Superhard Research Articles

Dense nanocrystalline ReB2 bulk with ultrahigh hardness

Microcrystalline ReB2 with a high hardness of 48 GPa measured under a small load of 0.49 N has been reported as a superhard material. Nanocrystalline ReB2 is expected to achieve better mechanical properties than its microcrystalline counterparts. Here we report dense nanocrystalline ReB2 bulks synthesized by modified mechanical alloying and high-pressure sintering techniques. The modified mechanical alloying technique reduces the content of WC impurity from 7.75 wt% to 1.59 wt% in as-synthesized nanocrystalline ReB2 powders with an average grain size of 6.6 ± 2.0 nm. The extrinsic high-pressure (6 GPa) and the intrinsic nanocrystalline structure greatly promote the densification process of the ReB2 bulks during sintering. Our optimized nanocrystalline ReB2 bulk is ∼99% dense and with an average crystallite size of 40 nm, resulting in a high hardness of ∼33 GPa under a load of 9.8 N and indentation fracture toughness of ∼5 MPa m1/2. The high hardness can be ascribed to the synergistic effect of high density and nanocrystalline structure of ReB2 bulks. This work provides promising strategies to largely enhance the mechanical properties of different ceramics for practical applications.

Testing for Abrasion Resistance of WC-Co Composites for Blades Used in Wood-Based Material Processing.

Commonly used tool materials for machining wood-based materials are WC-Co carbides. Although they have been known for a long time, there is still much development in the field of sintered tool materials, especially WC-Co carbides and superhard materials. The use of new manufacturing methods (such as FAST-field-assisted sintering technology), which use pulses of electric current for heating, can improve the properties of the materials used for cutting tools, thereby increasing the cost-effectiveness of machining. The ability to increase tool life without the downtime associated with tool wear allows significant cost savings, particularly in mass production. This paper presents the results of a study of the effect of grain size and cobalt content of carbide tool sinters on the tribological properties of the materials studied. The powders used for consolidation were characterised by irregular shape and formed agglomerates of different sizes. Tribological tests were carried out using the T-01 (ball-on-disc) method. In order to determine the wear kinetics, the entire friction path was divided into 15 cycles of 200 m and the weight loss was measured after each stage. In order to determine the mechanism and intensity of wear of the tested materials under technically dry friction conditions, the surface of the tested sinters was observed before the test and after 5, 10, and 15 cycles. The conclusions of the study indicate that the predominant effect of surface cooperation at the friction node is abrasion due to the material chipping that occurs during the process. The results confirm the influence of sintered grain size and cobalt content on durability. In the context of the application of the materials in question for cutting tools, it can be pointed out that sintered WC(0.4)_4 has the highest potential for use in the manufacture of cutting tools.

Superhard BN allotropes with tunable hybridization sp2/sp3 ratios by compressed nanotubes

The superhard BN materials with tunable properties present potential applications in multifunctional devices under extreme conditions. Eight novel BN structures of which band gaps ranging from 2.34 to 6.18 eV due to the tunable hybridization sp2/sp3 ratios, are proposed by the polymerization of BN nanotubes (BNNTs), including two fully sp2 hybridization structures (tP14-BN and tP16-BN), two fully sp3 hybridization structures (hP24-BN and hP48-BN) and four sp2/sp3 hybridization structures (mP44-BN, oI28-BN, mP76-BN and mP84-BN). There are four superhard sp2/sp3 BN (B:N = 1:1) structures are the firstly reported. Notably, the mP76-BN with the hardness of 52.55 GPa is the hardest structure in all the reported sp2/sp3 hybrid BN structures. Moreover, the oI28-BN, mP76-BN and mP84-BN exhibit the high fracture toughness property. This study systematically reveals the transition mechanism of BNNTs under high pressure and theoretically presents an efficient route to modulate the sp2/sp3 hybridization ratios in superhard BN allotropes realizing tunable band gaps for the potential applications in electronic devices or mechanical materials.

High pressure and high temperature phase transformations of covalent triazine-based frameworks

A monolithic two-dimensional (2D) biphenyl-linked triazine-based framework (CTF-2) showing merely slight crystallinity was prepared by Lewis acid-promoted polymerisation of 4,4′-biphenyldicarbonitrile in liquid TiCl4 at moderate temperature to avoid unwanted carbonization. The transformations of CTF-2 induced by high pressure (up to 60 GPa) and temperature (up to 2400 K) were studied by IR and in situ Raman spectroscopy as well as TEM microscopy. In contrast to graphitic C3N4 materials, CTF-2 exhibits high optical density and reduced fluorescence at high pressure, allowing laser-induced sample heating. Spectral data show the formation of sp3 hybridised carbon atoms and triazine ring distortion in CTF-2 material at pressures above 17 GPa. According to the pressure Raman effect, the bulk modulus of the high-pressure phase of CTF-2 (high-pressure phase CTF-2) was found to be 570 ± 10 GPa, which is a typical value for superhard carbon materials. Structural studies revealed nitrogen preservation in the specimen after high-pressure treatment and stability of the disordered sp3 states of the high-pressure phase CTF-2 in the quenched specimen. This behaviour is quite different from that of the quenched disordered sp3 states of high pressure pure carbon phases, which convert to the sp2 state at pressures below 3–7 GPa.

Thermal stability of ReB2‐type transition metal diborides

AbstractThe thermal stability of metastable ReB2‐type transition metal diborides (TMB2), which are considered as new type of superhard material, is of vital importance to obtain bulk samples. In the present work, thermal stability of four kinds of ReB2‐type TMB2 powders, ReB2, OsB2, Os1–xRexB2, and Os1‐xWxB2, were synthesized with varied transition metal (TM)‐to‐B molar ratio by mechanochemical methods and the subsequent annealing was compared. The as‐synthesized powders were then consolidated using a pressureless sintering technique. The results showed that the B content required to obtain the pure hexagonal ReB2‐type Os1–x(TM)xB2 phase varied, which indicated different thermal stabilities, such as OsB2 < Os0.1W0.1B2 < Os0.9Re0.1B2 < Os0.8W0.2B2 < ReB2 < Os0.6W0.4B2 and Os0.5W0.5B2. Among them, Os0.6W0.4B2 and Os0.5W0.5B2 were found to be relatively thermally stable and could be synthesized with a stoichiometric molar ratio of (Os + W):B = 1:2. It was also found that the thermal stability of TMB2 with a hexagonal ReB2 structure could be mainly governed by the length of lattice constant c. The results have guiding significance for the design and preparation of new type of TM borides. In addition, the hardness of TMB2 can be increased by tailoring the B content in the raw materials more precisely.

Specific Features of Measuring the Elasticity Modulus and the Decrement of Oscillations in Superhard Material Polycrystals by the Dynamic Method

Specific Features of Measuring the Elasticity Modulus and the Decrement of Oscillations in Superhard Material Polycrystals by the Dynamic Method

Crystal structure and physical properties of Ti2B5 predicted by first principles calculations

Applying the advanced CALYPSO structure searching method and first principles calculations, we predict a novel superhard structure for Ti2B5, which belongs to the monoclinic C2/m space group. The previously known Hf2B5, W2B5, and δ-V2O5-type structures are also selected as the potential structures for Ti2B5. It is shown that the predicted C2/m structure is the most stable phase among these considered structures within the range of 0-100GPa. The calculations of the phonon dispersion and elastic constants affirm that the predicted C2/m phase is dynamically and mechanically stable. The predicted high shear modulus (249 GPa) and large hardness (46.9 GPa) show that it should be anunderlying superhard material. In addition, the elastic anisotropy of Ti2B5 is also explored by investigating the directional dependence of the Young's modulus, shear modulus, and Poisson's ratio as well as some well anisotropy indices. The particular analyses of the density of states and electron charge density distribution verify that the strong B-B and Ti-B covalent bonds in the predicted C2/m-Ti2B5 phase playan importantpartin its hardness and structural stability.

Residual stress effects on toughening of ultrafine-grained B4C-SiC ceramics

The attainment of both enhanced toughness and super hardness is crucial for B4C ceramics. Refining grains to the nanoscale is known to be an effective strategy to address the above issue, but remains a great challenge. In this work, B4C and SiC nanoparticles were synthesized and used for the preparation of ultrafine-grained B4C-SiC ceramics. The grain size effect on the mechanical properties of composites was systematically investigated. It is found that, contrary to the conventional view that there is almost no residual thermal stress in the B4C-SiC interface, residual stress arises as B4C grains reach a nanostructured state, and the toughness can be considerably improved to 5.13 MPa m1/2. The B4C-SiC composite also demonstrates excellent hardness of 36.6 GPa, even superior to that of monolithic B4C ceramics. This study has important implications for the development of toughening mechanisms for B4C-SiC ceramics and presents a new strategy for producing high-performance ceramics.

Screening outstanding mechanical properties and low lattice thermal conductivity using global attention graph neural network

Mechanical and thermal properties of materials are extremely important for various engineering and scientific fields such as energy conversion and energy storage. However, the characterization of these properties via high throughput screening at the quantum level, although highly accurate, is inefficient and very time- and resource-consuming. In contrast, prediction at the classical level is highly efficient but less accurate. We deploy scalable global attention graph neural network for accurate prediction of mechanical properties which bridge the gap between the accuracy at the quantum level and efficiency at the classical level. Using 10,158 elastic constants as training data, we trained the models on 5 mechanical properties, namely bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and hardness. With the trained model, we predicted 775,947 data in search of materials with ultrahigh hardness. We further verify the recommended ultrahigh hardness materials by high precision first principles calculations, and we finally identify 20 structures with extreme hardness close to diamond, the hardest material in nature. Among those, two super hard materials are completely new and have not been reported in literature so far. We further recommend potential materials from bulk modulus prediction to search low lattice thermal conductivity, and we verify the thermal conductivity of 338 structures with first principles. Our results demonstrate that one can find materials with extreme mechanical properties recommended by graph neural network and low thermal conductivity material from bulk modulus prediction with minimal first principles calculations of the structures (only 0.04%) in the large-scale materials pool.

Sintering pure polycrystalline boron carbide bulks with enhanced hardness and toughness under high pressure

Boron carbide ceramics possess high hardness, but they generally are subjected to low toughness due to strong covalent bonds. However, the fabrication of dense and pure polycrystalline boron carbide bulks with excellent mechanical properties by conventional methods remains a significant challenge. Here, the fine-grained and pure polycrystalline boron carbide bulks have been successfully fabricated utilizing submicron-sized boron carbide powder as starting materials under the condition of high pressure (5.5 GPa) and low temperature range (1000–1400 °C). The well-sintered boron carbide bulk recovered at 1000 °C exhibits a mean grain size of 205.2 ± 191.2 nm and relative density of up to ∼97%, and the measured Vickers hardness and fracture toughness achieve 35.0 ± 2.8 GPa and 4.6 ± 0.5 MPa m0.5, respectively. The outstanding trade-off between hardness and toughness demonstrated that the synergistic effect between fine grain size and dislocation, and the interplay between amorphous grain-boundary phases and pores via grain-boundary sliding have critical role in contributing to improve hardness and toughness, respectively. This study has essential implications in sintering fine-grained and pure polycrystalline engineering ceramics and superhard materials, which provide a tuning approach to enhance the mechanical properties.

Investigation of the stability of samples in the form of a sphere with a hole from a semi-finished product obtained by injection molding of plastic ceramic material based on powders SiC, WC, AlN

Experimental work was carried out according to the concept of shape stability. Developed at the Institute of Superhard Materials. VM Bakul NASU injection molding plant made a semi-finished product in the form of a ball with a hole of thermoplastic mass based on powders SiC, WC, AlN according to the established technological parameters, optimized by the results of computer modeling. Based on the developed scheme of conducting mechanical tests on the installation of FP-10 samples from the injected semi-finished product and analyzed the nature of the destruction. An increase in the compaction of the thermoplastic mass around the injection point has been experimentally proven. Based on experimental data, a scheme of the actual destruction of the crack distribution at intermediate stages of testing to complete destruction of the samples was created. It is shown that the failure occurs in the area of the calculated junction lines, which have become stress concentrators.

Front Cover: (Z. Anorg. Allg. Chem. 11‐12/2023)

Graphical Abstract ZAAC is an international scientific journal which publishes original papers on new relevant research results from all areas of inorganic chemistry, solid state chemistry, and co-ordination chemistry.The contributions reflect the latest findings in these research areas and serve the development of new materials, such as super-hard materials, electrical superconductors, or intermetallic compounds. Up-to-date physical methods for the characterization of new chemical compounds and materials are also described.

First-principles investigation on the high-pressure phase transitions and elastic constants of chromium nitrides

We performed first-principles calculations on the structural and electronic properties of chromium nitrides under high pressure. Based on crystal structure predictions, we identified several novel chromium nitrogen compounds and pressure-induced phase transitions in the Cr–N system from ambient pressure to 200 GPa. The dynamic stable compounds—CrN2, CrN6, and Cr2N3—are studied through both electronic and phononic calculations. This work provides a theoretical basis for the phase transition of Cr–N compounds under different pressures, points out the direction of future research on Cr–N systems, and may stimulate the synthesis of superhard and high energy density materials in the future.

Superconductivity at 117 K in H-doped diamond

To explore the higher-temperature superconductivity at ambient pressure or near ambient pressure in diamond, we investigate the structure, electronic states, dynamics, and electron-phonon interactions of diamond doped by H atoms based on the first-principles calculations. The data on phonon spectra indicates that H-doped diamond is dynamically stable above 5 GPa. The hybridization between H and C induces the metallization transition. The analysis on electron-phonon interactions determines that H-doped diamond is superconducting above 5 GPa. The intercalatrion of H introduces more soft phonon modes, resulting in stronger superconductivity than other doped diamonds and hard materials. We predict that the superconducting transition temperature of H-doped diamond is 117.7 K at 5 GPa, which confirms that the superconductivity of diamond-like compounds can be further improved. Our study also shows that it is feasible for hard or superhard materials to obtain the high-temperature superconductivity at near ambient pressure.

Evolution of shear amorphization in superhard cubic silicon carbide

Shear amorphization induced by high stress in cubic silicon carbide (β-SiC) has not yet been experimentally observed till-to-date. In this letter, we report the detailed evolution of stress-induced amorphization in superhard β-SiC using transmission electron microscopy (TEM) and molecular dynamics (MD) simulations. From experimental observations using TEM, shearing by stacking faults along (111) crystallographic plane triggers the onset of amorphization. Further, an increase in the stress results in highly localized strains within the amorphous band led by the coalescence of vicinity dislocations. Based on the MD simulations, it has been verified that the existence of stacking faults decreases the amount of shear stress and shear strain necessary to trigger amorphization compared to the pristine β-SiC, which is in line with experimental observations. This study underscores a detailed understanding of the amorphization process in superhard materials through which stress is alleviated in high-stress environments.

Superhard and superconductive polycrystalline boron doped diamond synthesized with different concentration of B4C

Boron doped diamond are potential superhard materials with good electric conductivity. In this work, polycrystalline boron doped diamond (p-BDD) with different boron concentration were synthesized from the mixture of graphite and boron carbide under ultra-high pressure (15 GPa) and temperature (2500–2700 K). The results indicate that no more boron atoms can be inserted into diamond when the boron concentration is above 4 at%. The EELS spectra indicated that the boron atoms have been doped into the diamond grains, and surplus boron carbide locate at the grain boundary. The hardness measurements reveal that the p-BDD doped with 1 at% and 23 at% boron atoms exhibit high hardness of 78.5 GPa and 45 GPa, respectively, which is equivalent to the hardness of single crystal diamond and cubic boron nitride. The p-BDD is a semiconductor and transforms into superconductive state below 2.5 K. According to microstructure measurements, highly dense p-BDD and B4C bonded diamond exhibit superhard behavior because of the strong bonding of diamond grains by B4C additives. The oxidation temperature of these p-BDD specimens is around 1538 K in the air, which is far higher than diamond. B4C is demonstrated as a superior additive to sinter superhard and electric conductive p-BDD, which are extremely useful in areas where high hardness and electric conductive are highly desired.

Is the hardness of material harder than diamond reliable?

With the development of new synthesis methods and chemistries, a number of new superhard materials have been reported to be harder than diamond. While such materials are highly desirable due to their wide-ranging applications, there are some inherent uncertainties in the methods utilized to determine and define the hardness of such materials. In this paper, we employed the standard Vickers diamond indenter and substitute indenters with the same shape to measure the hardness of nine ceramics and superhard materials within well-defined criteria and methodology, for the assessment of consistency in the hardness testing. The findings and the developed testing method in the current study have broad implications in characterizing new and emerging superhard materials, leading to new discoveries.

Preparation of Ti3Si0.8Al0.2C2 Bonded Diamond Composites and Their Friction Properties Coupled with Different Counterfaces

Polycrystalline diamonds were sintered with the binder of Ti3Si0.8Al0.2C2 to solve the shortcomings of conventional metal binders and ceramic binders. Ti3Si0.8Al0.2C2 bonded polycrystalline diamond composites have been synthesized from a mixture of Ti3Si0.8Al0.2C2 powder (40 wt%) and diamond powder under 4.5–5.5 GPa at 1050–1300°C by high press and high-temperature technology. The characteristic peaks of TiC were observed at 4.5 GPa and above 1100°C. From the microstructure analysis, the diamond particles are equally dispersed throughout the Ti3Si0.8Al0.2C2 matrix and firmly bound to the matrix. The effects of the sintering conditions, test parameters, and counterparts on the friction properties of diamond composites were systematically analyzed. The friction coefficient between diamond composite and glass counterpart is between 0.18 and 0.33 and increases significantly at the speed of 400 rpm/min. SEM and EDS analysis show Ti3Si0.8Al0.2C2 has a strong holding force on the diamond particles. The wear mechanism is mainly abrasive, and there are obvious grooves on the surface of the glass ball. The friction coefficient of diamond composite sintered at 1050°C with POM decreases monotonically with increasing load and reaches the minimum value of 0.42 at 12 N. The higher sintering temperature (1150°C) resulted in lower friction coefficient for diamond/POM pairs. The friction coefficient of diamond/PP pairs decreases first and then increases with increasing load, reaching a minimum value of 0.431 at 10 N. The wear mechanism of PP and POM is a combination of abrasive wear and adhesive wear, while the abrasive chips of POM are small salt-like particles and the abrasive chips of PP are long flocs. The wear track of PP balls has a smoother surface than that of POM balls. For an Al pair, the friction coefficient of diamond composite sintered at 1150°C is higher than that of the composite sintered at 1050°C except for the 10 N load. The friction coefficient of the diamond composites with Cu pairs varies slightly with increasing load, with values ranging from 0.443 to 0.518 and decreases with increasing speed, reaching a minimum value of 0.396 at 600 rpm/min. The wear mechanism of Cu and Al is mainly based on adhesive wear. The presence of the diamond second phase improves the stability of Ti3Si0.8Al0.2C2 compared to the high-pressure treatment of single phase Ti3SiC2. The analysis of friction properties indicates that Ti3Si0.8Al0.2C2 bonded diamond composites are promising candidates for superhard tool materials.

Exploring chemical bonding nature, weak exciton effect, electronic, optical and thermoelectric properties of NaReN2 via DFT computations

We performed first principles calculations on the recently synthesized NaReN2. This compound is the main product of the reaction between Re and NaN3 in forming the super hard material ReN2. Theoretical calculations of sodium rhenium nitride, such as band structure and density of states (DOS) were performed. The partial density of states (PDOS) was also included to determine the contribution of individual atoms to the electronic states. The theoretical data reveal that the material possesses a rather narrow bandgap of 0.26 eV. Semi-conducting materials with such low bandgap values have interesting applications in various fields. The optical properties were also studied indicating possible application of NaReN2 in opto-electronic devices. Excitonic study reveals the weak excitonic behavior of this material. Thus, first principles calculations on this material provide necessary theoretical information on physicochemical properties of NaReN2.

Investigation of Situational Correlations of Wire Electrical Discharge Machining of Superhard Materials with Acoustic Emission Characteristics

The purpose of this research is to find relationships between the parameters of the acoustic emission signals accompanying the electroerosive processing with a wire electrode of metals and hard alloys and the most important process indicators. These indicators include an increase in the concentration of erosion products in the interelectrode gap, an increase in the probability of wire electrode breakage, the efficiency of the supplied energy, the current productivity. This article presents the results of the study of acoustic emission signals during the processing of hard alloys with a cutting machine. The main focus is on the period preceding the breakage of the wire electrode. Changes in the parameters of acoustic emission a few seconds before failure are shown, and the possibility of preventing wire breakage by monitoring the parameters of acoustic emission signals is established. To evaluate the efficiency of the energy supplied to the processing zone, a dynamic model is proposed, with the help of which the processing efficiency is estimated by changing the transmission coefficients in one or several frequency ranges. To explain the situation that occurs in the processing zone with an increase in the concentration of erosion products, the article draws a parallel between electroerosive and laser processing, related to technologies of processing with concentrated flows of energy. Studies have shown that acoustic emission signals can be used to search for rational processing modes and improve automatic control systems for electroerosive equipment.