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The effect of chromium diboride addition on the densification process and oxidation behavior of two ZrB2-MoSi2 and ZrB2-SiC baseline systems was studied. CrB2 was beneficial in lowering the sintering temperature owing to the tendency of its oxide to react with MoSi2 and SiC forming low-melting phases that helped the powder consolidation. Oxidation at 1500 °C induced the formation of further boron oxide as first consequence. In one case, when CrB2 was combined with MoSi2, an improved oxidation resistance was observed due to the stabilization of Cr-borides in the subscales saturated with B2O3. In the other case, when it was combined with SiC, the excessive low viscosity of the borosilicate glass facilitated the consumption of a thicker portion of materials as compared to the ZrB2-SiC reference.

High-entropy boride ceramics of (Zr0.2Ta0.2Ti0.2Nb0.2Hf0.2)B2 and (Zr0.2W0.2Ti0.2Mo0.2Hf0.2)B2 were prepared by combining powder synthesis via borothermal reduction process at 1600 °C and subsequent ceramic densification by spark plasma sintering at 2000 °C in argon. High-entropy diboride powdered products exhibited ultra-fine particle size (

High entropy ceramics have attracted much attention in recent years, however, few open literature on the composites in which high entropy phases were used as matrix can be found. In this work, high entropy diboride powder was firstly synthesized, and then monolithic high entropy diboride ceramics of (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 (sample HEB) and high entropy particulate composites of (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2-20 vol% SiC (sample HEB-SiC) were prepared by SPS at different temperatures. As-synthesized high entropy diboride powders demonstrated good sinterability in the temperature range from 1600 °C to 1900 °C. The grain growth rate of the high entropy phase was comparable to that in the ZrB2 system, but the addition of SiC second phase particles inhibited the grain growth of the high entropy diboride phase. Due to the grain growth inhibition by the introduced SiC particles, the hardness for the particulate composites increased. Moreover, the SiC particles improved the fracture toughness through crack deflection and branching.

The effect of alumina fibers and nanosize graphite particles on the morphology and hardness of coatings based on silicon–boron carbide–zirconium diboride composites heat-treated in air atmosphere is considered.

A family of twelve monometallic diborides, containing transition metals from group IVB elements to group VIII elements, and Ni-W bimetallic diborides are synthesized via a molten salt-assisted method. Their trend in activity for the hydrogen evolution reaction is studied, with Ni-W bimetallic diborides acting as the most efficient, nonprecious electrocatalysts.

The production of composite electrolytic coatings on a nickel substrate using binary titanium–chromium diboride obtained by mechanical synthesis was studied. The influence of heat treatment parameters on the phase composition and structure of the coatings was examined. It is shown that these coatings substantially increase the wear resistance of structural medium-carbon steel.

The compressive yield stress and bending strength of NiAl–CrB2 composites are studied in the 20–800°C temperature range. The effect of the composite phase ratio and experimental temperature on the composite strength for different loading patterns is established. During tensile stresses, the introduction of 15 wt.% CrB2 into the intermetallic is optimal to ensure the maximum bending strength of the composite at 500°C. The bending strength of NiAl–15 wt.% CrB2 composite at the experimental temperature 500°C exceeds that of the starting NiAl, by a factor of 3.

The synthesis of early transition nanocrystals using NaBH4and the respective metal oxides at atmospheric pressure was studied at temperatures between 400 and 1000°C. Reaction products were analyzed by x‐ray diffraction, the crystallite size was determined after Rietveld refinement of diffraction patterns, while the morphology was analyzed by scanning and transmission electron microscopy. For all the investigated systems the lowest temperature to complete the synthesis was 700°C and the reaction occurred in three subsequent steps: (i) decomposition of NaBH4, (ii) formation of crystalline ternary species Na–M–O and Na–B–O, (iii) conversion of intermediary species to MB2and NaBO2. Syntheses carried out at T>700°C only caused coarsening of the powders. The synthetized boride powders had the morphology of highly agglomerated nanocrystals. TiB2had a specific surface area of 33.5m2/g and crystallite diameter of 12nm. Both ZrB2and HfB2had a platelet‐like morphology with crystallite diameter around 45nm and specific surface area of 25.0 and 36.4m2/g, respectively. Finally, NbB2and TaB2powders had a crystallite diameter around 5nm with specific surface area of 21.1 and 11.4m2/g, respectively. The goal of this synthesis is the use of cheap raw materials and moderate temperature conditions.

Friction and wear behaviour of the hot pressed monolithic chromium diboride (CrB 2 ) of 94% theoretical density were studied at different loads (3, 5 and 10 N) and frequencies (10 and 15 Hz) under dry reciprocative sliding condition against cemented tungsten carbide (WC–Co) ball. Coefficient of friction and wear rate were calculated. While the value of coefficient of friction (COF) is decreased from 0.50 to 0.36 with increasing load from 3 N to 10 N; it is observed to be nearly independent with respect to sliding frequency. The measured wear volume is in the range of 0.67 × 10 −3 –2.24 × 10 −3 mm 3 for the investigated regime. The lowest specific wear rate of 4.63 × 10 −7 mm 3 /N m was observed at 10 N load and 15 Hz frequency. Abrasive and tribo-oxidative wear with delamination were the active mechanisms manifested in both flat and counter body materials.

The influence of chromium diboride additions on the tribotechnical characteristics of NiAl–CrB2 composite coatings in friction against a counterface made of NiAl plasma-sprayed coating is studied. The friction surfaces of the plasma-sprayed composite NiAl coatings with different chromium diboride contents (5, 15, and 30 wt.%) are examined. It is established that wear substantially depends on the amount of refractory component.

Cermet coatings based on Ni–Al alloys and titanium–chromium diboride are electrospark-deposited onto 40Kh steel. It is analyzed how the metal-to-refractory phase ratio in the electrode material and the deposition conditions influence the mechanical and tribotechnical characteristics of the coatings. The wear resistance and friction coefficients of the electrospark-deposited coatings are shown to depend on the friction speed and loading. The cermet coatings show high tribotechnical characteristics (I = 1–5 μm/km, f = 0.2–0.25) at speeds to 12 m/sec and loads to 4 MPa in dry friction conditions. These characteristics can be controlled by varying electrode chemical composition and deposition conditions.

The effect of TiB2 content on mechanical properties of silicon carbide–titanium diboride ceramic composites was studied. The hardness of the ceramics decreased from 27.8 GPa for nominally pure SiC to 24.4 GPa for nominally pure TiB2. In contrast, fracture toughness of the ceramics increased from 2.1 MPa m 1/2 for SiC to ∼6 MPa m 1/2 for SiC with TiB2 contents of 40 vol.% or higher. Flexure strengths were measured for three composites containing 15, 20, and 40 vol.% TiB2 and analyzed using a two parameter Weibull analysis. The Weibull modulus increased from 12 for 15 vol.% TiB2 to 17 for 20 and 40 vol.% TiB2. Microstructural analysis revealed microcracking in the ceramics containing 20 and 40 vol.% TiB2. The ceramic containing 40 vol.% TiB2 had the best combination of properties with a fracture toughness of 6.2 MPa m 1/2 , hardness of 25.3 GPa

An aluminum-copper matrix composite reinforced with aluminum diboride particles was studied at high temperature via thermomechanometry experiments. The matrix contained 2 wt% Cu, whereas the amount of boron forming AlB2 ranged from 0 to 4 wt%, i.e., 0 to 8.31 vol% of diboride particles. In the first segment of the research, we demonstrated that larger amounts of AlB2 particles raised the composite hardness even at 300°C. To assess the material creep behavior, another set of specimens were tested under 1 N compression at 400°C and 500°C for 12 h. Higher levels of AlB2 allowed the composites to withstand compression creep deformations at those temperatures. By using existing creep models developed for metal matrix composites we were able to determine that viscous slip deformation was the dominant deformation mechanism for the temperatures and stress levels used in our experiments. Additionally, the computed creep activation energy for these aluminum matrix composites were found comparable to the energies reported for other similar materials, for instance, Al/SiCp composites.

The paper examines the wear mechanisms of the VT-3 titanium alloy and TBKhN composite in fretting corrosion conditions. It is shown that the oxide film formed on the titanium alloy during friction fails and further wear occurs on the juvenile surface. This leads to seizure and removal of the material from the surface, indicating that adhesive wear prevails in friction of the titanium alloy. The TBKhN40 composite is characterized by oxidative wear. The oxide film with grain-oriented fine structure forms in the friction process through selective oxidation of refractory and metal components.

In this study, AlB 2 powders were synthesized by using a combined method of mechanical alloying (MA) and annealing of elemental aluminum (Al) and boron (B) powders. Milling was performed in a planetary ball-mill (Fritsch™ Pulverisette 7 Premium Line) up to 15 h under argon (Ar) atmosphere. Annealing process was carried out in a tube furnace at 650 °C for 6 h under Ar atmosphere. The effects of MA durations on the annealing process and AlB 2 formation were investigated. The conversion of Al and B powders to AlB 2 starts after only MA for 3 h or after MA for 1 h and subsequent annealing. A slight formation of AlB 12 occurs at 242 °C for as-blended powders and it shifts to about 272 °C for MA’d powders. Al–B powder blends MA’d for 9 h and annealed have AlB 2 particles in size between 35 and 75 nm in the presence of Al 13 Fe 4 , Fe 3 B and Fe 2 B contaminations.

The resistance to electrochemical corrosion and high-temperature oxidation of titanium-chromium diboride and materials with a metal binder based upon it is studied. The mechanism of processes that occur in compacted specimens and powders is studied, and the composition of reaction products in relation to specimen heating temperature is determined. It is established that the oxide films that form at a specimen surface during oxidation are protective and prevent further material oxidation.

We study structural, elastic, and electronic properties for three RuB 2 phases using ab initio total-energy calculations within the density functional theory. The orthorhombic and hexagonal structures are mechanically stable. More precisely, the orthorhombic is more stable than the hexagonal form. Results of bulk modulus, which are in good agreement with experimental data, show that the considered structures are potentially highly compressible materials. This is confirmed by the calculation of the hardness, indicating that RuB 2 is an ultracompressible material, but not a superhard material.

The effect of a bias potential applied to the substrate on the structure formation, substructural characteristics, and physicomechanical properties of tantalum diboride nanocrystalline films obtained by the method of nonreactive high-frequency magnetron sputtering has been investigated. It has been shown that the sign and magnitude of the bias potential exert a substantial influence on the structure, composition, and physicomechanical properties of arising coatings. Upon the application of a positive bias potential to the substrate, textured films with a columnar structure, whose greatest hardness was ∼45 GPa and the elasticity modulus was only moderate, ∼350 GPa, have been obtained. Upon the application of a negative bias potential to the substrate, the amorphous cluster films with a hardness of ∼11.5 GPa and an elasticity modulus of ∼144 GPa were condensed.

A study of the structural, elastic and electronic properties of BeB2, a promising diboride in view of the lighter mass of Be, has been made using self-consistent Density Functional Theory (DFT). The five independent elastic constants of BeB2 are predicted for the first time. The elastic behaviour of the compound is compared with those of the superconducting MgB2 and NbB2. We also evaluate the electronic structure of BeB2 in order to obtain further insight into its surprising difference from the superconducting MgB2.

We have carried out a thermodynamic analysis of chemical processes in the Cr – CrB2 – CrSi2 system at 1400 K to 1800 K. We have established that chromium diboride fulfills the function of a protective layer, while chromium disilicide protects against reaction with air at temperatures up to 1473 K.

Results are provided for a study of the reaction of MoSi2 with niobium and tantalum diborides. It is established that sections of MoSi2–Nb(Ta)B2 within the quaternary system Mo–Si–B–Nb(Ta) are described by eutectic composition diagrams with a eutectic temperature equal to 1870 and (1890 ± 30)°C. The diboride phase content is 20 and 14 mol.% respectively in systems with NbB2 and TaB2.

Boride ceramics were directionally solidified around LaB 6 –(Ti x Zr 1− x )B 2 eutectic compositions. The directionally solidified rods were composed of LaB 6 rich concentric belts surrounding large eutectic regions with a dense and uniform distribution of (Ti x Zr 1− x )B 2 fibers of 0.6 μm in diameter inside a LaB 6 matrix. The di- and hexa-boride mole fractions at eutectic points have been more accurately defined for x = 0–0.76. Fiber growth directions were modified by the addition of titanium. Diboride (subscript 2) fibers grew along [0 0 0 1] 2 for x = 0 and along 〈 1 ¯ 1 0 0 〉 2 for x ≠ 0. The orientation of the hexaboride (subscript 6) matrix was unchanged with or without titanium addition. The favored crystallographic orientation relationship was: [0 0 1] 6 //[0 0 0 1] 2 and ( 1 1 0 ) 6 / / ( 1 1 2 ¯ 0 ) 2 and ( 1 ¯ 1 0 ) 6 / / ( 1 ¯ 1 0 0 ) 2 for any x value. The dominant interfaces were { 1 1 0 } 6 / / { 1 1 2 ¯ 0 } 2 for x = 0 and (0 0 1) 6 //(0 0 0 1) 2 for x ≠ 0, the latter being expected to be energetically more favorable, as predicted by a 2D geometrical model.

Magnetocaloric effect has been investigated in TbB(2) in the vicinity of a second-order magnetic transition at 144K. The maximum magnetic entropy change -Delta S(M) is 12.7J kg(-1) K(-1) for a field change from 0 to 7T. The relative cooling power and adiabatic temperature change Delta T(ad) are 256J kg(-1) T(-1) and 29.5 K for a field change from 0 to 5T. For a low-field change from 0 to 2 T, considerable -Delta S(M) (5.2 J kg(-1) K(-1)) and Delta T(ad) (14.7 K) are obtained with a cooling power of 67.6 J kg(-1) T(-1), suggesting TbB2 a good candidate for magnetic refrigerant around 144 K. (C) 2010 Elsevier B.V. All rights reserved.

It has been shown that by directional crystallization of pseudobinary compositions of molybdenum disilicide (MoSi 2 ) with some other refractory compounds, particularly with diborides of transition metals (composite materials having a disilicide matrix and extended in crystallization direction diboride inclusions) can be obtained that may improve the mechanical properties of the matrix material.

The phase relations of the molybdenum diboride compounds MoB 2− x and Mo 2 B 5− y and the structure of Mo 2 B 5− y have been re-examined with samples prepared between 1400 and 1800°C. Two types of six-membered boron ring, nearly planar and puckered, occur within the Mo 2 B 5− y structure. The sites at the centers of the puckered rings may be unoccupied (K′ layers) or occupied by boron atoms (K layers). Neutron diffraction refinements show that these sites are completely unoccupied (K′ layers) in Mo 2 B 5− y samples of several batch compositions. This result, combined with the insensitivity of the unit cell dimensions of Mo 2 B 5− y to batch composition, shows that this phase has a narrow single phase region, with y close to unity, between 1400 and 1800°C. MoB 2− x also has a narrow single-phase region centered at about 61 at.% boron over this temperature range.

Nanocrystalline titanium diboride (TiB 2 ) powders were successfully synthesized at 650 °C by using TiCl 4 , Mg and MgB 2 as the reactants. Transmission electron microscopy images show that typical TiB 2 crystallites are composed of uniform particles with the average grain size of about 40 nm.

Sliding friction experiments were conducted on SiC, Si3N4 B4C-TiB2 and Al2O3 ceramics under the condition of sliding contact with themselves in air. The test conditions included temperature up to 800°C., maximum surface velocity of 1.14m/min, and normal forces ranging from 4.31N(0.44kgf) to 104.57N(10.67kgf). Friction coefficient as low as 0.09 to 0.1 was obtained at 800°C for boron carbide-titanium diboride. The wear rates of Si3N4, SiC and Al2O3 ceramics increased with increasing temperature. However, the wear rate of B, C-TiB, decreased with increasing temperature. For most of the sliding pairs, the wear rate was in the range of 10-5mm3/Nm to 10-4mm3/N-m, the single exception being a value of 10-8mm3/N-m recorded for B4C-TiB2 at 800°C. The implications of wear mechanism for these results were shown.

It is shown that on cocrystallization of hexaboride (of f-transition metals) and diboride (of d-transition metals) phases in eutectic mixtures the crystallographic relation between rigid boron networks in both phases is kept constant and is defined by mutual joining of boron atom pairs in both structures, independently of the direction of crystallization.

In this letter we address the discrepancy in the literature concerning the enthalpy of formation of AlB 2 by means of density functional calculations. The literature values vary from 23 to 151 kJ/mol. Our calculated value of 16 kJ/mol clearly favors the lower value with which it is in agreement within the expected accuracy of DFT.

Niobium diboride was prepared by chemical vapour deposition from a gas mixture of NbCl 5 , BCl 3 , H 2 and Ar on a quartz substrate heated to a temperature of 950–1200°C. Homogeneous films were formed from 950 to 1050°C, and pillar crystal from 1050 to 1200°C. Niobium diboride was found to react with the quartz substrate above 1200°C to form a silicide, Nb 5 Si 3 . The optimum molar compositions of the gas mixture used to grow crystals of NbB 2 were 2.5–5.0 vol% of BCl 3 , and 2.5–5.5 vol% of NbCl 5 . In the presence of impurities of nickel regular hexagonal shaped pillar crystals of maximum 10 μm in a edge and 50 μm in length were obtained.

The conditions of growth of TiB 2 single crystals from iron, cobalt and nickel solutions and those of ZrB 2 and HfB 2 from iron solution have been investigated and the quality of the crystals grown by this method have been characterized. The diboride crystals were formed only when the mole percent of the metal diboride and solvent metal were higher than a critical value in each case. TiB 2 crystals grown from iron contained less than 0.2 wt% Fe, those from cobalt contained about 1 wt% Co and those from nickel contained more than 1.5 wt% Ni. ZrB 2 and HfB 2 crystals from iron solution included less than 0.5 wt% Fe. Etch-pit densities of the (0001) plane of TiB 2 single crystals grown from iron were lower than those from cobalt or nickel. It was possible to obtain single crystals of low etch-pit density in the case of ZrB 2 and HfB 2 . Lang X-ray topographs of TiB 2 , ZrB 2 and HfB 2 crystals grown from iron showed images representing low internal stresses. In contrast, the topographs of TiB 2 crystals grown from cobalt and nickel showed high internal stress images.

A study was made of the zones of diffusional reaction of chromium diboride with carbon steels, cast iron, and Armco iron, forming under glow-discharge conditions. An attempt was made to discover the laws governing the formation of diffusion layers in these systems. It was established that with rise in carbon concentration in the steel the width of the diffusional reaction zone grows, attaining a maximum for a steel with a carbon content of 0.8%. At a carbon concentration in the steel of 1.6% or more, the width of the diffusion zone sharply decreases. In a CrB2-steel system a strong inhibiting influence on diffusion is exerted by the higher chromium carbide Cr3C2.

VB 2 and NbB 2 crystals were grown by a radio frequency floating zone method and characterized. Materials having a nearly stoichiometric composition and total metallic impurities less than 0.9 wt.% were obtained for both diborides. Products containing single crystals of 3–5 mm in diameter were obtained. In both diboride single crystals, etching sections had numerous etch pits (densities: 10 6 −10 7 /cm 2 along 〈11 2 0〉 directions in the (0001) plane, and etch pits and line patterns along the [0001] direction in {10 1 0} and {11 2 0} planes, respectively.