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Compared with common monoanionic high entropy ceramics, polyanionic high entropy ceramics, with better performance, are more difficult to prepare. Since transition metal carbides have high hardness and transition metal nitrides have good toughness, it is hoped that high entropy carbonitride ceramics can combine the advantages of the two. In this paper, a series of novel (Ti 0.25 Nb 0.25 Hf 0.25 Ta 0.25)C x N 1–x (x = 0.3, 0.4, 0.5, 0.6, and 0.7) high entropy ceramics were successfully synthesized for the first time using spark plasma sintering at 1850 °C with metal carbides and metal nitrides as raw materials. The phase evolution, microstructure, hardness and fracture toughness of high carbonitride entropy ceramics were investigated. X-ray diffraction indicates the ceramics with the single-phase face-centered cubic structure were successfully prepared. It was found that the high entropy ceramics sintered at 1850 °C with four cationic components have excellent performance on fracture toughness (KIC = 7.1746 MPa m1/2). With the increase of carbide proportion, the hardness of the ceramics increased, but the fracture toughness and bending strength decreased slightly. And it was found that the differences of grain size and porosity content were the main reasons for the mechanical properties difference. The excellent hardness properties, good bending strength and outstanding fracture toughness of the high entropy ceramics provide the possibility for the application of high entropy ceramics in aerospace and other applications requiring high performance. [ABSTRACT FROM AUTHOR]

The relationship between the chemical elements of high-entropy boride (HEB) ceramics and their hardness is important for the prediction of high-hardness HEB ceramics. In this work, by designing four HEB ceramics with different chemical elements, the effect of lattice parameter difference factor (δ, represents the difference-degree in lattice parameters among the five individual constitute diborides) on phase composition and lattice-distortion, and the effect of rule of mixture (ROM) average hardness and lattice-distortion on HEB ceramics hardness were studied. The results indicated that, as δ value increases, more severe lattice-distortion occurs inside the HEB ceramics, and a single solid-solution is difficult to be formed. Furthermore, lattice-distortion and the ROM average hardness codetermine the HEB ceramics hardness. The greater lattice-distortion brings about significantly higher hardness for HEB ceramics than their ROM average hardness. Among the four HEBs, (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2)B 2 exhibits the high hardness of 26 GPa measured using a load of 9.8 N. [ABSTRACT FROM AUTHOR]

The composition and synthesis approach of high entropy ceramics have significant influences on their microstructures and mechanical properties. A new (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2)B 2 high entropy diboride ceramic with excellent mechanical properties was successfully prepared by spark plasma sintering (SPS) at a relatively "low temperature" (1800 °C) in this work. The effects of two mixing strategies, grinding and ball milling, on the solid solubility of the calcined powders were studied. Furthermore, the effects of these two mixing strategies on the phase and morphology of the calcined powders and the mechanical properties of the SPS sintered ceramics were analyzed. Experimental results indicated that, compared with a single solid solution phase within the high entropy boride powders obtained by ball milling, incomplete solid-solution presented in the high entropy boride powders obtained using grinding treatment. It promoted the solid-state diffusion and in-situ reaction of the diborides in the ceramics during the sintering process, thereby improving the densification and the hardness of the high entropy diboride ceramics. By grinding treatment, (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2)B 2 high entropy ceramic with a relative density of 94% and a hardness of up to 25.34 ± 1.5 GPa at an indentation load of 9.8 N was obtained at 1800 °C. This work not only expanded the family of high-entropy diboride ceramics but also proved the advantage of grinding treatment in preparation of high entropy diboride ceramics. [ABSTRACT FROM AUTHOR]

In this paper, BNNSs/Al 2 O 3 composite powder was prepared by in-situ reaction using borate nitridation method and BNNSs/Al 2 O 3 composite ceramics were prepared by hot-pressing sintering. This method achieves uniform mixing of BNNSs and Al 2 O 3 ceramic matrix and reduces the introduction of impurities in the processing process. The BNNSs/Al 2 O 3 composite ceramics have excellent bending strength (549.4 MPa), fracture toughness (5.18 MPa m1/2) and hardness (21.3 GPa). The high hardness of composite ceramics is attributed to high grain boundary strength and density. The reinforcing mechanisms of ceramics include BNNSs pull-out, BNNSs bridging, crack deflection as well as the transgranular fracture and intergranular fracture of Al 2 O 3 matrix. [ABSTRACT FROM AUTHOR]

Alumina is one of the most commonly used and researched structural ceramic because of its excellent properties. However, its intrinsic brittleness is the fatal drawback, which hinders it from wider applications. How to improve its fracture toughness as well as the bending strength is always challenging for material researchers. In this paper, alumina matrix composites were fabricated by hot-pressing, in which some additives, including zirconia, alumina platelets, and MXene, were incorporated. The influence of the introduced additives on their microstructure and mechanical properties was investigated. Compare with the monolithic alumina, both bending strength and fracture toughness of all samples were improved greatly. Incorporation of zirconia was beneficial to the mechanical properties due to the phase-transformation strengthening and toughening mechanism. While alumina platelets resulted in high fracture toughness because of the self-toughening of elongated grains. The synergistic effect of alumina platelets and MXene enormously improved the fracture toughness from 2.9 ± 0.3 MPa·m1/2 for monolithic alumina to 7.5 ± 0.4 MPa·m1/2 for the composite, which was increased by 159%. This work will provide useful references for the fabrication of high-strength and high-toughness alumina ceramics by introducing additives properly. [ABSTRACT FROM AUTHOR]