Superhard Metal Research Articles

Anomalous lattice stiffening in tungsten tetraboride solid solutions with manganese under compression

Tungsten tetraboride (WB4)-based solid solutions represent one of the most promising superhard metal candidates; however, their underlying hardening mechanisms have not yet been fully understood. Here, we explore the lattice compressibility of WB4 binary solid solutions with different manganese (Mn) concentrations using high-pressure x-ray diffraction (XRD) up to 52 GPa. Under initial compression, the lattices of low and high Mn-doped WB4 alloys (i.e. W0.96Mn0.04B4 and W0.84Mn0.16B4) are shown to be more and less compressible than pure WB4, respectively. Then, a c-axis softening is found to occur above 39 GPa in WB4, consistent with previous results. However, an anomalous sudden a-axis stiffening is revealed at ~36 GPa in W0.96Mn0.04B4, along with suppression of c-axis softening observed in WB4. Furthermore, upon Mn addition, a simultaneous stiffening of a- and c-axes is demonstrated in W0.84Mn0.16B4 at ~37 GPa. Speculation on the possible relationship between this anomalous stiffening and the combined effects of valence-electron concentration (VEC) and atomic size mismatch is also included to understand the origin of the nearly identical hardness enhancement in those two solid solutions compared to WB4. Our findings emphasize the importance of accurate bonding and structure manipulation via solute atoms to best optimize the hardness of WB4 solid solutions.

Nonrandomly Distributed Tungsten Vacancies and Interstitial Boron Trimers in Tungsten Tetraboride

Tungsten tetraboride (WB4) and its solid solutions represent one of the most promising candidates for superhard metals; however, the structural and bonding uncertainties regarding the fractionally ...

Structural, mechanical and electronic properties study on group 5 transition metals ternary mononitrides from first-principles calculations

Various possible structures of ternary mononitrides of group 5 transition metals (V0.5Ta0.5N, V0.5Nb0.5N and Nb0.5Ta0.5N) are systematically explored in 0–300 GPa pressure range through CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) method. Three possible phases with Pmm2, Cmc21, and Fm-3m space group have been uncovered. Their dynamical, mechanical and thermodynamic stabilities are checked through calculations of phonon spectra, elastic constants and formation enthalpy. The elastic anisotropies are fully investigated by the dependence of linear bulk, Young's and shear moduli on the crystal orientations. To study the strength of these three compounds, the compressive, tensile and indentation shear strengths are systematically studied from the first-principles calculations. The mononitrides with Pmm2 and Cmc21 structures exhibit much better mechanical property relative to VN, NbN and TaN with traditional B1 (NaCl-like) structure. Our results reveal that the Pmm2 and Cmc21 phases of these three compounds possess the strength values up to 30 GPa under the Vickers indentation simulation calculations, which are comparable to well-known WB3, WB4 and ReB2 reported as potential superhard transition metal borides. Additionally, the electronic structures of these mononitrides are also studied to explore the intrinsic chemical bonding nature, and a novel transition of semiconductor-to-conductor from V0.5Ta0.5N, V0.5Nb0.5N to Nb0.5Ta0.5N is exposed for the Fm-3m phases of these compounds.

Preparation core/shell-type microparticles consisting of cBN cores aluminum coating via composite method

The mechanically mixed aluminum and cubic boron nitride (cBN) powder are important raw materials for preparing super-hard metal cutting tools. So aluminum shell-coated cBN may have significant economic potential. A Brownian motion based electrostatic self-assembly route in liquid salt of high temperature was developed using silica nanolayer bridge joint for the aluminum coating of cBN microparticles. In this approach, piranha solution method was first used to activate the surface of cBN cores, the silica nanolayer via sol-gel method served as the substrate for an outer aluminum coating of cBN cores. Coating of aluminum on cBN cores was accomplished by a self-assembly process from a liquid state solution of molten salt and aluminum powders. To distinguish small amounts of non-reacted aluminum powders which acted as aluminum source with the core - shell structure product, bromoform (CHBr3) was used to separate both of them due to the density differences. Samples were characterized by SEM, XRD, EDX and DSC. The contribution of our work lies in the creation of a novel strategy to fabricate a light metal element coating on the inert material particles in view of the versatility of sol-gel process and the controllability of the molten salt process.

Perspective: Superhard metal borides: A look forward

Previous efforts toward the synthesis of materials with superior mechanical properties (e.g., superhardness, high bulk modulus, and high shear modulus) have focused on binary metal boride structures with short covalent bonds. In this paper, we discuss the methods for enhancing the intrinsic and extrinsic hardening factors commonly used to achieve these properties. As the realm of binary metal borides has all but been exhausted, there is a need to move to ternary and higher borides as well as to achieve greater morphological control at the nanoscale. The use of ternary and higher borides as starting materials provides a multitude of diverse compositional and structural combinations. These compounds merit further investigation as they offer the potential for the optimization of mechanical properties.

A novel superhard tungsten nitride predicted by machine-learning accelerated crystal structure search

Transition metal nitrides have been suggested to have both high hardness and good thermal stability with large potential application value, but so far stable superhard transition metal nitrides have not been synthesized. Here, with our newly developed machine-learning accelerated crystal structure searching method, we designed a superhard tungsten nitride, h-WN6, which can be synthesized at pressure around 65 GPa and quenchable to ambient pressure. This h-WN6 is constructed with single-bonded armchair-like N6 rings and presents ionic-like features, which can be formulated as W2.4+N62.4−. It has a band gap of 1.6 eV at 0 GPa and exhibits an abnormal gap broadening behavior under pressure. Excitingly, this h-WN6 is found to be the hardest among transition metal nitrides known so far (Vickers hardness around 57 GPa) and also has a very high melting temperature (around 1,900 K). Additionally, the good gravimetric (3.1 kJ/g) and volumetric (28.0 kJ/cm3) energy densities make this nitrogen-rich compound a potential high-energy-density material. These predictions support the designing rules and may stimulate future experiments to synthesize superhard and high-energy-density material.

Designing superhard metals: The case of low borides

The search for new superhard materials has usually focused on strong covalent solids. It is, however, a huge challenge to design superhard metals because of the low resistance of metallic bonds against the formation and movement of dislocations. Here, we report a microscopic mechanism of enhancing hardness by identifying highly stable thermodynamic phases and strengthening weak slip planes. Using the well-known transition-metal borides as prototypes, we demonstrate that several low borides possess unexpectedly high hardness whereas high borides exhibit an anomalous hardness reduction. Such an unusual phenomenon originates from the peculiar bonding mechanisms in these compounds. Furthermore, the low borides have close compositions, similar structures, and degenerate formation energies. This enables facile synthesis of a multiphase material that includes a large number of interfaces among different borides, and these interfaces form nanoscale interlocks that strongly suppress the glide dislocations within the metal bilayers, thereby drastically enhancing extrinsic hardness and achieving true superhard metals. Therefore, this study not only elucidates the unique mechanism responsible for the anomalous hardening in this class of borides but also offers a valid alchemy to design novel superhard metals with multiple functionalities.

Spark plasma sintering of WC-based cermets/titanium and vanadium added composites: A comparative study on the microstructure and mechanical properties

In an attempt to develop the composition and properties of W2C-(W,Ti)C-TiC and WC-WC1-x-VC-V super hardmetals, spark plasma sintering (SPS) method was implemented. WC powders were mixed separately with 10 wt% Ti and 10 wt% V in a high energy mixer mill and sintering processes were performed at temperatures of 2150 and 2000 °C, respectively. XRD investigations revealed the formations of TiC and (Ti,W)C as the reaction products in WC-10 wt% Ti composite. Moreover, the interfacial reaction between WC and V led to the formation of WC1-x and VC compounds. A higher bending strength (613 ± 25 MPa) and fracture toughness (4.1 ± 0.58 MPa m1/2) were obtained for WC-10 wt% V samples compared to WC-10 wt% Ti, While the WC-10 wt% Ti composite showed a higher value of hardness (3128 ± 42 Vickers) in comparison to WC-10 wt% V (2632 ± 39 Vickers), which can act as a super hard cermet.

Реакция организма пациента на использование имплантатов с покрытием нитридами сверхтвердых металлов при реконструкции тазобедренного сустава

Реакция организма пациента на использование имплантатов с покрытием нитридами сверхтвердых металлов при реконструкции тазобедренного сустава

Extraordinary Indentation Strain Stiffening Produces Superhard Tungsten Nitrides.

Transition-metal light-element compounds are a class of designer materials tailored to be a new generation of superhard solids, but indentation strain softening has hitherto limited their intrinsic load-invariant hardness to well below the 40GPa threshold commonly set for superhard materials. Here we report findings from first-principles calculations that two tungsten nitrides, hP4-WN and hP6-WN_{2}, exhibit extraordinary strain stiffening that produces remarkably enhanced indentation strengths exceeding 40GPa, raising exciting prospects of realizing the long-sought nontraditional superhard solids. Calculations show that hP4-WN is metallic both at equilibrium and under indentation, marking it as the first known intrinsic superhard metal. An x-ray diffraction pattern analysis indicates the presence of hP4-WN in a recently synthesized specimen. We elucidate the intricate bonding and stress response mechanisms for the identified structural strengthening, and the insights may help advance rational design and discovery of additional novel superhard materials.

Near-isotropic elastic properties and high shear deformation resistance: Making low symmetry and open structured YbAlB14, LuAlB14 and ScMgB14 superhard

Superhard materials are usually found in high symmetry and high bond density compounds like diamond and cubic BN. Herein, using a combination of first-principles calculations and the microscopic and macroscopic models for hardness prediction, three new possible superhard boron-rich metal borides with low symmetry and open structure are predicted. The predicted hardness values are 37.9 GPa, 37.5 GPa and 41.3 GPa for YbAlB14, LuAlB14 and ScMgB14, respectively, from the macroscopic model and 45.1 GPa, 50.8 GPa and 50.4 GPa, respectively, from the microscopic model. The reliability of this work is supported by comparing the measured hardness of AlMgB14 and Y0.62Al0.71B14 with the calculated Vickers hardness of AlMgB14 and YAlB14. The metal elements are found to influence the anisotropy of BB bonds and elastic properties beside transferring valence electrons to the boron framework and stabilizing the crystal structure. From elastic property angle, it is the near-isotropic elastic modulus and high shear deformation resistance underpinned by the nearly isotropic BB bonding that warrant these low symmetry and open structured ternary borides superhard.

Special features of the interface between glass–metal coatings of superhard materials powders and metal matrices

The main results of the investigation of special features of the formation of the interface between the glass coating on the diamond and cBN powders with metals, that are bonds of grinding tools are described and generalized. It has been shown that the mutual diffusion occurring in the contact zone results in the increase of the adhesion at the interphase boundary, which ensures a strong fastening of a metal coating on a glass aggregate and the glass aggregate itself in a metal bond.

Superhard W0.5Ta0.5B nanowires prepared at ambient pressure

The primary focus of superhard materials development has relied on chemical tuning of the crystal structure. While these intrinsic effects are invaluable, there is a strong possibility that hardness can be dramatically enhanced using extrinsic effects. Here, we demonstrate that the superhard metal W0.5Ta0.5B can be prepared as nanowires through flux growth. The aspect ratios of the nanowires are controlled by the concentration of boride in molten aluminum, and the nanowires grow along the boron-boron chains, confirmed via electron diffraction. This morphology inherently results from the crystal habit of borides and can inspire the development of other nanostructured materials.

Superhard Mixed Transition Metal Dodecaborides

Solid solutions of mixed metal dodecaborides of ZrB12, YB12, and ScB12 were prepared by arc-melting and studied for their mechanical properties. Zr1–xYxB12 formed an essentially perfect solid solution, closely following Vegard’s law. Zr1–xScxB12 and Y1–xScxB12 undergo a face centered-cubic to body-centered tetragonal transition at 90–95 at. % Sc as determined by powder X-ray diffraction and transmission electron microscopy. The compounds Zr0.5Y0.5B12, Zr0.5Sc0.5B12, and Y0.5Sc0.5B12 are superhard (Vickers hardness ≥ 40 GPa) and demonstrate an increase in hardness to 45.8 ± 1.3, 48.0 ± 2.1, and 45.2 ± 2.1 GPa under a load of 0.49 N, respectively, compared to 40.4 ± 1.8, 40.9 ± 1.6, and 41.7 ± 2.2 GPa for pure ZrB12, YB12, and ScB12, respectively. In addition, Zr0.5Y0.5B12, Zr0.5Sc0.5B12, and Y0.5Sc0.5B12 solid solutions show a substantial increase in oxidation resistance to approximately 630, 685, and 695 °C, respectively, when compared to other superhard metal borides (e.g., ∼400 °C for WB4) and their all...

Ultraincompressible, Superhard Materials

The search for new ultraincompressible, superhard materials has been performed largely through trial and error. Despite the difficulties in synthesizing such materials, interest in this field has blossomed with recent experimental and theoretical results. Such progress has led to the development of superhard metal borides, ultraincompressible nitrides, and strong carbides. Our previous parameters of high electron density and high covalent bond density had served us well in synthesizing ultraincompressible, superhard metals, but after a decade of research, a more detailed understanding is needed. By studying the origin of strength from previously discovered incompressible, hard materials, we hope to gain new insight into this rapidly growing field. In particular, we propose a focus on bonding and structure to understand why some materials are ultraincompressible and superhard.

Superhard Monoborides: Hardness Enhancement through Alloying in W1- x Tax B.

In tungsten monoboride (WB), the boron atoms are linked in parallel serpentine arrays, with tungsten atoms in between. This lattice is metallic, unlike conventional covalent superhard materials such as diamond or cubic boron nitride. By selectively substituting tungsten atoms with tantalum, the Vickers hardness can be increased to 42.8 GPa, creating a new superhard metal.

Extrinsic Hardening of Superhard Tungsten Tetraboride Alloys with Group 4 Transition Metals.

Alloys of tungsten tetraboride (WB4) with the group 4 transition metals, titanium (Ti), zirconium (Zr), and hafnium (Hf), of different concentrations (0-50 at. % on a metals basis) were synthesized by arc-melting in order to study their mechanical properties. The phase composition and purity of the as-synthesized samples were confirmed using powder X-ray diffraction (PXRD) and energy dispersive X-ray spectroscopy (EDS). The solubility limit as determined by PXRD is 20 at. % for Ti, 10 at. % for Zr, and 8 at. % for Hf. Vickers indentation measurements of WB4 alloys with 8 at. % Ti, 8 at. % Zr, and 6 at. % Hf gave hardness values, Hv, of 50.9 ± 2.2, 55.9 ± 2.7 and 51.6 ± 2.8 GPa, respectively, compared to 43.3 GPa for pure WB4 under an applied load of 0.49 N. Each of the aforementioned compositions are considered superhard (Hv > 40 GPa), likely due to extrinsic hardening that plays a key role in these superhard metal borides. Furthermore, these materials exhibit a significantly reduced indentation size effect, which can be seen in the plateauing hardness values for the W1-xZrxB4 alloy. In addition, W0.92Zr0.08B4, a product of spinoidal decomposition, possesses nanostructured grains and enhanced grain hardening. The hardness of W0.92Zr0.08B4 is 34.7 ± 0.65 GPa under an applied load of 4.9 N, the highest value obtained for any superhard metal at this relatively high loading. In addition, the WB4 alloys with Ti, Zr, and Hf showed a substantially increased oxidation resistance up to ∼460 °C, ∼510 °C, and ∼490 °C, respectively, compared to ∼400 °C for pure WB4.

Crystal Field Splitting is Limiting the Stability and Strength of Ultra-incompressible Orthorhombic Transition Metal Tetraborides.

The lattice stability and mechanical strengths of the supposedly superhard transition metal tetraborides (TmB4, Tm = Cr, Mn and Fe) evoked recently much attention from the scientific community due to the potential applications of these materials, as well as because of general scientific interests. In the present study, we show that the surprising stabilization of these compounds from a high symmetry to a low symmetry structure is accomplished by an in-plane rotation of the boron network, which maximizes the in-plane hybridization by crystal field splitting between d orbitals of Tm and p orbitals of B. Studies of mechanical and electronic properties of TmB4 suggest that these tetraborides cannot be intrinsically superhard. The mechanical instability is facilitated by a unique in-plane or out-of-plane weakening of the three-dimensional covalent bond network of boron along different shear deformation paths. These results shed a novel view on the origin of the stability and strength of orthorhombic TmB4, highlighting the importance of combinational analysis of a variety of parameters related to plastic deformation of the crystalline materials when attempting to design new ultra-incompressible, and potentially strong and hard solids.

Structure of superhard tungsten tetraboride: A missing link between MB2 and MB12 higher borides

Superhard metals are of interest as possible replacements with enhanced properties over the metal carbides commonly used in cutting, drilling, and wear-resistant tooling. Of the superhard metals, the highest boride of tungsten--often referred to as WB4 and sometimes as W(1-x)B3--is one of the most promising candidates. The structure of this boride, however, has never been fully resolved, despite the fact that it was discovered in 1961--a fact that severely limits our understanding of its structure-property relationships and has generated increasing controversy in the literature. Here, we present a new crystallographic model of this compound based on refinement against time-of-flight neutron diffraction data. Contrary to previous X-ray-only structural refinements, there is strong evidence for the presence of interstitial arrangements of boron atoms and polyhedral bonding. The formation of these polyhedral--slightly distorted boron cuboctahedra--appears to be dependent upon the defective nature of the tungsten-deficient metal sublattice. This previously unidentified structure type has an intermediary relationship between MB2 and MB12 type boride polymorphs. Manipulation of the fractionally occupied metal and boron sites may provide insight for the rational design of new superhard metals.

Change of acute stage reactants level in experimental intramedullary osteosynthesis

Aim. To assess the plasma protein fractions as organism’s reaction to osteosynthesis by medical devices made of medical steel and of medical steel coated by titanium and gafnium nitrides mixture. Methods. Diaphyseal tibial fracture was modeled on 30 rabbits with further assessment of animal organism reaction to osteosynthesis by pins made of medical steel and of medical steel coated by super-hard metal nitrides mixture. The change of blood plasma protein fractions and acute phase reactants was used as an indicator. Measurements were performed before the osteosynthesis and on the 5th and 180th day after the surgery. Results. Together with the stable level of total blood protein, dysproteinemia characteristic for acute phase of the inflammation was observed after the trauma and reparatory process induction. Two-waived reaction was registered. The first developed at the 5th day after the surgery and was characterized by the drop of blood albumin level together with α- and β2-globulin levels increase, the reaction was observed in the intervention group earlier compared to control group. The second wave developed at the 180th day and was characterized by β2- and γ-globulin levels increase. The registered changes corresponded with the phases of acute non-specific response, which includes the overproduction of «positive» and decreased production of «negative» acute phase reactants by liver, influenced by cytokines, as well as later adaptive immune response. Conclusion. Overall, the intensity of acute phase reactions was lower in the intervention group, indicating less damage associated with titanium and gafnium nitrides coated implants use.