provided to achieve the high densification and fine grain size by suppressing the rapid grain growth in the final sintering stage. the ultrafine-grained HEZC with an average grain size of 162 nm was prepared by TSS4, the sample showed that the compact microstructure with homogeneous ultrafine grains was obtained. In addition to retarding the rapid grain growth and obtaining the finer grain size, two-step sintering also could help to achieve a uniform microstructure with a narrower grain-size distribution compared to the conventional sintering. Two-step sintered sample (TSS4) has a lower σ (0.31, the ratio of the standard deviation Σ of grain-size distribution to the average grain size Gavg) than that of the conventionally sintered sample (CS4, σ=0.84). The Vickers hardness and fracture toughness of the HEZC sample prepared by CS4 were 11.0 GPa and 2.0 MPa·m1/2, respectively. Meanwhile, these values of sample prepared by TSS4 were 12.5 GPa and 2.4 MPa·m1/2, increased by 13.6% and 20.0%, respectively. The hardness and fracture toughness of samples prepared by two-step sintering were much better than those of conventional sintering, owing to the ultra-fined and homogeneous microstructure. Conclusions Pressureless two-step sintering process is successfully used to prepare the dense and ultrafine-grained high-entropy (Ce0.2Nd0.2Sm0.2Gd0.2Y0.2)2Zr2O7 ceramics with a relative density of 99.0% and an average grain size of 162 nm, as the phase transformation from defective fluorite to pyrochlore occurred over 1 400 ℃. In addition, this process could achieve the higher grain-size uniformity and finer grain size, as compared to that of the conventional sintering. Owing to the ultrafine grain size and high microstructural uniformity, the high-entropy (Ce0.2Nd0.2Sm0.2Gd0.2Y0.2)2Zr2O7 ceramics possessed the excellent mechanical properties with higher Vickers hardness of 12.5 GPa and fracture toughness of 2.4 MPa·m1/2. It was believed that pressureless two-step sintering method could be used to prepare other high-entropy ceramics with fine grain size and high quality. © 2024 Chinese Ceramic Society. All rights reserved.
