Search

A simple and efficient microwave-assisted preparation of ReB2-based material is reported utilizing ammonium perrhenate (NH4ReO4), magnesium boride (MgB12) reactants and carbon as an absorber of microwave irradiation. The investigation of microwave irradiated NH4ReO4 +MgB12 +C mixtures, thermal analysis results and electron microscopy examination reveals that NH4ReO4 decomposition produces ReO3 at early stages of the process. The ReO3 then exothermically reacts with MgB12 forming the nanoscale Re3B phase, which converts into ReB2 upon further irradiation. The coupling of microwave energy with exothermic reactions significantly accelerates the formation of ReB2. The product primarily consists of ReB2 as well as B4C and minor carbon phases. Structural characterization reveals that the average particle size of ReB2 is ~ 50 nm.

As the limits of what can be achieved with conventional hard compounds, such as tungsten carbide, are nearing reach, super-hard materials are an area of increasing industrial interest. The refractory hard metal borides, such as ReB2 and WB4, offer an increasingly attractive alternative to diamond and cubic boron nitride as a next-generation tool material. In this Thesis, a thorough discussion is made of the progress achieved by our laboratory towards understanding the synthesis, structure, and properties of these extremely hard compounds. Particular emphasis is placed on structural manipulation, solid solution formation, and the unique crystallographic manifestations of what might also be called "super-hard metals".

The applications of super hard material have been increased in aerospace, marine and automobile industries. In the present, the investigation deals with rhenium diboride (ReB2)-based molybdenum metal matrix which is fabricated through stir casting method. Rhenium diboride is mixed to the molybdenum with various weight percentages such as 3, 5 and 8%. The material properties are evaluated for the sample specimens. Based on the comparison of material properties, molybdenum with 8% of rhenium diboride has been considered for these experimental investigations. Due to high hardness and strength, conventional machining methods are not suitable. Among all the unconventional machining processes, the laser beam machining process is the most suitable for high hardness materials. Various input and output parameters are considered. The responses such as the material removal rate (MRR) and surface roughness (SR) are measured. The effects and contribution of parameters are confirmed through analysis of variance (ANOVA) and Pareto charts.

An emerging class of superhard materials for extreme environment applications are compounds formed by heavy transition metals with light elements. In this work, ultrahigh pressure experiments on transition metal rhenium diboride (ReB2) were carried out in a diamond anvil cell under isothermal and non-hydrostatic compression. Two independent high-pressure experiments were carried out on ReB2 for the first time up to a pressure of 241 GPa (volume compression V/V0 = 0.731 &plusmn, 0.004), with platinum as an internal pressure standard in X-ray diffraction studies. The hexagonal phase of ReB2 was stable under highest pressure, and the anisotropy between the a-axis and c-axis compression increases with pressure to 241 GPa. The measured equation of state (EOS) above the yield stress of ReB2 is well represented by the bulk modulus K0 = 364 GPa and its first pressure derivative K0&acute, = 3.53. Corresponding density-functional-theory (DFT) simulations of the EOS and elastic constants agreed well with the experimental data. DFT results indicated that ReB2 becomes more ductile with enhanced tendency towards metallic bonding under compression. The DFT results also showed strong crystal anisotropy up to the maximum pressure under study. The pressure-enhanced electron density distribution along the Re and B bond direction renders the material highly incompressible along the c-axis. Our study helps to establish the fundamental basis for anisotropic compression of ReB2 under ultrahigh pressures.

The properties of structure, elasticity, and electron for ReB2 and MoB2 are investigated, using first-principles calculations, as well as the role of chemical bonds between metal layer and boron la...

Rhenium diboride is an established superhard compound that can scratch diamond and can be readily synthesized under ambient pressure. Here, we demonstrate two synergistic ways to further enhance the already high yield strength of ReB

Shear strength measurements have been carried out on rhenium diboride, ReB2, to a pressure of 74 GPa using a Radial X-ray Diffraction (R-XRD) technique in a diamond anvil cell using platinum as an internal x-ray pressure standard. The R-XRD result has provided a unique insight into the deformation of hexagonal crystal lattice under non-hydrostatic compression and variation of shear strength with increasing pressure. From R-XRD data, we have estimated hydrostatic component of compression to determine an equation of state of rhenium diboride yielding a bulk modulus of K0 = 366 ± 25 GPa with a pressure derivative K 0 ′ = 4.3 ± 0.5 in good agreement with hydrostatic density functional theory calculations. The average lower bound of shear strength (τ) from various diffraction planes was then calculated using the measured interplanar d-spacing (dm) and hydrostatic component of d-spacing (dp) to be shown to approach 6.7 ± 0.4 GPa at 70 GPa. Our results show that the anisotropic compression effects observed in ReB2 under hydrostatic compression are correlated to electronic structure changes under compression as predicted by theoretical calculations.

Rhenium diboride (ReB2) in its P 6 3 / mmc crystalline structure, is widely known as a super-hard material, and has been studied many times using the Density Functional Theory (DFT) approach. In this work the same chemical composition was studied in three additional unreported hypothetical crystallographic phases by means of DFT with the hybrid functional approach, and the elastic constants of each phase were calculated. The elastic behavior of ReB2 was analyzed by means of elastic moduli calculations. Additionally, the velocities of the elastic waves of each phase were calculated, along with the Debye’s temperatures, also elastic anisotropy is studied. Semi-empirical and empirical models of hardness were used to estimate qualitatively which phases are or are not hard. It has been determined that the elastic moduli of two out of the three hypothetical phases are desirable and the elastic waves move very slow ( 2 km/s) in one of them. These results and the analysis of the bond critical points (bcp) of each phase allow us to conclude that one of them is soft while the other two are hard. The synthesized phase of ReB2 P 6 3 / mmc was studied in order to compare and confirm the results.

Three mechanically and dynamically stable polymorphs of rhenium diboride (ReB 2 ) (space group: P6 3 /mmc, No: 194), (space group: R-3m, No: 166) and (space group: Pmmn, No: 59) were thoroughly analysed within the framework of Density Functional Theory from the structural, mechanical, optical, thermodynamical and phonon properties point of view. The calculated hardness of rhombohedral structure suggests that it can be even harder than well known hexagonal ReB 2 .

Rhenium diboride (ReB2) thin films have been deposited by a nanosecond pulsed laser deposition method on Si (100) substrate heated to 570 °C. The coatings were formed in the ablation process of SPS sintered ReB2 target. The effect of laser wavelength, energy density and postannealing on the films' properties was studied. Investigated wavelengths were 355 and 1064 nm of Nd:YAG nanosecond laser. Laser beam energy density varied from 2.1 to 6.1 J cm− 2 and from 4.1 to 9.4 J cm− 2 at 355 and 1064 nm, respectively. Layer thickness was of the order of several hundred nanometres. Deposition efficiency increases with an energy density in quasi linear way and strongly grows for shorter wavelength. The layers consist of two characteristic microstructures: a smooth basis and stick debris (typical diameters of several hundred nanometres). Lower energy density of laser beam and longer wavelength favour dominance of smooth basis and minimization of debris. The XRD analysis of all samples indicate the crystalline ReB2 with preferred (002) orientation and fine grain size of about 20 nm. Shorter wavelength and higher energy density foster stronger (002) orientation. Moreover, an annealing right after the deposition (25 min in 350 °C) causes minimization of degree of orientation and decrease of hardness. The Vickers hardness of ReB2 films is at about 60 GPa and is reduced to about 40 GPa after the annealing process. Deposition efficiency and physical and chemical structures of layers produced under variety of conditions were studied and compared.

Rhenium diboride (ReB2) powder was prepared by mechanochemical processing of Re–B powder mixtures with subsequent annealing at temperatures of 600 °C to 1200 °C. Reactive evolution during the synthesis was investigated; furthermore, the effects of the amount of excess B on the reactions that occurred during the synthesis were assessed. The substantial reaction of Re with B occurred at 700 °C to form Re7B3 with a small amount of ReB2. At 800 °C, Re7B3 converted into ReB2; this conversion was enhanced with increasing temperature and increasing amount of excess B. At 1000 °C or above, single-phase ReB2 powder without trace quantities of Re7B3 was obtained for compositions with 15 wt% or greater excess B. The synthesized ReB2 powder particles were submicrometer with the vast majority being ∼500 nm. In addition, the resulting ReB2 powders were consolidated by hot pressing or spark plasma sintering to examine the sinterability of the powders.

Using first-principles calculations, the elastic constants, thermodynamic property and structural phase transition of rhenium diboride under pressure are investigated by means of the pseudopotential plane-waves method, as well as the effect of metallic bond on its hardness. Eight candidate structures of known transition-metal compounds are chosen to probe for rhenium diboride ReB2. The calculated lattice parameters are consistent with the experimental and theoretical values. Based on the third order Birch–Murnaghan equation of states, the transition pressure Pt between the ReB2–ReB2 and MoB2–ReB2 phases is firstly determinate. Elastic constants, shear modulus, Young’s modulus, Poisson’s ratio and Debye temperature are derived. The single-bonded B–B feather remains in ReB2 compounds. Furthermore, according to Mulliken overlap population analysis, a semiempirical method to evaluate the hardness of multicomponent crystals with partial metallic bond is presented. Both strong covalency and a zigzag topology of interconnected bonds underlie the ultraincompressibilities. In addition, the superior performance and large hardness (39.1 GPa) of ReB2–ReB2 indicate that it is a superhard material.

WB2 nanocomposite coatings were synthesized by dc magnetron sputtering. The morphology, chemical composition, chemical bonding state, microstructure, hardness, and tribological properties were investigated. The columnar-structured coatings are dense and uniform. The as-deposited coatings were found to have a metastable AlB2-type structure as characterized by X-ray diffraction and further confirmed by high-resolution transmission electron microscopy. Coatings exhibit superhardness about 43.2 +/- 5GPa (taken from the plateau region) obtained by nanoindentation. In addition, the steady-state friction coefficient mu=0.23 and wear rate K=6.5x10-6mm3N-1m-1 were obtained under dry sliding condition in ambient environments. These characteristics indicate AlB2-type WB2 coatings have high potential application as superhard and low wear coatings. Moreover, the AlB2-type WB2 can hardly be prepared using the conventional powder metallurgy technology without high pressure, and this study proposes a new practical approach to prepare it.

Rhenium diboride (ReB2), containing corrugated layers of covalently bonded boron, is a superhard metallic compound with a microhardness reaching as high as 40.5 GPa (under an applied load of 0.49 N). Tungsten diboride (WB2), which takes a structural hybrid between that of ReB2 and AlB2, where half of the boron layers are planar (as in AlB2) and half are corrugated (as in ReB2), has been shown not to be superhard. Here, we demonstrate that the ReB2-type structure can be maintained for solid solutions of tungsten in ReB2 with tungsten content up to a surprisingly large limit of nearly 50 atom %. The lattice parameters for the solid solutions linearly increase along both the a- and c-axes with increasing tungsten content, as evaluated by powder X-ray and neutron diffraction. From micro- and nanoindentation hardness testing, all of the compositions within the range of 0–48 atom % W are superhard, and the bulk modulus of the 48 atom % solid solution is nearly identical to that of pure ReB2. These results furth...

Structural relaxations, electronic properties, and surface energies of ReB2 (001) and (110) surfaces with various terminations are investigated with a first-principles method. It is found that the surface interatomic spacings of ReB2 (001) and (110) surfaces are different from those of the bulk structure. The vertical spacings between the first and second layers of the studied surfaces are contracted. The (001)-Re surface is likely to be stable without introducing a large relaxation. Among these surfaces, only the (110) surface has surface rumpling, and the Re atoms on its first layer are apt to move inward. After atomic relaxation, some covalent bonds formed by the outmost atoms of the relaxed surfaces are shorter than those of the bulk system, which indicates that the covalent B–B and Re–B bonds of the surface layer have been strengthened. An analysis of surface energies shows that after relaxation, the (001)-Re surface is more stable than other types of surfaces.

We have presented an efficient method to predict the anisotropic hardness of crystalline materials along different crystallographic directions or on different crystallographic planes in terms of electronegativity. Bond stretching and bending strengths, respectively, are proposed to characterize the ability of a chemical bond to resist stretching and bending deformation, which are the main microscopic deformations in single crystals when measuring indentation hardness. Good agreement between the calculated and experimental values of anisotropic hardness for a large range of crystalline materials has been achieved, including sphalerite, wurtzite and rocksalt structured materials, as well as oxides (e.g. alpha-SiO(2) and LaGaO(3)) and graphite. The anisotropic hardness values of other important materials, such as B(12) analogs, group IVA nitrides, tungsten carbide structured materials, and transition metal di- and tetra-borides, were quantitatively predicted. We found that materials with the same crystal structure have the same or similar hardness anisotropy. Furthermore, the more orderly bond arrangement in single crystals and the greater bond ionicity often result in greater hardness anisotropy. This work shines a light on the nature of hardness and on studies of the anisotropy of other macroscopic properties of crystalline materials. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Mixed-metal borides are promising superhard materials (Kaner et al. (2005) [1] ). In this Letter, density functional computations have been applied to the structural, electronic and elastic properties of mixed-metal diborides Re 0.5 Ir 0.5 B 2 , Re 0.5 Tc 0.5 B 2 , Os 0.5 W 0.5 B 2 and Os 0.5 Ru 0.5 B 2 . The elastic moduli decrease from pure metal diboride ReB 2 to Re 0.5 Ir 0.5 B 2 and on the contrary increase from OsB 2 to Os 0.5 W 0.5 B 2 because boron–metal interactions are contaminated by the occupied anti-bonding orbitals. Alloying ReB 2 (OsB 2 ) with Tc (Ru) decreases the elastic moduli owing to the relativistic effects. Mixed-metal effects on elastic deformations focus on bonding strengths, which effectively tune the elastic properties of metal diborides.

The five independent moduli required to construct the complete monocrystal elastic modulus tensor of the hexagonal-symmetry superhard compound ReB(2) were measured from 308 to 5 K using resonant ultrasound spectroscopy on a special-texture polycrystal. This is possible because, confirmed by X-ray diffraction, the specimen measured was composed of grains with hexagonal axes parallel so that its polycrystal elastic response is identical to a monocrystal and because hexagonal-symmetry solids are elastically isotropic in the plane perpendicular to the hexagonal axis. Along the hexagonal (c) axis, C(33) (0) = 1021 GPa, nearly equal to C(11) of diamond, and consistent with the superhard properties. However, in the (softer) isotropic plane, C(11) (0) = 671 GPa, much lower than diamond. The changes of C(ij) with temperature are very small and smooth. The Debye temperature was computed to be 738 K, and using a high-temperature approximation, the Grüneisen parameter is γ = 1.7.

Dense transition metal borides have recently been identified as superhard materials that offer the possibility of ambient pressure synthesis compared to the conventional high pressure, high temperature approach. This feature article begins with a discussion of the relevant physical properties for this class of compounds, followed by a summary of the synthesis and properties of several transition metal borides. A strong emphasis is placed on correlating mechanical properties with electronic and atomic structure of these materials in an effort to better predict new superhard compounds. It concludes with a perspective of future research directions, highlighting some recent results and presenting several new ideas that remain to be tested.

This paper investigates the structural and electronic properties of rhenium diboride by first-principles calculation based on density functional theory. The obtained results show that the calculated equilibrium structural parameters of ReB2 are in excellent agreement with experimental values. The calculated bulk modulus is 361 GPa in comparison with that of the experiment. The compressibility of ReB2 is lower than that of well-known OsB2. The anisotropy of the bulk modulus is confirmed by c/a ratio as a function of pressure curve and the bulk modulus along different axes along with the electron density distribution. The high bulk modulus is attributed to the strong covalent bond between Re-d and B-p orbitals and the wider pseudogap near the Fermi level, which could be deduced from both electron charge density distribution and density of states. The band structure and density of states of ReB2 exhibit that this material presents metallic behavior. The good metallicity and ultra-incompressibility of ReB2 might suggest its potential application as pressure-proof conductors.

The elastic and physical characteristics of ReB2 crystal have been predicted through a method of density functional theory within the generalized gradient approximation (GGA). Five independent elastic constants are C11=662 GPa, C12=150 GPa, C13=146 GPa, C33=1090 GPa and C44=263 GPa. The bulk modulus (B), shear modulus (G), Young s modulus (E), Poisson s ratio (γ) and the ratio of linear compressibility coefficient along the a- and c-axis crystal direction (Ka/Kc) are 356 GPa, 305 GPa, 711 GPa, 0.167 and 1.758, respectively. In addition, the dependence of bulk modulus (B) on temperature (T) and pressure (p) as well as the coefficient of thermal expansion (αL) at various temperatures are evaluated and discussed. The coefficient of thermal expansion is consistent with the famous Gruneisen s law when the temperature is less than 1500 K. Our results agree well with the other experimental results.

Recently, the superhardness of rhenium diboride films was reported. In this study the first successful preparation and characterization of ruthenium boride films is presented. The morphology, topography, microstructure and hardness of films, prepared by pulsed laser deposition, were investigated. The films, which are 0.7 μm thick, have a dense grain texture, and are composed of two phases Ru2B3 (main phase, 65% volume fraction) and RuB2 (35%). The RuB2 phase does not show any preferred orientation, while Ru2B3 is textured preferentially along the (1 1 4) and (1 0 5) directions, with crystallite growth parallel within 1.9° of average mismatch. The composite Vickers microhardness of the film–substrate systems was measured, and the intrinsic hardness of the films was separated using an area law-of-mixtures approach. The obtained films were found to be superhard, the intrinsic film hardness value (49 GPa) being much higher than that for the RuB2 bulk used as the target for film deposition and than that for the Ru2B3 bulk.

The simultaneous synthesis and densification of rhenium diboride is investigated starting from Re and B as reactants by using the Spark Plasma Sintering (SPS) apparatus. It is shown that SPS represents an effective technique to synthesize ReB 2 bulk samples with high purity and density. In particular, a dense product with traces of secondary phases (Re 7 B 3 ) is obtained in 35 min of total processing time by applying a maximum temperature of 1600 °C and a mechanical pressure of 20 MPa.

First-principles calculations of the crystal structures of rhenium diboride (ReB2) have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The quasi-harmonic Debye model, using a set of total energy versus molar volume obtained with the first-principles calculations, is applied to the study of the thermal and vibrational effects. The structural parameters, thermal expansions, heat capacities, Gruneisen parameters and Debye temperatures dependence on the temperature and pressure are obtained in the whole pressure range from 0 to 70 GPa and temperature range from 0 to 2000 K as well as compared with available data.

Superhard materials are usually composed of light elementssuch as boron, carbon, nitrogen, and oxygen. These lightelemental atoms have the ability to form covalently bonded,three-dimensional networks of high atomic density withextreme resistance to external shear. Most researchers agreeonthedefinitionaccordingtowhich‘‘superhard’’materialsarethosewithVickershardness(Hv)higherthan40GPa

Single crystals of ReB(2) have been prepared from an aluminum flux under inert gas flow. The crystals are typically 1-3 mm in diameter and 500 microm thick, growing along the [002] direction with a distinct hexagonal morphology. Vickers microhardness and nanoindentation testing indicate that the (002) plane possesses the highest hardness with measured values of 40.5 and 36.4 GPa, respectively. The elastic anisotropy was examined and the indentation moduli of the basal plane and an (hk0) plane of unknown indices are 675 and 510 GPa, respectively. Four-probe electrical resistivity measurements demonstrate that ReB(2) is the hardest material known to exhibit metallic behavior. Thermogravimetric analysis indicates that the crystals are stable in air up to 1000 degrees C due to the formation of a protective boron oxide coating.

Recently, the superhardness of rhenium diboride was discovered. This study presents a first successful preparation and characterization of thin ReB2 films. The films were deposited by the pulsed laser deposition (PLD) technique. The morphology, microstructure, and hardness of the films were investigated. The films are compact and continuous, with a preferred (002) orientation. The composite Vickers hardness of the film-substrate systems was measured, and the intrinsic hardness of the films was separated using an area law-of-mixtures approach taking into account the indentation size effect. The obtained films are found to be superhard: the intrinsic film hardness value (52 GPa) is close to that of the ReB2 bulk.

▪ Abstract The synthesis of the two currently used superhard materials, diamond and cubic boron nitride, is briefly described with indications of the factors influencing the quality of the crystals obtained. The physics of hardness is discussed and the importance of covalent bonding and fixed atomic positions in the crystal structure, which determine high hardness values, is outlined. The materials investigated to date are described and new potentially superhard materials are presented. No material that is thermodynamically stable under ambient conditions and composed of light (small) atoms will have a hardness greater than that of diamond. Materials with hardness values similar to that of cubic boron nitride (cBN) can be obtained. However, increasing the capabilities of the high-pressure devices could lead to the production of better quality cBN compacts without binders.

By electron and x-ray diffractionwe establish that the CrB4 compound discovered over 40 years ago crystallizes in the oP 10 (Pnnm) structure, in disagreement with previous experiments but in agreement with a recent first-principles prediction. The 3D boron network in this structure is a distorted version of the rigid carbon sp(3) network proposed recently for the high-pressure C-4 allotrope. Our systematic density functional theory analysis of the electronic, structural, and elastic properties in ten related transition metal TMB4 tetraborides (TM = Ti, V, Cr, Mn, Fe and Y, Zr, Nb, Mo, Tc) identifies CrB4 as the prime candidate to be a superhard material. In particular, the compound's calculated weakest shear and tensile stresses exceed 50 GPa, and its Vickers hardness is estimated to be 48 GPa. We compare the reported and estimated Vickers hardness for notable (super) hard materials and find that the CrB4 calculated value is exceptionally high for a material synthesizable under standard ambient-pressure conditions.

Cubic BC2N was synthesized from graphite-like BC2N at pressures above 18 GPa and temperatures higher than 2200 K. The lattice parameter of c-BC2N at ambient conditions is 3.642(2) A, which is larger by 1.48% than would be expected based on ideal mixing between diamond and cubic boron nitride. The bulk modulus of c-BC2N is 282 GPa which is one of the highest bulk moduli known for any solid, and is exceeded only by the bulk moduli of diamond and c-BN. The hardness of c-BC2N is higher than that of c-BN single crystals which indicates that the synthesized phase is only slightly less hard than diamond.

The unit-cell volume of rhenium diboride $({\text{ReB}}_{2})$ has been measured by synchrotron x-ray diffraction at pressures and temperatures of up to 7.5 GPa and 1100 K with a cubic anvil apparatus. From the pressure $(P)$-volume $(V)$-temperature $(T)$ measurements, thermoelastic parameters were derived for ${\text{ReB}}_{2}$ based on a modified high-$T$ Birch-Murnaghan equation of state and a thermal-pressure approach. With the pressure derivative of the bulk modulus, ${K}_{0}^{\ensuremath{'}}$, fixed at 4.0, we obtain: the ambient bulk modulus ${K}_{0}=334(23)\text{ }\text{GPa}$, temperature derivative of bulk modulus at constant pressure ${(\ensuremath{\partial}K/\ensuremath{\partial}T)}_{P}=\ensuremath{-}0.064(6)\text{ }\text{GPa}\text{ }{\text{K}}^{\ensuremath{-}1}$, volumetric thermal expansivity ${\ensuremath{\alpha}}_{T}({\text{K}}^{\ensuremath{-}1})=a+bT$ with $a=1.33(25)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\text{ }{\text{K}}^{\ensuremath{-}1}$ and $b=1.48(64)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }{\text{K}}^{\ensuremath{-}2}$, pressure derivative of thermal expansion ${(\ensuremath{\partial}\ensuremath{\alpha}/\ensuremath{\partial}P)}_{T}=\ensuremath{-}5.76(95)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\text{ }{\text{GPa}}^{\ensuremath{-}1}\text{ }{\text{K}}^{\ensuremath{-}1}$, and temperature derivative of bulk modulus at constant volume ${(\ensuremath{\partial}K/\ensuremath{\partial}T)}_{V}=\ensuremath{-}0.049(11)\text{ }\text{GPa}\text{ }{\text{K}}^{\ensuremath{-}1}$. The ambient bulk modulus derived from this work is comparable to previous experimental and theoretical results. These results, including the ambient bulk modulus and other thermoelastic parameters determined in present study, extend our knowledge of the fundamental thermophysical properties on ${\text{ReB}}_{2}$ and are important to the development of theoretical and computational modelings of hard materials.

Recently, the superhardness of rhenium diboride was discovered. This study presents a first successful preparation and characterization of thin ReB2 films. The films were deposited by the pulsed laser deposition (PLD) technique. The morphology, microstructure, and hardness of the films were investigated. The films are compact and continuous, with a preferred (002) orientation. The composite Vickers hardness of the film−substrate systems was measured, and the intrinsic hardness of the films was separated using an area law-of-mixtures approach taking into account the indentation size effect. The obtained films are found to be superhard: the intrinsic film hardness value (52 GPa) is close to that of the ReB2 bulk.

Chung et al . (Reports, 20 April 2007, p. 436) reported the synthesis of superhard rhenium diboride (ReB 2 ) at ambient pressure. We show that ReB 2 , first synthesized at ambient pressure 45 years ago, is not a superhard material. Together with the high cost of Re, this makes the prospect for large-scale industrial applications of ReB 2 doubtful.

Considering the effect of spin-orbit coupling, we have studied the equation of state, elastic properties, and the electronic structures of $\mathrm{Re}{\mathrm{B}}_{2}$ by first-principles plane-wave basis pseudopotential calculations. Our calculated results show that $\mathrm{Re}{\mathrm{B}}_{2}$ has high bulk modulus $(356\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$, large shear modulus $(293\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$, and great elastic constant ${c}_{44}$ $(284\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$, and thus support that $\mathrm{Re}{\mathrm{B}}_{2}$ is ultraincompressible and a potentially superhard material. Besides the compact crystal structure, the electronic mechanisms of its hardness can be understood from the highly directional covalent bonds and the optimal filling of the bonding states.

The quest to create superhard materials rarely strays from the use of high-pressure synthetic methods, which typically require gigapascals of applied pressure. We report that rhenium diboride (ReB 2 ), synthesized in bulk quantities via arc-melting under ambient pressure, rivals materials produced with high-pressure methods. Microindentation measurements on ReB 2 indicated an average hardness of 48 gigapascals under an applied load of 0.49 newton, and scratch marks left on a diamond surface confirmed its superhard nature. Its incompressibility along the c axis was equal in magnitude to the linear incompressibility of diamond. In situ high-pressure x-ray diffraction measurements yielded a bulk modulus of 360 gigapascals, and radial diffraction indicated that ReB 2 is able to support a remarkably high differential stress. This combination of properties suggests that this material may find applications in cutting when the formation of carbides prevents the use of traditional materials such as diamond.

Rhenium diboride is a recently recognized ultra-incompressible superhard material. Here we report the electronic (e), phonon (p), e-p coupling and thermal properties of ReB$_2$ from first-principles density-functional theory (DFT) calculations and neutron scattering measurements. Our calculated elastic constants ($c_{11}$ = 641 GPa, $c_{12}$ = 159 GPa, $c_{13}$ = 128 GPa, $c_{33}$ = 1037 GPa, and $c_{44}$ = 271 GPa), bulk modulus ($B$ $\approx$ 350 GPa) and hardness ($H$ $\approx$ 46 GPa) are in good agreement with the reported experimental data. The calculated phonon density of states (DOS) agrees very well with our neutron vibrational spectroscopy result. Electronic and phonon analysis indicates that the strong covalent B-B and Re-B bonding is the main reason for the super incompressibility and hardness of ReB$_2$. The thermal expansion coefficients, calculated within the quasi-harmonic approximation and measured by neutron powder diffraction, are found to be nearly isotropic in $a$ and $c$ directions and only slightly larger than that of diamond in terms of magnitude. The excellent agreement found between calculations and experimental measurements indicate that first-principles calculations capture the main interactions in this class of superhard materials, and thus can be used to search, predict, and design new materials with desired properties., Comment: submitted to prb

First-principle calculations are performed to investigate the structural, elastic, and electronic properties of ReB2 and WB2. The calculated equilibrium structural parameters of ReB2 are consistent with the available experimental data. The calculations indicate that WB2 in the P6(3)/mmc space group is more energetically stable under the ambient condition than in the P6/mmm. Based on the calculated bulk modulus, shear modulus of polycrystalline aggregate, ReB2 and WB2 can be regarded as potential candidates of ultra-incompressible and hard materials. Furthermore, the elastic anisotropy is discussed by investigating the elastic stiffness constants. Density of states and electron density analysis unravel the covalent bonding between the transition metal atoms and the boron atoms as the driving force of the high bulk modulus and high shear modulus as well as small Poisson's ratio.