Progress in densification and toughening of high entropy carbide ceramics

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 (Ti0.25Nb0.25Hf0.25Ta0.25)CxN1–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. 

Preparation, characterization and performance of high-entropy carbonitride ceramics

Investigation of vacuum active brazing of (HfZrTiTaNb)C high-entropy carbide ceramic to Nb with TiZrCuNi filler alloy

Regulation of nitrogen content and phasic composition can be directly controlled by adjusting raw carbon allocation based on the certain cationic component in high-entropy carbide and carbonitride. By adopting this method, a series of high-entropy ceramics with the nominal composition of (Ti0.2V0.2Nb0.2Ta0.2W0.2)(CxNy) were synthesized. Within, chemical composition, phase evolution, phasic formation mechanism, mechanical performance and the strengthening mechanism were systematically probed. The matrix has undergone the evolution from double phases to single phase and then to the complex phases with the decrease of carbon and the increase of nitrogen. The monophasic carbide exhibits the best hardness (24.59 ± 1.20 GPa), and the single-phase carbonitride with 8:1 carbon-nitrogen ratio shows the best flexure strength (282.4 ± 16.2 MPa). Precipitation of (W,M)2C also contributes to the crack deflection and the fine-grain strengthening and enhances fracture toughness (for the best of 5.44 ± 0.24 MPa∙m1/2). 

Regulating nitrogen fraction, phasic composition and mechanical properties in (Ti0.2V0.2Nb0.2Ta0.2W0.2)(CxNy) high-entropy ceramic via carbon stoichiometry

A novel technique to simultaneously lower the synthesis temperature of high‐entropy carbides and maintain their high mechanical properties was proposed. Certain amount of carbon vacancies was first introduced to significantly lower the temperature down to 2000°C for uniform elemental distributions in high‐entropy carbides. Those carbon vacancies were then fully eliminated through the reaction between high‐entropy carbides and certain amount of boron carbide. Concomitantly, the high‐entropy boride phase was formed. The elimination of carbon vacancies and the formation of high‐entropy boride phase significantly improved the mechanical properties of the high‐entropy carbides. A high mechanical strength over 500 MPa can be obtained by phase optimization.

hIn situ reaction synthesis of high‐entropy carbide/diboride composite with high mechanical properties

Preparation of high-entropy carbides in bulk quantities and in powder form via simple methods at relatively low temperature is challenging. Here, we report the first synthesis of (ZrTiTaNbMo)C high-entropy carbide (HEC) powders via magnesiothermic reduction process. In this, a mixture of individual metallic oxides, graphite and magnesium is treated at a temperature of 1350 °C under Ar atmosphere. The (ZrTiTaNbMo)C powders are obtained after acid leaching, deionized water cleaning and drying process, and synthesized powders shows single-phase face-centered cubic (fcc) structure with the oxygen content of only 0.35 wt% and particle-size range of 5–60 µm. Such powders are in potential demand for additive manufacturing techniques, while high dense HEC bulk ceramic is fabricated by hot pressing sintering using ball-milled HEC powders in preliminary researches. The successful synthesis of (ZrTiTaNbMo)C powders may guide the way towards synthesis of many other high-entropy carbide powders via magnesiothermic reduction process. 

Synthesis of single-phase (ZrTiTaNbMo)C high-entropy carbide powders via magnesiothermic reduction process

Spherical (Zr0.2Ti0.2Ta0.2Nb0.2Mo0.2)B2 powders with a uniform particle size distribution are successfully prepared using a novel industrial approach, which combines spray drying process and thermal plasma sintering technology together. In this, single-phase (Zr0.2Ti0.2Ta0.2Nb0.2Mo0.2)B2 powders are firstly synthesized via borothermal reduction process using a mixture of individual metallic oxides and boron powders as starting materials. The influence of boron powders content on the structure of prepared powders is researched. Then, (Zr0.2Ti0.2Ta0.2Nb0.2Mo0.2)B2 granules are prepared after wet-grinding and spray-drying process, which exhibit spherical shape and homogeneous elements distribution. RF induction thermal plasma is finally used to sinter the granulated particle, and the apparent density of sintered spherical powders is increased to 2.57 g/cm3 from 1.43 g/cm3. Such powders are in potential demand for additive manufacturing techniques, and the successful synthesis of spherical (Zr0.2Ti0.2Ta0.2Nb0.2Mo0.2)B2 powders may guide the way towards preparation of many other spherical high-entropy diboride powders. This article is protected by copyright. All rights reserved 

Synthesis of single‐phase high‐entropy diboride powders and their spheroidization process

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Synthesis of single-phase (ZrTiTaNbMo)C high-entropy carbide powders via magnesiothermic reduction process

Synthesis of single‐phase high‐entropy diboride powders and their spheroidization process