High-entropy Carbides Research Articles

Mechanical and tribological performance of (TiNbTaWMo)C high-entropy ceramics in a wide temperature range

High-entropy carbide ceramics have attracted extensive attentions for potential applications as high temperature structural materials, the mechanical and tribological performance of (TiNbTaWMo)C high-entropy carbides over wide temperature range combined with the oxidation behavior were investigated in the present study. (TiNbTaWMo)C high-entropy carbides were successfully prepared from five kinds of carbide powders by spark plasma sintering at 2000 °C. The mechanical properties of (TiNbTaWMo)C and the tribological performance at room temperature –800 °C were studied. The Vickers hardness, bending strength and fracture toughness of the sintered (TiNbTaWMo)C high-entropy carbides are 21.31 ± 0.43 GPa, 477.15 ± 34.17 MPa, and 6.41 ± 0.85 MPa m1/2, respectively. At room temperature, the friction coefficient is stable at 0.35, and the wear rate is as low as 3.39 × 10−7 mm3/Nm. With temperature increasing, friction coefficient increases firstly and decreases above 600 °C, which is contributed by the formation of oxide films at high temperatures. The wear mechanisms of (TiNbTaWMo)C high-entropy ceramics at different temperatures were discussed in detail by microstructures and phase analysis of the worn surfaces together with the oxidation behaviors of (TiNbTaWMo)C at 600 °C and 800 °C.

Unveiling the Transporting Mechanism of (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)C at Room Temperature

High-entropy materials have been widely researched in recent years, and more work on their thermal and electrical properties is still needed. Herein, we fabricate a high-entropy carbide (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)C ceramic and report the thermal and electrical conductivity at room temperature using first-principles calculations and experiments. The movement of phonons is suppressed in high-entropy carbides when analyzing the thermal and electrical conductivity at room temperature, but the movement of electrons is not. After the first-principles calculations on the electronic structure and lattice vibration and experiments, we give the reasons why the rule of mixture can predict electrical conductivity but not thermal conductivity at room temperature. Finally, we outline the cause of the similar lattice patterns between TaC and (Ti0.2Zr0.2Nb0.2Hf0.2Ta0.2)C.

Processing and Mechanical Properties of (Hf-Ta-Zr-Nb-Ti)C

The influence of the processing route on the microstructure characteristics and mechanical properties of high-entropy carbides was investigated. The experimental materials were prepared by a combination of ball milling and spark plasma sintering at the temperature of 2100 °C. The effect of sintering time (5 min, 10 min, and 20 min) on the microstructure development and properties of the investigated systems has been studied. The microstructure analysis was performed using scanning electron microscopy + EDAX analysis on the polished surfaces. Nanoindentation was used for the nano-hardness and Young’s modulus measurements. Micro-hardness was measured using Vickers method and indentation method using a Vickers indenter was used to determine the indentation fracture resistance – fracture toughness. The measured grain size values were in the range from 5 µm to 14 µm. The nano-hardness values changed from 36 GPa to 39 GPa and the indentation modulus of elasticity from 558 GPa to 577 GPa, respectively. The measured micro-hardness values were in the range from 18.93 GPa to 21.95 GPa and the indentation fracture resistance changed from 2.70 MPa.m1/2 to 3.50 MPa.m1/2, respectively.

A (Hf0.2Mo0.2Nb0.2Ta0.2Ti0.2)C system high-entropy ceramic with excellent mechanical and tribological properties prepared at low temperature

It is a substantial challenge to prepare high-performance high-entropy carbides at low temperature. In this study, a system of (Hf0.2Mo0.2Nb0.2Ta0.2Ti0.2)C high-entropy ceramics with excellent mechanical and tribological properties were prepared at low temperature by using mixture with Aluminum. The results indicate that sintering temperature of specimen with 7 wt% Aluminum (HEC7A) can be reduced by 150 °C. Meanwhile, the synergy effect of the solid solution hardening and grain refinement enables the samples with a higher microhardness (≥18.1 GPa). Moreover, the addition of Aluminum can enhance the wear resistance. Especially for HEC7A at 900 °C, the wear rate is three times less than that of the pure sample. The primary anti-wear mechanism is the formation of Al2O3 tribo-films by the tribo-chemical reaction.

ADASYN-assisted machine learning for phase prediction of high entropy carbides

The lack of appropriate data and data imbalance hindered the development of ML models for identifying novel high-entropy ceramics. To circumvent data imbalance for ML-based ceramic design, we build a semi-synthetic database of high entropy carbides using literature data, atomic environment mapping-based structure plots and adaptive synthetic sampling (ADASYN) technique. A 5-fold cross-validated kNN classifier was trained on both original and balanced datasets. The kNN model trained on a balanced dataset has 95% testing accuracy while controlling for overfitting. SHAP describes the relationship between characteristics and goal variables. This paper shows a new ML approach with a decreased bias to predict high-entropy single-phase carbides preemptively.

Synthesis of high-entropy carbides from multi-metal polymer precursors

Three compositions of high-entropy carbides, (TiHfVNbTa)C, (TiZrHfNbTa)C and (TiZrNbTaW)C were synthesized via a modified Pechini process, in which citric acid served as both a cation chelating agent and a carbon source. Through pyrolysis and spark plasma sintering, single phase high-entropy carbides were formed from the homogeneous precursors at a relatively low temperature of 1800 °C. The dispersion of cations in the polymer precursor facilitated shorter diffusion distances in polymer-derived materials, and thus compositional homogeneity was significantly improved relative to materials produced by a solid-state method, as quantified by a defined coefficient of variation applied to energy-dispersive X-ray spectroscopy. A finer microstructure in polymer-derived materials results in improved fracture toughness with a KIC value of 4.29 MPa·m1/2 achieved for (TiHfVNbTa)C.

Electrolysis Synthesis of Carbides and Carbon Dioxide Capture in Molten Salts.

The application of carbides in catalysis, batteries, aerospace fields, etc. has been continuously expanded and deepened, which is attributed to the diversified physicochemical properties of carbides via a tune-up of their morphology, composition, and microstructure. The emergence of MAX phases and high entropy carbides with unparalleled application potential undoubtedly further stimulates the research upsurge of carbides. The traditional pyrometallurgical or hydrometallurgical synthesis of carbides inevitably faces the shortcomings of complex process, unacceptable energy consumption, extreme environmental pollution, and beyond. The molten salt electrolysis synthesis method with the superiorities of straightforward route, high efficiency, and environmental friendliness has demonstrated its validity in the synthesis of various carbides, which naturally initiates more research. In particular, the process can achieve CO2 capture while synthesizing carbides based on the excellent CO2 capture capability of some molten salts, which is of great significance for carbon neutralization. In this paper, the synthesis mechanism of carbide by molten salt electrolysis, the process of CO2 capture and carbides conversion, the latest research progress in the synthesis of binary, ternary, multi-component, and composite carbides are reviewed. Finally, the challenges, development perspectives, and research directions of electrolysis synthesis of carbides in molten salts are featured.

High-entropy carbides designed to resist cavitation erosion-corrosion in an acidic environment: Surface engineering guided by first-principles calculations and experiments

In this study, the influence of chemical composition on phase stability, mechanical and electronic properties of (TixZr0.2Nb0.2Ta0.2Mo0.4-x)C (x = 0.3, 0.2, 0.1) high entropy carbide ceramics (HECCs) were first explored by first-principles calculations. The results showed that the composition (Ti0.3Zr0.2Nb0.2Ta0.2Mo0.1)C has higher values of electron work function (EWF) value and theoretical hardness than that of the other HECCs with lower Ti concentrations. Following this investigation, a (Ti0.3Zr0.2Nb0.2Ta0.2Mo0.1)C HECC coating was deposited onto a commercially pure titanium substrate by a reactive-sputtering method. The as-prepared HECC coating had a uniform thickness of ∼17 μm and consisted of equiaxed nanocrystals with a mean diameter of ∼7 nm. The cavitation erosion-corrosion behavior of the HECC coating was studied by combining electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) measurements in a 0.05 M H2SO4 solution. The EIS results revealed that with increasing time of cavitation exposure, the resistance value of the HECC coating was an order of magnitude larger than that for uncoated CP-Ti for a given cavitation time. EN analysis indicated that under cavitation conditions, uncoated CP-Ti was more vulnerable to localized corrosive attack than the HECC coating in an acidic environment.

Electromagnetic wave absorbing properties of high-entropy transition metal carbides powders

The electromagnetic wave absorption performance of high-entropy transition metal carbides, including (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C, (Ti0.2Zr0.2Mo0.2Nb0.2Ta0.2)C and (Ti0.2Zr0.2Cr0.2Nb0.2Ta0.2)C, was investigated in the frequency from 2 GHz to 18 GHz. The electromagnetic parameter measurement and subsequent electromagnetic wave absorption evaluation showed that the high-entropy transition metal carbides possessed good electromagnetic wave absorbing properties, which had a great relation to the solid-solution elements. The (Ti0.2Zr0.2Mo0.2Nb0.2Ta0.2)C with a 1.50 mm thickness displayed a minimal reflection loss of -32.1 dB, and the effective absorption bandwidth (<-10 dB) was in the frequency range of 13–18 GHz. Intriguingly, the minimum reflection loss reached -36.6 dB at the thickness of 2.50 mm. The electromagnetic wave absorption performance was mainly associated with conductivity loss and dipole polarization, which originated from intrinsic conductivity of the high-entropy carbides and lattice distortion resulting from solid solution of multiple elements, respectively. The experimental results show that the high-entropy transition metal carbides may be promising structural-functional integrated ceramics in wide applications.

Synthesis and mechanical and elevated temperature tribological properties of a novel high-entropy (TiVNbMoW)C 4.375 with carbon stoichiometry deviation

High-entropy carbides are a nascent group of ceramics that are promising for high-temperature applications due to the combination of good stability, high hardness (<i>H</i>), high strength, and superior creep resistance that they display. Due to high melting points and low lattice diffusion coefficients, however, the high-entropy carbides are usually difficult to consolidate to a nearly full density. To cope with this challenge, herein, binary carbides including TiC, V₈C₇, NbC, Mo₂C, and WC with different carbon stoichiometry were used to prepare dense high-entropy (TiVNbMoW)C_4.375, and the influence of carbon vacancy on formation ability and mechanical properties of carbon-deficient high-entropy (TiVNbMoW)C_4.375 were investigated. Intriguingly, although the starting binary carbides have different crystal structures and carbon stoichiometry, the as-prepared high-entropy material showed a rock-salt structure with a relatively high density (98.1%) and good mechanical properties with hardness of 19.4±0.4 GPa and fracture toughness (<i>K</i>_IC) of 4.02 MPa·m^1/2. More importantly, the high-entropy (TiVNbMoW)C_4.375 exhibited low coefficient of friction (COF) at room temperature (RT) and 800 ℃. Wear rate (<i>W</i>) gradually increased with the temperature rising, which were attributed to the formation of low-hardness oxidation films at high temperatures to aggravate wear. At 800 ℃, lubricating films formed from sufficient oxidation products of V₂O₅ and MoO₃ effectively improved tribological behavior of the high-entropy (TiVNbMoW)C_4.375. Wear mechanisms were mainly abrasive wear resulting from grain pullout and brittle fracture as well as oxidation wear generated from high-temperature reactions. These results are useful as valuable guidance and reference to the synthesis of high-entropy ceramics (HECs) for sliding parts under high-temperature serving conditions.

Carbon-deficient high-entropy (Zr0.17Nb0.2Ta0.2Mo0.2W0.2)C0.89: A potential high temperature and vacuum wear-resistant material

High-entropy carbides are promising materials as sliding structural components in ultra-high temperature environments due to their good phase stability and excellent mechanical properties. However, the tribological behaviors under coupling conditions of high temperature and vacuum environment have not been deeply evaluated. To fill such a gap, the high-temperature tribological properties of carbon-deficient high-entropy (Zr0.17Nb0.2Ta0.2Mo0.2W0.2)C0.89 (HEC0.89) prepared by using five binary carbides (ZrC, NbC, TaC, Mo2C, and WC) as starting materials were investigated. The microhardness and fracture toughness of HEC0.89 were 21.3 ± 0.5 GPa and 4.5 ± 0.2 MPa·m1/2. Most importantly, HEC0.89 exhibited low wear rate (lower than an order of 10-6 mm3/Nm) under vacuum environments from room temperature (RT) to high temperatures. Moreover, the lowest friction coefficient (0.31 ± 0.01) was obtained at 900 °C, which is attributed to the transferring of films consisting of low hardness oxidation products. These results clearly demonstrate the prospective for the application of high-entropy carbides as sliding parts in high-temperature and vacuum environments.

Machine learning-driven synthesis of TiZrNbHfTaC5 high-entropy carbide

Synthesis of high-entropy carbides (HEC) requires high temperatures that can be provided by electric arc plasma method. However, the formation temperature of a single-phase sample remains unknown. Moreover, under some temperatures multi-phase structures can emerge. In this work, we developed an approach for a controllable synthesis of HEC TiZrNbHfTaC5 based on theoretical and experimental techniques. We used Canonical Monte Carlo (CMC) simulations with the machine learning interatomic potentials to determine the temperature conditions for the formation of single-phase and multi-phase samples. In full agreement with the theory, the single-phase sample, produced with electric arc discharge, was observed at 2000 K. Below 1200 K, the sample decomposed into (Ti-Nb-Ta)C, and a mixture of (Zr-Hf-Ta)C, (Zr-Nb-Hf)C, (Zr-Nb)C, and (Zr-Ta)C. Our results demonstrate the conditions for the formation of HEC and we anticipate that our approach can pave the way towards targeted synthesis of multicomponent materials.

Ablation threshold and temperature dependent thermal conductivity of high entropy carbide thin films

High entropy carbides (HECs) are a promising new class of ultra-high temperature ceramics that could provide novel material solutions for leading edges of hypersonic vehicles, which can reach temperatures &gt; 3,500 �C and experience extreme thermal gradients. Although the mechanical and thermal properties of HECs have been studied extensively at room temperature, few works have examined HEC properties at high temperatures or considered these materials� responses to thermal shock. In this work, we measure the thermal conductivity of a five-cation HEC up to 1200 �C. We find that thermal conductivity increases with temperature, consistent with trends demonstrated in single-metal carbides. We also measure thermal conductivity of an HEC deposited with varying CH4 flow rate, and find that although thermal conductivity is reduced when carbon content surpasses stoichiometric concentrations, the films all exhibited the same temperature dependent trends regardless of carbon content. To compare the thermal shock resistance of HECs with a refractory carbide, we conduct pulsed laser ablation measurements to determine the fluence threshold the HECs can withstand before damaging. We find that this metric for the average bond strength trends with the theoretical hardness of the HECs as expected.

Features of SHS of multicomponent carbides

Combustion of powders of transition metals of titanium PTS (average particle size 57 μm), zirconium PCRK-1 (12 μm), tantalum Ta PM-3 (8 μm), hafnium GFM-1 (4 μm), niobium NBP-1a (21 μm) with carbon black grade P-803 dispersion 1–2 μm was studied. The combustion process of the compressed samples (mass 2.5–6.9 g, height 1.2–1.7 cm, relative density 0.55–0.61) was performed in an inert argon medium at a pressure of 760 mmHg in the constant pressure chamber. Combinations were studied, Me1 + Me2 + Me3 + Me4 + 4C, Me1 + Me2 + Me3 + Me4 + Me5 + 5C. XRD patterns of the mixtures were recorded on a DRON-3М diffractometer (CuKα-radiation). Combustion product sections were studied using a LEO 1450 VP scanning electron microscope (Carl Zeiss, Germany). The fractional composition and particle size distribution of the mixture were determined according to standard procedure using a Microsizer-201C laser particle size analyzer. Combustion velocity, elongation of samples, phase composition of products were determined. The maximum combustion temperature of the mixture (Ti + Hf + Zr + Nb + Ta) + 5C was measured experimentally for the first time. The morphology and microstructure of the reaction products were also observed. Combustion products of mixtures (Ti + Zr + Nb + Ta) + 4C and (Ti + Zr + Nb + Hf) + 4C contain high entropy carbides, which are solid solutions with the same structural type B1 (space group Fm-3m) and having different cell parameters. Product samples of mixtures (Ti + Zr ++ Hf + Ta) + 4C and (Ti + Hf + Zr + Nb + Ta) + 5C contain high entropy and medium entropy carbides, also representing solid solutions with the same structural type B1 (space group Fm-3m). The results of this work can be used in the production of high-entropy and medium-entropy multicomponent carbides.

Low thermal conductivity of dense (TiZrHfVNbTa)Cxhigh-entropy carbides by tailoring carbon stoichiometry

Transition metal carbides are promising candidates for thermal protection materials due to their high melting points and excellent mechanical properties. However, the relatively high thermal conductivity is still a major obstacle to its application in an ultra-high-temperature insulation system. In this work, the low thermal conductivity of dense (TiZrHfVNbTa)C_<i>x</i> (<i>x</i> = 0.6–1) high-entropy carbides has been realized by adjusting the carbon stoichiometry. The thermal conductivity gradually decreases from 10.6 W·m⁻¹·K⁻¹ at room temperature to 6.4 W·m⁻¹·K⁻¹ with carbon vacancies increasing. Due to enhanced scattering of phonons and electrons by the carbon vacancies, nearly full-dense (97.9%) (TiZrHfVNbTa)C_0.6 possesses low thermal conductivity of 6.4 W·m⁻¹·K⁻¹, thermal diffusivity of 2.3 mm²·s⁻¹, as well as electrical resistivity of 165.5 μΩ·cm. The thermal conductivity of (TiZrHfVNbTa)C_0.6 is lower than that of other quaternary and quinary high-entropy carbide ceramics, even if taking the difference of porosity into account in some cases, which is mainly attributed to compositional complexity and carbon vacancies. This provides a promising route to reduce the thermal conductivity of high-entropy carbides by increasing the number of metallic elements and carbon vacancies.

Machine learning of carbon vacancy formation energy in high-entropy carbides

Carbon vacancies as intrinsic defects in high-entropy carbides (HECs) strongly affect their physical and chemical properties, it is important to understand the details of vacancy formation at an atomic level. In this work, 1280 carbon vacancies are investigated via high-throughput density functional theory (HT-DFT) calculations and machine learning (ML) models on their formation energies. We found that more than half of the carbon vacancies are preferentially surrounded by three types of cations, with Hf less likely to be present. The role of metal cations in promoting the formation of carbon vacancies is: Nb＞Ta＞Zr＞Hf, indicating the possibility to modify the carbon vacancy concentration by change the cation elemental ratios. A random forest model was trained on DFT results to predict vacancy formation energies with a mean absolute error of 54 meV. The feature importance analysis is consistent with the analysis of the DFT results.

Indentation size effect in the hardness measurements of high entropy carbides

The indentation load-size effect was investigated during hardness testing of high entropy carbides with different hardness, applying indentation loads from 50 mN to 10 N. The experimental systems were recently developed (Hf-Ta-Zr-Nb-Ti)C and (Mo–Nb–Ta–V–W)C high-entropy carbides, prepared by ball milling and a spark plasma sintering, with single-phase and high relative density of 99.4% and 99.0%, respectively. The load dependence of hardness was analysed using the traditional Meyer's law, the proportional specimen resistance model and the modified proportional specimen resistance model. The best correlation (R2> 0.99) between the measured values and the used models was achieved using the modified proportional specimen resistance model. This resulted in the so-called load-independent hardness of (Hf-Ta-Zr-Nb-Ti)C and (Mo–Nb–Ta–V–W)C systems 21.96 and 14.81 GPa in the case of analyses for 50 mN–10 N load range and 25.42 GPa and 14.13 GPa in the case of 50 mN–1 N load range, respectively. The different deformation and damage mechanisms detected as a potential reason for the origin of the load-size effect were microcracks at grain boundaries during the micro/macro-indentation and plastic deformation/nanocrack formation at the indents during nano/micro-indentation.

Synthesis and oxidation behavior of the materials based on carbon fibers and ultra-high temperature binary and high-entropy carbides

The paper reports the results obtained in the experimental studies of the oxidation behavior of powders based on carbon fibers (graphite) and carbides of metals of groups IV and V of the periodic table, including high-entropy carbide (HEC) TiZrNbHfTaC5. The materials were synthesized by vacuum-free electric arc method using X-ray fibrous amorphous carbon and metal oxides as a starting material. X-ray diffractometry of the synthesis products identified graphite, titanium, zirconium, niobium, hafnium, and tantalum carbides, and HEC TiZrNbHfTaC5. Scanning electron microscopy of the synthesis products showed carbon fibers with the surface modified by the above-mentioned carbides. Differential thermal analysis was performed to determine the parameters of the oxidation process of these materials; the main difference between the synthesized materials is the oxidation rate, with the lowest one demonstrated by the high-entropy carbide containing material.

Machine learning-assisted mechanical property prediction and descriptor-property correlation analysis of high-entropy ceramics

High-entropy ceramics have attracted extensive attention due to their unique properties. However, the development of novel ceramics has been hindered by extensive trial-and-error strategies, along with insufficient knowledge and computational power. In this work, we develop machine learning (ML) models based on the chemical attributes of constituent elements and metal carbides of high-entropy carbides (HECs) for predicting their related Young's modulus, hardness and wear resistance values. Our models demonstrate low mean absolute errors (15.3 GPa for modulus and 1.1 GPa for hardness), high R2 scores (0.969 and 0.963) and excellent agreement with experimental measurements, indicating high model robustness. We further establish a database of 230,230 HECs and analyse the correlations between chemical descriptors and their properties, especially for those containing transition metals from Groups IV, V and VI. Our models can rapidly explore the mechanical properties of HECs and help guide descriptor-property correlation analysis in a low-cost and reliable manner, which provides an efficient method for accelerating the design of novel high-entropy materials with desired performance characteristics.

Plasmonic high-entropy carbides

Discovering multifunctional materials with tunable plasmonic properties, capable of surviving harsh environments is critical for advanced optical and telecommunication applications. We chose high-entropy transition-metal carbides because of their exceptional thermal, chemical stability, and mechanical properties. By integrating computational thermodynamic disorder modeling and time-dependent density functional theory characterization, we discovered a crossover energy in the infrared and visible range, corresponding to a metal-to-dielectric transition, exploitable for plasmonics. It was also found that the optical response of high-entropy carbides can be largely tuned from the near-IR to visible when changing the transition metal components and their concentration. By monitoring the electronic structures, we suggest rules for optimizing optical properties and designing tailored high-entropy ceramics. Experiments performed on the archetype carbide HfTa4C5 yielded plasmonic properties from room temperature to 1500K. Here we propose plasmonic transition-metal high-entropy carbides as a class of multifunctional materials. Their combination of plasmonic activity, high-hardness, and extraordinary thermal stability will result in yet unexplored applications.