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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) 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.

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.

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.

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.

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.

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.

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.

The need for wear- and scratch-resistant materials drives the quest for new superhard materials. In this work, we apply two design parameters to identify ultra-incompressible, superhard materials-high valence electron density and high bond covalency. Our first example of such a material is OsB2. The bulk modulus of OsB2 was measured using in situ high-pressure X-ray diffraction and was determined to be in the range of 365-395 GPa. While this value is slightly less than that of the bulk modulus of diamond, due to the anisotropic crystal structure of OsB2, the axis compressibility in the orthorhombic c-direction is less than the axis compressibility found in diamond. OsB2 also scratches the surface of a sapphire window, indicating that the hardness of OsB2 exceeds 2000 kg/mm2.

The elastic moduli of polycrystalline rhenium diboride are measured as a function of temperature between 5 and 325 K. The room temperature results show that ReB2 has very high values for both the bulk and shear modulus, confirming the incompressible and superhard nature of this material. With decreasing temperature, the moduli increase, with a hint of softening below 50 K.

The ultraincompressible transition metal diborides RuB 2 , OsB 2 , and ReB 2 were synthesized by arc melting the elemental metals and boron under an argon atmosphere at ambient pressure. The hardness and Young’s modulus were measured using nanoindentation with a Berkovich diamond indenter. The bulk modulus and shear modulus were derived based on an isotropic model and then plotted as a function of hardness. A strong correlation is observed between the hardness and shear modulus for these transition metal diborides. © 2008 American Institute of Physics. DOI: 10.1063/1.2946665

Dubrovinskaia et al . question our demonstration that rhenium diboride (ReB 2 ) is hard enough to scratch diamond. Here, we provide conclusive evidence of a scratch through atomic force microscopy depth profiling and elemental mapping. With high hardness, high-bulk modulus, and the ability to withstand extreme differential stress, ReB 2 and related materials should be investigated regardless of their cost, which is not prohibitive.