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High hardness and high toughness (Ti,Nb,Ta,Mo,W)(C,N)-based ceramics were prepared using pre-synthesized high-entropy carbonitride (HECN) powder and high-entropy alloy (HEA) via spark plasma sintering. By HEA addition of 5 vol%, dense (Ti,Nb,Ta,Mo,W)(C,N)-based ceramics with fine microstructure was obtained at 1450 °C. Due to the presence of the precipitated (Mo,W) 2 C phase within the HECN matrix, which forms a solid solution with the added HEA, the ceramics exhibited the high Vickers hardness (24.5 ± 1.3 GPa) and fracture toughness (6.1 ± 0.4 MPa m1/2), arise from the fine grain structure of the ceramics and the pronounced effect of crack deflection. This study introduced a new strategy for the preparation of high-entropy carbonitride-based ceramics, which significantly improved the fracture toughness while maintained high hardness. [ABSTRACT FROM AUTHOR]

Based on the carbide boronizing reaction, B 4 C-(Hf,Zr,Ta,Nb,Ti)B 2 composites were prepared by reactive and non-reactive spark plasma sintering using self-synthesized (Hf,Zr,Ta,Nb,Ti)C and compared with previously reported B 4 C-(Hf,Zr,Ta,Nb,Ti)B 2 composites prepared by reactive spark plasma sintering using corresponding 5 commercial individual transition metal carbides. Irrespective of sintering methods, B 4 C-(Hf,Zr,Ta,Nb,Ti)B 2 prepared by using the self-synthesized (Hf,Zr,Ta,Nb,Ti)C powder presented higher relative densities, smaller grain sizes, a more homogeneous elemental distribution and higher Vickers' hardness than the materials available in the literature. However, compared with reactive spark plasma sintering, B 4 C-(Hf,Zr,Ta,Nb,Ti)B 2 prepared by non-reactive spark plasma sintering had a more uniform phase distribution, leading to the higher Vickers hardness up to about 30 GPa. [ABSTRACT FROM AUTHOR]

In this study, a dual charge-compensator formulation was developed for zirconolite with the nominal composition Ca 1-x Ce x ZrTi 2-2x Fe x Cr x O 7 (x = 0–0.30). The design strategy here was such that trivalent Fe and Cr were both targeted to substitute across the Ti site(s) to charge balance an inventory of CeO 2 included as a structural analogue for Pu. The targeted solid solution was prepared by sintering constituent oxides at 1400 °C for 10 h under an air atmosphere. By means of powder XRD refinement and selected area electron diffraction analysis, the dominant zirconolite polytype was confirmed to be 2M across the solid solution. The obtained product density was significantly increased when compared to the previously discussed Ca 1-x Ce x ZrTi 2-2x Cr 2x O 7 system, suggesting that the partial inclusion of Fe in a 1:1 molar ratio with Cr may improve sintering behaviour. The limit of solid solution was reached at approximately x = 0.30, for which the segregation of CeO 2 and Cr 2 O 3 phases was clearly evidenced. An evaluation of obtained chemical compositions and bulk oxidation states was performed to inform the solid solution mechanism of Ce, Cr and Fe within zirconolite. [ABSTRACT FROM AUTHOR]

In this study, the mechanism of the effect of ZrB 2 on phase transformation of Si 3 N 4 at a low temperature and the influence of its content on Si 3 N 4 -based ceramics were investigated. Previous study has shown that oxide impurities, i.e., B 2 O 3 and ZrO 2 on ZrB 2 particles, alone cannot contribute to phase transformation of Si 3 N 4 at a low temperature. But, the introduction of 0.5 vol% ZrB 2 into Si 3 N 4 ceramics can promote the α-β phase transformation of Si 3 N 4 , which is confirmed to be the role of boron by comparison of the experimental results obtained from the addition of 0.5 vol% Zr and 0.5 vol% B. Increasing the ZrB 2 content from 0 vol% to 2.5 vol% intensifies the α-β phase transformation while decreasing the α phase content of Si 3 N 4 -based ceramics, accompanied by a slight grain growth, leading to a decrease in hardness. At the same time, aspect ratio and the quantities of elongated grains per square micron increase, and thus the fracture toughness increases significantly. However, when the content of ZrB 2 increases to 5 vol%, the Si 3 N 4 -based ceramics not only have a substantial decrease in hardness, but also the fracture toughness fails to be effectively improved due to high porosity and the decrease in aspect ratio and the quantity of elongated grains per square micron. The current study demonstrates that the dense Si 3 N 4 -based ceramics with high hardness and toughness (hardness ∼19.9 ± 0.2 GPa, toughness ∼6.27 ± 0.19 MPa m1/2) can be prepared successfully at 1600 °C by introducing 0.5 vol% ZrB 2. [ABSTRACT FROM AUTHOR]