Effects of (Ta0.2W0.2Nb0.2Mo0.2V0.2)C high-entropy carbide content on the microstructure and properties of Ti(C0.7N0.3)-based cermet

Using pre-synthesized high-entropy (Ta_0.2W_0.2Nb_0.2Mo_0.2V_0.2)C carbide as the reinforcing phase, Ti(C_0.7N_0.3)-based cermets were prepared by pressureless sintering at 1600 °C. The results revealed that due to the solid solution reaction between the mono-carbide and (Ta_0.2W_0.2Nb_0.2Mo_0.2V_0.2)C, only one set of face-centered-cubic diffraction peaks in XRD was detected in the as-sintered cermets, alongside the typical core-rim structure. Compared to the Ti(C_0.7N_0.3)-based cermets without high-entropy reinforcing phase, the Vickers hardness was increased from 17.06 ± 0.09 GPa to 18.42 ± 0.33 GPa and the fracture toughness was increased from 9.21 ± 0.31 MPa m^1/2 to 12.56 ± 0.23 MPa m^1/2 by adding 10 wt% (Ta_0.2W_0.2Nb_0.2Mo_0.2V_0.2)C. The wear resistance of the cermet was enhanced significantly with increasing (Ta_0.2W_0.2Nb_0.2Mo_0.2V_0.2)C content. This work provided a potential that the high-entropy carbide can be applied as an effective reinforcing phase in the preparation of high-performance Ti(C_0.7N_0.3)-based cermets.     

Effect of carbon content on the microstructure and mechanical properties of high-entropy (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)Cx ceramics

High-entropy (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x ceramics, with different carbon contents (x=0.55?1), were prepared by spark plasma sintering using powders synthesized via a carbothermal reduction approach. Single-phase, high-entropy (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_x ceramics could be obtained when using a carbon content of x=0.70?0.85. Combined ZrO₂ and Mo-rich carbide phases, or residual graphite, existed in the ceramics due to either a carbon deficiency or excess at x=0.55 and 1, respectively. With the carbon content increased from x=0.70 to x=0.85, the grain size decreased from 4.36 ± 1.55 μm to 2.00 ± 0.91 μm, while the hardness and toughness increased from 23.72 ± 0.26 GPa and 1.69 ± 0.21 MPa·m^1/2 to 25.45 ± 0.59 GPa and 2.37 ± 0.17 MPa·m^1/2, respectively. This study showed that the microstructure and mechanical properties of high-entropy carbide ceramics could be adjusted by the carbon content. High carbon content is conducive to improving hardness and toughness, as well as reducing grain size.     

Hardness and toughness improvement of SiC-based ceramics with the addition of (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2

The influences of different contents ranging 0–15 wt% of high-entropy boride (HEB) (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ on the mechanical properties of SiC-based ceramics using Al₂O₃-Y₂O₃ sintering additives sintered by spark plasma sintering process were investigated in this study. The results showed that the introduction of 5 and 10 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ could facilitate the densification and the grain growth of SiC-based ceramics via the mechanism of liquid phase sintering. However, the grain growth of SiC-based ceramics was inhibited by the grain boundary pinning effect with the addition of 15 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂. The SiC-based ceramics with 15 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ showed the enhanced hardness (21.9±0.7 GPa) and high toughness (4.88±0.88 MPa·m^1/2) as compared with high-entropy phase-free SiC-based ceramics, which exhibited a hardness of 16.6 GPa and toughness of 3.10 MPa·m^1/2. The enhancement in mechanical properties was attributed to the addition of higher hardness of HEB phase, crack deflection toughening mechanism, and presence of residual stress due to the mismatch of coefficient of thermal expansion.     

First-principles prediction,fabrication and characterization of (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)B2 high-entropy borides

The microstructure and mechanical properties of (Hf_0.2Nb_0.2Ta_0.2Ti_0.2Zr_0.2)B₂ high-entropy boride (HEB) were first predicted by first-principles calculations combined with virtual crystal approximation (VCA). The results verified the suitability of VCA scheme in HEB studying. Besides, single-phase (Hf_0.2Nb_0.2Ta_0.2Ti_0.2Zr_0.2)B₂ ceramics were successfully fabricated using boro/carbothermal reduction (BCTR) method and subsequent spark plasma sintering (SPS); furthermore, the effects of different amounts of B₄C on microstructure and mechanical properties were evaluated. Due to the addition of B₄C and C, all samples formed single-phase solid solutions after SPS. When the excess amount of B₄C increased to 5 wt%, the sample with fine grains exhibited superior comprehensive properties with the hardness of 18.1 ± 1.0 GPa, flexural strength of 376 ± 25 MPa, and fracture toughness of 4.70 ± 0.27 MPa m^1/2. Nonetheless, 10 wt% excess of B₄C coarsened the grains and decreased the strength of the ceramic. Moreover, the nanohardness (34.5–36.9 GPa) and Young's modulus (519–571 GPa) values with different B₄C contents just showed a slight difference and were within ranges commonly observed in high-entropy diboride ceramics.     

The new complex high-entropy metal boron carbonitride: Microstructure and mechanical properties

In this contribution, the ternary BCN anion systems of high-entropy ceramics (HEC) are consolidated by hot-pressing sintering and the impacts of sintering temperature and the content of amorphous BCN addition on microstructural evolution and mechanical performance were evaluated. Results confirmed that high-entropy, oxide, and BN(C) phases were precipitated for (Ta_0.2Nb_0.2Zr_0.2Hf_0.2Ti_0.2)(B, C, N) ceramics after sintering at 1900°C. With the decrease of BCN addition, a new phase of MiB₂ (Mi representing the metal atoms) occurred. The Vickers hardness, bending strength, elastic modulus, and fracture toughness of the optimized bulk HECs were investigated, obtained at 24.5 ± 2.3 GPa, 522.0 ± 2.6 MPa, 478.9 ± 11.1 GPa, and 5.36 ± 0.56 MPa m^1/2, respectively.     

High-entropy (HfTaTiNbZr)C and (HfTaTiNbMo)C carbides fabricated through reactive high-energy ball milling and spark plasma sintering

Powders of high-entropy Hf_0.2Ta_0.2Ti_0.2Nb_0.2Zr_0.2C (HEC_Zr) and Hf_0.2Ta_0.2Ti_0.2Nb_0.2Mo_0.2C (HEC_Mo) carbides were fabricated through the reactive high-energy ball milling (R-HEBM) of metal and graphite particles. It was found that 60 min of R-HEBM is adequate to achieve a full conversion of the initial precursors into a FCC solid solution for both compositions. The HEC_Zr powder possesses a unimodal particle size distribution (40% d ≤ 1 μm, 95% d ≤ 10 μm), and the HEC_Mo powder features a bimodal distribution with a slightly larger particle size overall (30% d ≤ 1 μm, 80% d ≤ 10 μm). Bulk high-entropy ceramics with a minor presence of an oxide phase were fabricated through the spark plasma sintering of these high-entropy powders at 2000 °C with a 10 min dwelling time. The HEC_Zr ceramics possess a relative density of up to 94.8%, hardness of 25.7 ± 3.5 GPa, Young's modulus of 473 ± 37 GPa, and thermal conductivity of 5.6 ± 0.1 W/m·K. HEC_Mo ceramics with a relative density of up to 93.8%, hardness of 23.8 ± 2.7 GPa, Young's modulus of 544 ± 48 GPa, and thermal conductivity of 5.9 ± 0.2 W/m·K were also fabricated. A comparison of the properties of the HECs produced in this study and those previously reported is also provided.     

Microstructure and mechanical properties of high-entropy borides derived from boro/carbothermal reduction

Starting from metal oxides, B₄C and graphite, a suite of high-entropy boride ceramics, formulated (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)B₂, (Hf_0.2Zr_0.2Mo_0.2Nb_0.2Ti_0.2)B₂ and (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ derived from boro/carbothermal reduction at 1600 °C were fabricated by spark plasma sintering at 2000 °C. It was found that the synthetic high-entropy boride crystalized in hexagonal structure and the yield of the targeting phase was calculated to be over 93.0 wt% in the sintered ceramics. Benefitting from the nearly full densification (96.3% ˜ 98.5% in relative density) and the refined microstructure, the products exhibited the relatively high Vickers hardness. The indentation fracture toughness was determined to be comparable with the single transition metal-diboride ceramics. It should be noted that the formation of high-entropy boride ceramics were featured with the relatively high hardness at no expense of the fracture toughness.     

Effects of SiC nanowires and whiskers on high-entropy carbide-based composites sintered at low and high temperatures

This study aimed to investigate the toughening effects of SiC nanowires (SiC_nw) and SiC whiskers (SiC_w) on high-entropy carbide based composites prepared at different temperatures (1600°C and 2000°C). At low temperature (1600°C), SiC_nw and SiC_w maintain their original morphology and properties, and exhibit the good toughening effects. The SiC_nw with larger aspect ratio and more curly wires exhibit a much stronger toughening effect on the (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 composites reinforced with 15 vol.% SiC_nw, which shows the highest value of fracture toughness about 6.7 MPa∙m^1/2. However, at high sintering temperature (2000°C), SiC_nw and SiC_w are prone to thermal-induced damages, which significantly reduces their mechanical properties, and thus, toughening effects on (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 composites. The addition of SiC_w, which have better thermal stability at 2000°C, results in the (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8–15 vol.% SiC_w composite exhibiting relatively better fracture toughness, about 3.7 MPa∙m^1/2. Based on the results of the current study, the critical influence of SiC_nw and SiC_w on the toughening of (Ti_0.2Zr_0.2Nb_0.2Ta_0.2Mo_0.2)C_0.8 composites is highly dependent on their high-temperature thermal stability.     

Pressureless sintering and properties of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramics: The effect of pyrolytic carbon

A novel (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy ceramic was successfully prepared by pressureless sintering at 2200 °C. With increasing content of resin-derived-carbon, the density, and mechanical and thermal properties increased up to a maximum content of 2～4 wt% resin addition, after which further addition was detrimental. All specimens showed high strength (≥347±36 MPa), with the highest value achieving 450±64 MPa, and fracture toughness significantly higher (>20 %) than those of the corresponding monocarbides and Ta_0.5Hf_0.5C, (Ta_1/3Zr_1/3Nb_1/3)C. The thermal conductivity was approximately equivalent to the lowest value of the corresponding mono-carbides, which was assumed to be due to the lattice distortion effect.     

Synthesis and thermal stability of novel high-entropy metal boron carbonitride ceramic powders

High-entropy metal boron carbonitride ceramic powders including (Ta_0.2Nb_0.2Zr_0.2Hf_0.2W_0.2)BCN, (Ta_0.2Nb_0.2Zr_0.2Hf_0.2Ti_0.2)BCN, and (Ta_0.2Nb_0.2Zr_0.2Ti_0.2W_0.2)BCN, were successfully synthesized via mechanical alloying at room temperature. Results show that for the first step of 10 h milling, the amorphous BCN phases are observed. After 24 h of second step milling, the as-synthesized high-entropy ceramics exhibit a single face-centered cubic solid solution structure with high compositional uniformity from nano-scale to micron-scale. When heated to 1500 °C for 30min in flowing Ar, the as-prepared high-entropy ceramic powders still show relatively high thermal stability; however, some metals oxides like HfO₂ and ZrO₂ are detected due to the pre-existing oxides on sample surfaces. After heat treatment, some amorphous phases are still retained. This work suggests a new processing route on the synthesis of high-entropy metal boron carbonitride ceramics.     

Fine-grained dual-phase high-entropy ceramics derived from boro/carbothermal reduction

In the current work, fine-grained dual-phase, high-entropy ceramics (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)B₂-(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C with different phase ratios were prepared from powders synthesized via a boro/carbothermal reduction approach, by adjusting the content of B₄C and C in the precursor powders. Phase compositions, densification, microstructure, and mechanical properties were investigated and correlated. Due to the combination of pinning effect and the boro/carbothermal reduction approach, the average grain size (～0.5?1.5 μm) of the dual-phase high-entropy ceramics was roughly one order of magnitude smaller than previously reported literature. The dual-phase high-entropy ceramics had residual porosity ranging from 0.3 to 3.2 % upon sintering by SPS and the material with about 18 vol% boride phase exhibited the highest Vickers hardness (24.2±0.3 GPa) and fracture toughness (3.19±0.24 MPam^1/2).     

Additive manufacturing of continuous carbon fiber reinforced high entropy ceramic matrix composites via paper laminating,direct slurry writing,and precursor infiltration and pyrolysis

In this study, continuous carbon reinforced C_f/(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C–SiC high entropy ceramic matrix composites were additively manufactured through paper laminating (PL), direct slurry writing (DSW), and precursor infiltration and pyrolysis (PIP). (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C high entropy ceramic (HEC) powders were synthesized by pressureless sintering and ball milling. A certain proportion of HEC powder, SiC powder, water, binder, and dispersant were mixed to prepare the HEC-SiC slurry. Meanwhile, BN coating was prepared on the 2D fiber cloth surface by the boric acid-urea method and then the cloth was cut into required shape. Additive manufacturing were conducted subsequently. Firstly, one piece of the as-treated carbon fiber cloth was auto-placed on the workbench by paper laminating (PL). Then, the HEC-SiC slurry was extruded onto the surface of the cloth by direct slurry writing (DSW). PL and DSW process were repeated, and a C_f/HEC-SiC preform was obtained after 3 cycles. At last, the preform was densified by precursor infiltration and pyrolysis (PIP) and the final C_f/HEC-SiC composite was prepared. The open porosity of the C_f/HEC-SiC composites, with the HEC volume fractions of 15, 30 and 45%, were 7.7, 10.6, and 11.3%, respectively. And the density of the C_f/HEC-SiC composites, with the HEC volume fractions of 15, 30 and 45%, were 2.9, 2.7 and 2.3 g/cm³, respectively. The mechanical properties of the C_f/HEC-SiC composites increased firstly and then decreased with the HEC content increase, reaching the maximum value when the HEC volume fraction was 30%. The mechanical properties of the C_f/HEC-SiC composites containing 45, 30 and 15% HEC were as follows: flexural strength (180.4 ± 14 MPa, 183.7 ± 4 MPa, and 173.9 ± 4 MPa), fracture toughness (11.9 ± 0.17 MPa m^1/2, 14.6 ± 2.89 MPa m^1/2, and 11.3 ± 1.88 MPa m^1/2), and tensile strength (71.5 ± 4.9 MPa, 98.4 ± 12.2 MPa, and 73.4 ± 8.5 MPa). From this study, the additive manufacturing of continuous carbon fiber reinforced high entropy ceramic matrix composites was achieved, opening a new insight into the manufacturing of ceramic matrix composites.     

Thermophysical and mechanical properties of novel high-entropy metal nitride-carbides

In this work, a novel (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)(N_0.5C_0.5) high-entropy nitride-carbide (HENC-1) with multi-cationic and -anionic sublattice structure was reported and their thermophysical and mechanical properties were studied for the first time. The results of the first-principles calculations showed that HENC-1 had the highest mixing entropy of 1.151R, which resulted in the lowest Gibbs free energy above 600 K among HENC-1, (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)N high-entropy nitrides (HEN-1), and (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy carbides (HEC-1). In this case, HENC-1 samples were successfully fabricated by hot-pressing sintering technique at the lowest temperature (1773 K) among HENC-1, HEN-1 and HEC-1 samples. The as-fabricated HENC-1 samples showed a single rock-salt structure of metal nitride-carbides and high compositional uniformity. Meanwhile, they exhibited high microhardness of 19.5 ± 0.3 GPa at an applied load of 9.8 N and nanohardness of 33.4 ± 0.5 GPa and simultaneously possessed a high bulk modulus of 258 GPa, Young's modulus of 429 GPa, shear modulus of 176 GPa, and elastic modulus of 572 ± 7 GPa. Their hardness and modulus are the highest among HENC-1, HEN-1 and HEC-1 samples, which could be attributed to the presence of mass disorder and lattice distortion from the multi-anionic sublattice structure and small grain in HENC-1 samples. In addition, the thermal conductivity of HENC-1 samples was significantly lower than the average value from the “rule of mixture” between HEC-1 and HEN-1 samples in the range of 300-800 K, which was due to the presence of lattice distortion from the multi-anionic sublattice structure in HENC-1 samples.     

A novel high-entropy monoboride (Mo0.2Ta0.2Ni0.2Cr0.2W0.2)B with superhardness and low thermal conductivity

A new high-entropy monoboride (Mo_0.2Ta_0.2Ni_0.2Cr_0.2W_0.2)B ceramic with a WB-type orthogonal structure was designed and synthesised by in-situ reactive hot pressing at 2000 °C and 30 MPa for 1.5 h under an argon atmosphere. The microstructure of the sintered samples was comprehensively characterised, and the formation of a high-entropy monoboride (Mo_0.2Ta_0.2Ni_0.2Cr_0.2W_0.2)B ceramic was confirmed. Owing to the high density of the dislocations and strengthening metal-boron bonds, the high-entropy (Mo_0.2Ta_0.2Ni_0.2Cr_0.2W_0.2)B ceramic exhibited a hardness of 48.51 ± 4.07 GPa, which enabled it to be classed as a new superhard material. In addition, the thermal conductivity (2.05 ± 0.10 W/(m·K) at 400 °C) and electric conductivity (132.30 S/cm) were determined.     

Fabrication and properties of Cf/(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C-SiC high-entropy ceramic matrix composites via precursor infiltration and pyrolysis

In this work, C_f/(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C-SiC high-entropy ceramic matrix composites were reported for the first time. Based on the systematic study of the pyrolysis and solid-solution mechanisms of (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C precursor by Fourier transform infrared spectroscopy, TG-MS and XRD, C_f/(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C-SiC with uniform phase and element distribution were successfully fabricated by precursor infiltration and pyrolysis. The as-fabricated composites have a density and open porosity of 2.40 g/cm³ and 13.32 vol% respectively, with outstanding bending strength (322 MPa) and fracture toughness (8.24 MPa m^1/2). The C_f/(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C-SiC composites also present excellent ablation resistant property at a heat flux density of 5 MW/m², with linear and mass recession rates of 2.89 μm/s and 2.60 mg/s respectively. The excellent combinations of mechanical and ablation resistant properties make the C_f/(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C-SiC composites a new generation of reliable ultra-high temperature materials.     

Microstructures and mechanical properties of high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C ceramics with the addition of SiC secondary phase

The relationships between microstructures and mechanical properties especially strength and toughness of high-entropy carbide based ceramics are reported in this article. Dense (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C (HEC) and its composite containing 20 vol.% SiC (HEC-20SiC) were prepared by spark plasma sintering. The addition of SiC phase enhanced the densification process, resulting in the promotion of the formation of the single-phase high-entropy carbide during sintering. The high-entropy carbide phase demonstrated a fast grain coarsening but SiC particles remarkably inhibited this phenomena. Dense HEC and HEC-20SiC ceramics sintered at 1900 °C exhibits four-point bending strength of 332 ± 24 MPa and 554 ± 73 MPa, and fracture toughness of 4.51 ± 0.61 MPa·m^1/2 and 5.24 ± 0.41 MPa·m^1/2, respectively. The main toughening mechanism is considered to be crack deflection by the SiC particles.     

Enhancing cutting performance of Ti(C,N)-based cermet tools on nodular cast iron by incorporating high-entropy carbide

In this study, we investigated the cutting performance and wear mechanisms of Ti(C,N)-based cutting tools containing varying weight percentages (0%, 5%, 10%, and 15%) of high-entropy carbide (HEC) (V_0.2Nb_0.2 Mo_0.2Ta_0.2W_0.2)C phase, when used for turning nodular cast iron. According to the turning test results, the cermet cutting tools containing 0 wt%, 5 wt%, 10 wt%, and 15 wt% HEC phases demonstrated effective cutting lives of 402, 720, 632, and 465 s, respectively. The tool with 5 wt% HEC phase showed the best cutting performance. When cutting nodular cast iron with cermet cutting tools, the main wear mechanisms observed were diffusion, oxidation, adhesion, and abrasion on the flank surface, along with diffusion, oxidation, and abrasion on the rake surface. The results of this study indicated that (V_0.2Nb_0.2Mo_0.2Ta_0.2W_0.2)C could be adopted as an effective reinforced phase in the cermet cutting tools.     

High-pressure sintering of ultrafine-grained high-entropy diboride ceramics

Herein the ultrafine-grained (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ high-entropy diboride ceramics were successfully fabricated by high-pressure sintering technology for the first time. The results showed that the grain size, relative density, and Vickers hardness of the as-fabricated samples all increased gradually with increasing sintering temperatures from 1373 K to 1973 K. The relative density and mean grain size of the as-sintered samples at 1973 K were 97.2% and 684 nm, respectively, and simultaneously they exhibited excellent comprehensive mechanical properties, combining a Vickers hardness of 26.2 GPa and a fracture toughness of 5.3 MPa·m^1/2, which were primary attributed to the fine grain strengthening mechanism and microcrack deflection toughening mechanism.     

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

Spherical (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.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 (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ powders are first synthesized via a borothermal reduction process using a mixture of individual metallic oxides and boron powders as starting materials. The influence of boron powder content on the structure of prepared powders is researched. Then, (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ granules are prepared after wet-grinding and spray-drying process, which exhibit a spherical shape and homogeneous element 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 (Zr_.2Ti_.2Ta_.2Nb_.2Mo_.2)B₂ powders may guide the way toward the preparation of many other spherical high-entropy diboride powders.     

The effect of submicron grain size on thermal stability and mechanical properties of high-entropy carbide ceramics

(Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy ceramics (HEC) with a submicron grain size of 400 to 600 nm were fabricated by spark plasma sintering using a two-step sintering process. Both X-ray and neutron diffractions confirmed the formation of single-phase with rock salt structure in the as-fabricated (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C samples. The effect of submicron grain size on the thermal stability and mechanical properties of HEC was investigated. The grain growth kinetics in the fine-grained HEC was small at 1300 and 1600°C, suggesting high thermal stability that was possibly related to the compositional complexity and sluggish diffusion in HEC. Compared to the coarse-grain HEC with a grain size of 16.5 µm, the bending strength and fracture toughness of fine-grained HEC were 25% and 20% higher respectively. The improvement of mechanical properties in fine-grained HEC may be attributed to micromechanistic mechanisms such as crack deflection.