Search

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 fracture toughness and oxidation resistance are crucial factors affecting the practical applications of high-entropy diboride ceramics (HEBs). In this study, silicon carbide whiskers (SiC w) were introduced as a reinforcement for HEBs for the first time. To achieve a uniform dispersion of SiC w within the HEB powders, a combination of simultaneous ultrasonic dispersion, mechanical agitation, and freeze-drying technology was employed. Consequently, a series of composites, (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2)B 2 -nSiC w (n = 0–20 vol%), were densified by spark plasma sintering. The effect of SiC w on the microstructure and mechanical properties of the HEBs were studied, as well as the toughening mechanism involved. The results revealed that the introduction of SiC w led to grain-size refinement in (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2)B 2 , along with significant improvements in relative density, hardness, fracture toughness, and oxidation resistance. The relative density, hardness and fracture-toughness of (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2)B 2 -SiC w composites exhibited an initial increase followed by a decrease with increasing SiC w content. Specifically, the (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2)B 2 –15 vol% SiC w composite simultaneously obtained the highest hardness of 25.6 GPa and fracture-toughness of 4.8 MPa·m1/2, representing a 26% improvement compared to (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2)B 2 ceramics. • The effect of SiC w on microstructure, mechanical properties and oxidation resistance of HEBs were studied for the first time. • The K Ic of (Hf 0.2 Zr 0.2 Ta 0.2 V 0.2 Nb 0.2)B 2 ceramic was increased by 26% without loss of hardness by introducing SiC w. • The oxidation resistance of HEBs was greatly enhanced due to the addition of SiC whiskers. [ABSTRACT FROM AUTHOR]

In this study, Zn 8 B 3 H 3 O 14 nanosheets were synthesized through a one-step coprecipitation process and employed as both a boron source and substrate templates for the fabrication of boron nitride nanosheets (BNNSs), which were synthesized efficiently at low temperatures by the borate nitration method using ammonia as the nitrogen source. The thickness of the synthesized BNNSs ranged from 3 to 10 nm with a flake dimension of about 2.56 μm. The prepared BNNSs using this method realized 98.8% boron source utilization and single batch yields up to gram level. In addition, BNNSs/SiO 2 microwave-transparent composites were prepared to investigate the potential for large-scale applications of the BNNSs produced by using this method. The composites demonstrated a remarkable increase in bending strength and fracture toughness, reaching 122.02 MPa and 2.31 MPa·m1/2, respectively, with the addition of only 0.5 wt% BNNSs. Compared with SiO 2 without BNNSs, the fracture toughness of the composites is increased by 106%. A novel crack plugging strong toughening mechanism of BNNSs is proposed. Simultaneously, the composites exhibit exceptional microwave-transparent properties, with dielectric constants below 4 in the real part and below 0.03 in the imaginary part. Therefore, this study has successfully achieved the low-temperature and efficient synthesis of micron-scale BNNSs, laying the groundwork for their large-scale application. [Display omitted] • Zn 8 B 3 H 3 O 14 nanosheets were used as substrate templates for the synthesis of BNNSs. • BNNSs were synthesized with high efficiency at low temperatures (900 °C). • The single batch yield of BNNSs reached up to the gram level. • The BNNSs/SiO 2 wave-transparent composites with ultrahigh toughness were obtained. • A novel mechanism for strengthening and toughening (BNNSs plugging) is proposed. [ABSTRACT FROM AUTHOR]