Refractory Metal Carbides Research Articles

(Digital Presentation) Electrochemical Conversion of CO2 into Tungsten Carbides in Molten Salts

Huge amounts of anthropogenic emissions of the greenhouse gas carbon dioxide into the Earth's atmosphere are one of the key factors causing global warming. To mitigate the consequences of the severe climate changes caused by this phenomenon, over the last two decades great efforts of researchers have been directed towards the development of sustainable, environmentally friendly, carbon neutral and, if possible, not very expensive (in terms of used energy and inexpensive consumables) technologies for capture, conversion and storage (CCS) of CO2.Electrochemical conversion of CO2 using molten salts can rightfully be classified as CCS technology. In this case, carbon dioxide from various sources of its generation (fossil fuel power plants, industrial enterprises with a high carbon footprint) can be captured by molten salt (as a result of its physical dissolution, or chemical absorption by molten salt), and then electrochemically be converted into high value-added carbon-containing compounds: (a) carbon monoxide [1]; (b) carbon allotropes of various structures and modifications [2]; (c) refractory metal carbides [3], and various composites based on them. The reaction path and composition of the cathode products will depend on the electrolysis conditions. Elemental carbon synthesis precursor can be – carbon dioxide, directly dissolved in the molten salt mixture (direct reduction of CO2), as well as the carbonate anion, formed as a result of carbon dioxide interaction with oxide ions which are presented in the electrolyte bath (indirect reduction of CO2).This work presents the result of research concerning the electrochemical synthesis of the powders of tungsten carbides (WC and W2C) in chloride melt NaCl-KCl (1:1) under carbon dioxide pressure at the temperature range 700 – 800 оС. Refractory metal precursors are its oxy-anions (WО3; W2O7 2-; Меn x[WO4]nx-2; WO3F3 3- where Me – Na; K; Li; Mg; Ca; n – valance of metal Me). The formation of the new forms of tungsten electrochemical active particles in electrolyte is realized by the changing (control) of acidity of the melt. Carbon source is CO2, which was introduced into the electrolyzer under the excessive pressure of 0.1 – 1.7 MPa. The creation of excessive gas pressure is necessary condition for the increasing of the rate of electrolytic process (current densities) throw the rise of CO2 solubility in chloride melt. The general scheme of the high-temperature synthesis of tungsten carbides by the method of Molten Salt Carbon Electrochemical Transformation (MS-CCT) is presented in Fig. 1.The electrochemical investigations of partial and joint electroreduction of tungsten carbide precursors were carried out by the method of cyclic voltammetry. The areas of potentials and current densities, where the joint electrochemical discharge of tungsten carbide precursors (a narrow range of deposition potentials) occurs up to refractory metal and carbon takes place were found.Electrolytical synthesis of nano-sized (10 – 30 nm) powders of tungsten carbides (WC, W2C) and composites WC-C (up to 5 wt % of free carbon); W2C-WC; WC-C-Pt was carried out from the melts of different chemical composition; and the characterization of obtained products was fulfilled by the methods of chemical analysis, X-ray diffraction, DTG, BET adsorption, scanning and transmission electron microscopy.Synthesized composite materials based on tungsten carbides of various compositions were investigated as a cathode material in the reaction of electrolytic splitting of water for hydrogen production in a sulfuric acid solution [4]. The obtained results showed that the best activity has a composite of tungsten monocarbide WC with a content of free carbon up to 5 wt.%. The hydrogen onset potential for this electrode is -0.02 V, the overvoltage of hydrogen release at ik = 10 mA/cm2 is -110 mV, the exchange current is 7.0×10-4 A/cm2, the Tafel slope – -85 mV/dec.The presence of free carbon on the surface of tungsten carbides electrode improves its catalytic activity, increasing the area of the active surface. The catalytic activity of electrodes made of tungsten monocarbide increases with the introduction of platinum (up to 10 wt %) into the composite.References Kaplan V, Wachtel E, Gartsman K et al (2010) Conversion of CO2 to CO by electrolysis of molten lithium carbonate. J Electrochem Soc 157:B552–B556.Novoselova I.A., Kushkhov Kh.B., Malyshev V.V., Shapoval V.I. (2001) Theoretical foundations and implementation of high-temperature electrochemical synthesis of tungsten carbides in ionic melts. Theor. Found. Chem. Eng. 35:175–187.Novoselova I.A., Kuleshov S.V., Volkov S.V. et al (2016) Electrochemical synthesis, morphological and structural characteristics of carbon nanomaterials produced in molten salts. Electrochim Acta 211:343–355.Novoselova I, Kuleshov S, Fedoryshena E et al (2018) Electrochemical synthesis of tungsten carbide in molten salts, its properties and applications. ECS Trans 86:81–94. Figure 1

PRODUCING METHOD OF HIGH ENTROPY CARBIDE IN AN IONIC MELT

Refractory metal carbides TiC, ZrC, HfC, NbC and TaC have excellent physical, chemical and mechanical properties as materials for ultra-high temperature ceramics. Of these, the most refractory are TaC and HfC, whose melting points approach 4000°C. It should be noted the high hardness, strength and wear resistance of refractory carbides. Hence, there is a natural interest in high-entropy carbides based on them, which are becoming an important class of new ceramic materials, since they potentially have more advanced applied properties. However, obtaining such materials by classical metallurgical methods is a difficult task. In modern research, samples of high-entropy carbides are most often synthesized using expensive special equipment (methods of plasma-spark sintering, high-energy planetary mills, etc.) and a relatively long preparation of precursors for sample production. This paper describes a new approach to the synthesis of multicomponent carbide (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C using an electrochemical process at a temperature not exceeding 1173 K. The method is based on the phenomenon of currentless metal transfer in molten salts. After the step-by-step transfer of metals, the sample was washed from the electrolyte, then sintered in a vacuum furnace. According to X-ray phase analysis, the resulting high-entropy carbide is a single-phase solid solution with an FCC structure. The diffraction pattern of the synthesized sample is in good agreement with the calculated diffraction pattern obtained by the Debye formula for a supercell of 64 000 atoms. A compact sample of high-entropy carbide was produced by pressing a tablet 10 mm in diameter with the addition of cobalt as a matrix metal. After vacuum sintering, the sample was ground to prepare for examination on a scanning electron microscope. Elemental mapping of the sample surface was performed, which showed a satisfactory distribution of metals that make up the high-entropy carbide. The measured microhardness of the sample turned out to be less than the values found in the publications of other authors, which may be due to some residual sample porosity.

Spark plasma sintering of Ti(C, N)-based cermet tool material doped with refractory rare metal carbides (TaC/NbC/VC): Core-rim structure, grains and mechanical properties

The secondary carbides have great impacts on microstructure and mechanical properties of Ti(C, N)-based cermets. In the study, Ti(C, N)-based cermets doped with different refractory rare metal carbides TaC/NbC/VC which was used for cutting tools were prepared by spark plasma sintering. The effects of TaC, NbC and VC on core-rim structure, grain size and mechanical properties of the cermets were studied. TaC/NbC/VC promotes the growth of solid solution phase, reduces the size of black core, increases the intergranular fracture and decreases the cleavage fracture. The solid solution effect and grain refinement effect of VC are stronger than that of NbC, while NbC is superior to TaC. Adding appropriate TaC/NbC/VC can improve the mechanical properties. The best comprehensive mechanical properties of Ti(C, N)-based cermets are achieved with the addition of 3 wt% VC, with a relative density of 99.5%, Vickers hardness of 18.4 ± 0.3GPa, fracture toughness of 10.7 ± 1.3 MPa·m1/2, and flexural strength of 947.4 ± 12.6 MPa.

Electrochemical Synthesis of Functional Coatings and Nanomaterials in Molten Salts and Their Application

Nanomaterials are widely used in modern technologies due to their unique properties. Developing methods for their production is one of the most important scientific problems. In this review, the advantages of electrochemical methods for synthesis in molten salts of nanostructured coatings and nanomaterials for different applications were discussed. It was determined that the nanostructured Mo2C coatings on a molybdenum substrate obtained by galvanostatic electrolysis have a superior catalytic activity for the water-gas shift reaction. The corrosion-resistant and wear-resistant coatings of refractory metal carbides on steels were synthesized by the method of currentless transfer. This method also was used for the production of composite carbon fiber/refractory metal carbide materials, which are efficient electrocatalysts for the decomposition of hydrogen peroxide. The possibility to synthesize GdB6 nanorods and Si and TaO nanoneedles by potentiostatic electrolysis was shown.

Decrease in the Starting Temperature of the Reaction for Fabricating Carbides of Refractory Metals When Using Carbon Nanoparticles as Precursors

Metal matrix composites with a matrix of refractory metals (niobium, tungsten) and reinforcing nanodiamond particles were prepared for studying the possibility of decreasing the starting temperature of carbide synthesis. The size of primary nanodiamond particles was 4–6 nm, but they were combined in large-sized agglomerates. Mechanical alloying was used for producing the composites by crushing agglomerates and distributing nanodiamonds evenly in the metal matrix. The initial and fabricated materials were investigated by X-ray diffraction, differential scanning calorimetry, and transmission and scanning electron microscopy. Thermal processing leads to the reaction for carbide synthesis. Studies have found that the usage of carbon nanoparticles (nanodiamonds) as precursors for fabricating carbides of refractory metals leads to a dramatic decrease in the synthesis temperature in comparison with macro-precursors: lower than 200 °C for tungsten and lower than 350 °C for niobium.

ЭЛЕКТРОХИМИЧЕСКОЕ ПОЛУЧЕНИЕ КОМПОЗИЦИОННЫХ МАТЕРИАЛОВ «КАРБИД ТУГОПЛАВКОГО МЕТАЛЛА – УГЛЕРОДНОЕ ВОЛОКНО» И ИЗУЧЕНИЕ ИХ СВОЙСТВ

The conditions and results of electrochemical synthesis of composite materials "refractory metal carbide - carbon fiber" (Me = Nb, Ta, Mo) are considered. The results of studies of the electrocatalytic activity of these composite materials in the hydrogen peroxide decomposition reaction in comparison with such traditional catalysts as copper and platinum are presented. The most active composite has been established.

(Digital Presentation) Modern State and Prospects of Electrochemical CO2 Conversion in Molten Salts

Over the last century, anthropogenic activity has led to a significant increase in the concentration of carbon dioxide in the Earth's atmosphere. Today, the annual increase in CO2 is 3200-3600 million tons. Carbon dioxide together with other greenhouse gases (water vapor H2O, methane CH4, nitrous oxide N2O, sulfur hexafluoride SF6, etc.) causes global warming, which leads to serious environmental degradation - increasing the acidity of sea water, melting glaciers, strengthening the water cycle, which in some regions leads to heavy rainfall and floods, and in others - to extreme droughts. Therefore, the effective carbon dioxide utilization is becoming an urgent scientific and environmental task.One of the ways of carbon dioxide utilization is the electrochemical method. It consists in the electrochemical reduction of CO2 at the cathode. Depending on the conditions of electrolysis (composition of the electrolytic bath, temperature, current density, voltage in the bath, electrode materials), the chemical composition of cathode products changes dramatically.Report is a review of the present state of research concerning the electrochemical conversion of carbon dioxide from conducting liquid and solid media of different chemical composition. They include solid, aqueous, non-aqueous (organic and ionic liquids), and molten salt electrolytes. It contains a comparative analysis of the effectiveness of using these electrolytes, as well as products obtained by the electrochemical splitting of CO2 and the prospects for their use. Special emphasis has been made on the electrochemical decomposition of carbon dioxide from molten salts, several variants of decomposition have been considered, the advantages and disadvantages of each variant have been analyzed. An analysis of literature data on this subject as well as research results of the author on the direct electrochemical conversion of CO2 into valuable solid-phase chemicals with added value is presented.A feature of the electrochemical reduction of CO2 dissolved in molten salts, in contrast to aqueous, organic, solid electrolytes and ionic liquids is the possibility of forming a solid carbon phase at the cathode. Knowledge of the laws of electrode processes involving CO2 is necessary to create a scientific basis and control for the processes of electrochemical synthesis of nanosized carbon-containing inorganic compounds - nanotubes, nanofibers, graphene, amorphous carbon, carbides of refractory metals and numerous composites based on them.The voltammetric study of carbon electrowinning mechanism from carbon dioxide under excessive pressure and lithium carbonate in different gas media (air, argon, carbon dioxide) dissolved in chloride melts at temperatures of 550–800 °С was conducted. The characterization of the chemical and phase compositions, morphological and structural features of nanosized carbon phases obtained by electrolysis in the studied melts has been carried out; a correlation between the structure of the products and the synthesis conditions is established.Thermodynamic analysis of possible ways of electroreduction of CO2 and carbonates of alkali and alkaline earth metals in the temperature range 700 - 1000 K showed that the cathode product can be: (1) - solid carbon phase; (2) - carbon monoxide; (3) - carbide phase. At temperatures below 1000 K, the first path will be thermodynamically advantageous.Electrochemical behavior of CO2 in molten salt mixtures NaCl–KCl (0.5: 0.5 mol%); NaCl–KCl–CsCl (0.3: 0.245: 0.455 mol%); NaCl–KCl–NaF (0.423: 0.423: 0.154 mol.%) under gas excess pressure (1-17 atm.) was studied by cyclic voltammetry at different potential scan rates (0.02-10 Vs-1) and reverse potentials at Pt, Au and GC electrodes. It was established that CO2 electroreduction takes place in two stages, the electrode process is controlled by the charge transfer rate and the rate of CO2 diffusion in the melts. The solubility of CO2 in the melts of Na,K|Cl and Na,K|Cl,F was determined and the implementation of Henry's law in the studied pressure range was shown. An ECE (electrochemical–chemical–electrochemical) mechanism of cathodic reaction has been proposed for the cathode discharge of CO2. Oxidation of oxide- and carbonate- anions takes place at the anode, with the release of oxygen.Electrochemical reduction of Li2CO3 in molten equimolar mixture of Na,K|Cl in various gaseous media (air, argon, CO2) was studied by CV. Reduction of Li2CO3 to carbon in air occurs through the stage of preliminary chemical reaction of acid-base type with the formation of two electrochemically active particles of CO2 and {LixCO3}2-x at potentials -0.8 and -1.7 V, respectively, relative to Pb|PbCl2 reference electrode . Both processes are irreversible, and the limiting stage of reduction {LixCO3}2-x is the diffusion of the depolarizer to the electrode surface. In the atmosphere of argon and CO2, only {LixCO3}2-x participates in the electroreduction process.Promising areas of application of electrolytic carbon are presented.

(Digital Presentation) Creation of Composite Materials “Refractory Metal Carbide – Carbon Fiber” in Molten Salts and Study Their Electrical Double Layer in Hydrogen Peroxide and Sodium Sulfate Solutions

NbC/C, TaC/C, and Mo2C/C composites were synthesized by the electrochemical method of currentless transfer in molten salts containing compounds of these refractory metals. The electrical double layer formed in solutions of sodium sulfate Na2SO4 and hydrogen peroxide H2O2 on the composite material "refractory metal carbide-carbon fiber" has been studied by electrochemical impedance spectroscopy.

A general method for rapid synthesis of refractory carbides by low-pressure carbothermal shock reduction

Refractory carbides are attractive candidates for support materials in heterogeneous catalysis because of their high thermal, chemical, and mechanical stability. However, the industrial applications of refractory carbides, especially silicon carbide (SiC), are greatly hampered by their low surface area and harsh synthetic conditions, typically have a very limited surface area (<200 m2 g-1), and are prepared in a high-temperature environment (>1,400 °C) that lasts for several or even tens of hours. Based on Le Chatelier's principle, we theoretically proposed and experimentally verified that a low-pressure carbothermal reduction (CR) strategy was capable of synthesizing high-surface area SiC (569.9 m2 g-1) at a lower temperature and a faster rate (∼1,300 °C, 50 Pa, 30 s). Such high-surface area SiC possesses excellent thermal stability and antioxidant capacity since it maintained stability under a water-saturated airflow at 650 °C for 100 h. Furthermore, we demonstrated the feasibility of our strategy for scale-up production of high-surface area SiC (460.6 m2 g-1), with a yield larger than 12 g in one experiment, by virtue of an industrial viable vacuum sintering furnace. Importantly, our strategy is also applicable to the rapid synthesis of refractory metal carbides (NbC, Mo2C, TaC, WC) and even their emerging high-entropy carbides (VNbMoTaWC5, TiVNbTaWC5). Therefore, our low-pressure CR method provides an alternative strategy, not merely limited to temperature and time items, to regulate the synthesis and facilitate the upcoming industrial applications of carbide-based advanced functional materials.

Elastic properties of solid-solution refractory metal carbides with vacancy from virtual crystal approximation and supercell method

The virtual crystal approximation (VCA), as an efficient approach, has been used to study the alloys composed of refractory metals. In this work, we estimate the VCA in the application of solid-solution refractory metal carbides with point defects within the framework of ab initio calculations. Results indicate the VCA predicted Cij are different from those from supercell method with the same stress-stain approach. The elastic constants C44 from VCA are close to the average values derived from the refractory metal carbides. For the solid-solution refractory metal carbides with the severe lattice distortion, Cij are very sensitive to the atomic displacement and their moduli are softened. The carbon vacancies have a large effect on the elastic constants, whereas product a small effect on the anisotropy and hardness of solid-solution refractory metal carbides.

Extraordinary toughening enhancement in nonstoichiometric vanadium carbide

• VC 1- x composites have a quasi-monophasic fcc structure with coherent precipitates. • A doubling of fracture toughness is obtained in VC 1- x without any second phase. • VC 1- x show an unadulterated transgranular fracture feature, leading to toughening. • Coherent toughening and amorphous bridging toughening make a prodigious toughening. Improving fracture toughness, which has gone through decades, is a long-standing topic and is particularly important for safety-critical applications. In refractory transition metal carbides (RTMCs), remarkable toughening is usually achieved by adding metallic binders, however, resulting in a drastic deterioration of hardness and thermal stability. Here, we report a novel self-toughening strategy for synthesizing high-toughness RTMCs. Using mechanical alloying (MA) and spark plasma sintering (SPS), we synthesized nonstoichiometric VC 1- x (0.5 ≤ 1- x ≤ 0.6) with a quasi-monophasic microstructure containing a carbon-rich matrix and carbon-poor precipitates. Significantly, the VC 0.5 sintered at 1400°C shows a good trade-off of high hardness of 20.5±0.5 GPa and fracture toughness of 7.1±0.2 MPa m 1/2 . The fracture toughness of VC 0.5 increases by more than 100% accompanied by 7% hardness loss, compared with that of stoichiometric VC. The microstructure characterization and fracture behavior analysis demonstrate that the extraordinary toughening enhancement is attributed to a self-toughening strategy combined with coherency toughening and amorphous bridging toughening, which may offer an efficient pathway for developing high-performance structural ceramics.

Elastic Properties of Solid-Solution Refractory Metal Carbides with Vacancy from Virtual Crystal Approximation and Supercell Method

Elastic Properties of Solid-Solution Refractory Metal Carbides with Vacancy from Virtual Crystal Approximation and Supercell Method

Synthesis of Refractory Metal Carbides on Carbon Fibers in Molten Salts and Their Electrocatalytic Properties

Coatings of TaC and NbC and Mo2C crystals on carbon fibers were obtained by currentless transfer in molten salts. Investigation of electrocatalytic properties of these compositions in the reaction of the hydrogen peroxide decomposition was carried out. It was determined that the hydrogen peroxide decomposition reaction has a zero order for all refractory metal carbides. Using the values of the rate constants at different temperatures the activation energies for the reaction of the hydrogen peroxide decomposition on refractory metal carbides were calculated using the Arrhenius equation. It has been established that coating of NbC on carbon fibers has a higher electrocatalytical properties in comparison with other carbides. The electrocatalytic properties of NbC/C, TaC/C and Mo2C/C composites were studied also by cyclic voltammetry.

Modulated and Incommensurate Superstructures of Atomic–Vacancy Ordering in Refractory Transition Metal Carbides

The formation of partially disordered modifications in refractory nonstoichiometric carbides prone to M6X5-type ordering is considered. In M6X5 superstructures the ray $${\mathbf{k}}_{9}^{{(3)}}$$ of the Lifshitz star {k9} with a current parameter μ9 = 1/2 in the disorder–order phase transition channel is replaced by the rays $${\mathbf{k}}_{5}^{{(6)}}$$ and $${\mathbf{k}}_{5}^{{(5)}}$$ of the non-Lifshitz star {k5} with a variable current parameter 0 < μ5 < 1/2. Depending on the specific value of the index μ5, this replacement leads to various modulated structures that differ by the concentration of vacancies in the defective ( $$1\bar {1}1$$ ) planes of the carbon fcc sublattice and by the modulation period. In the diffraction spectra the positions of the superstructure reflections due to the star {k5} at μ5 ≈ 0.473 correspond to the incommensurate ordered phase experimentally detected in a nonstoichiometric tantalum carbide. The incommensurate phase is close in short-range order in the first coordination sphere to the initial M6X5 superstructures with a reduced long-range order parameter η ≈ 0.6.

Synthesis of Refractory Metal Carbides on Carbon Fibers in Molten Salts and Their Electrochemical and Electrocatalytic Properties

Method of currentless transfer in molten salts was used for synthesis of tantalum and niobium carbides coatings and crystals of molybdenum carbide on carbon fibers. Temperature of process was varied from 800 up to 900 °C, and time of process from 0.5 up to 24 hours.Figure 1. Micrographs of carbon fiber with TaC and NbC coatings and Mo2C crystals (from left to right), obtained in molten salts melts by currentless transfer.The splicing of fibers with each other was not observed; coatings were uniform in a cross section as well as along the fiber. Thickness of TaC and NbC coatings on carbon fiber was 50 – 250 nm. Size of crystals of Mo2C was approximately several micrometers.Investigation of refractory metal (Ta, Nb, Mo) carbides on carbon fiber of the Carbopon-B-22 brand was carried out by impedance spectroscopy method. The values of the standard rate constants of charge transfer and capacities of the double electric layer were determined for synthesized materials in water solution with hydrogen peroxide.Coatings and crystals of refractory metal carbides on carbon fibers can be used as catalysts and electrocatalysts. The kinetics of the electrocatalytic decomposition of hydrogen peroxide on the surface of TaC/C, NbC/C and Mo2C/C composite materials was studied. This reaction was investigated on measuring the volume of released oxygen. The initial carbon fiber of the Carbopon-B-22 brand was used as the cathode. The composite materials made of carbon fiber coated with tantalum carbide TaC (NbC) or modified by molybdenum carbide Mo2C served as the anode. The reaction order was established, the rate constants and activation energies of the hydrogen peroxide decomposition were calculated. The composition with the highest electrocatalytic activity was determined. Figure 1

Synthesis of Refractory Metal Carbides on Carbon Fibers in Molten Salts and Their Electrochemical and Electrocatalytic Properties

TaC and NbC coatings and Mo2C crystals on the carbon fibers were obtained by the currentless transfer in molten salts. Investigation of electrocatalytic properties of these compositions in the reaction of the hydrogen peroxide decomposition was carried out. The kinetic parameters of this reaction were determined. It has been established that NbC on carbon fibers has a higher electrocatalytical properties in comparison with other carbides.

Electrical Explosion of Conductors to Produce Nanosized Carbides and to Apply Functional Coatings

An analytical review of recent studies in the field of electrical explosion of conductors (EEC) has been performed to select modes and conditions of high-resistivity EEC, which is used to produce functional wear-resistant hard-alloy carbide coatings containing refractory metals, their carbides, and nanocarbon particles. The influence of the environment in which the EEC is carried out on the composition of the products obtained is analyzed. The influence of the conductor’s electric explosion modes in gaseous carbon-containing media on the synthesis of refractory metal carbides is considered. The problems of producing the functional composite coatings of metal surfaces are also discussed.

Mechanism of Nucleation of Precipitates of Carbides of Refractory Metals Under Hot Rolling of Automotive Sheet Steel

The problem of formation of properties in microalloyed sheet steels is considered. The cause and the possible mechanism of precipitation of carbides of refractory metals on the boundary of the γ → α phase transition are suggested. A possible model of formation of a Fe3 C nucleus from austenite is presented.

Mechanical properties of tantalum carbide from high-pressure/high-temperature synthesis and first-principles calculations.

As a member of the refractory metal carbide family of materials, TaC is a promising candidate for ultra-high temperature ceramics (UHTC) with desirable mechanical strength. TaC sample quality and therefore mechanical properties are strongly dependent on synthesis method, and atomistic origins of mechanical failure are difficult to assign. Here, we have successfully synthesized high quality densified TaC samples at 5.5 GPa and 1400 °C using the high pressure and high temperature (HPHT) sintering method, with Vickers hardness determined to be 20.9 GPa. First-principles calculations based on the recently developed strain-stress method show that the ideal indentation strength of TaC is about 23.3 GPa in the (11[combining macron]0)[001] direction, in excellent agreement with experimental results. The detailed indentation shear deformation analysis and structural snapshots from the calculations indicate that the slip dislocations of TaC layers are the main structural deformation mode during the Vickers indentation process, and that the strong directional Ta-C bonds are responsible for the high mechanical strength of TaC. HPHT synthesis is shown to produce TaC samples with superior strength, and together with accurate first-principles calculations offers crucial insights for rational design and synthesis of novel and advanced UHTC materials.

Hardening tungsten carbide by alloying elements with high work function.

There is intensive searching for superhard materials in both theoretical and experimental studies. Refractory and transition metal carbides are typical materials with high hardness. In this study, first-principles calculations were performed first to analyze the electronic structures and mechanical properties of the tungsten-carbide-based compounds. The results indicated that tungsten carbide could be hardened by alloying elements with high work functions to tailor the Fermi level and electron density. Guided by the calculations, a new type of tungsten carbide alloyed with Re was synthesized. The Young's modulus and hardness of the Re-alloyed tungsten carbide are increased by 31% and 44%, respectively, as compared with those of tungsten carbide. This study provides a new methodology to design superhard materials on a feasible electronic base using work function as a simple guiding parameter.