Molybdenum Boride Research Articles

Tribological behavior of PS400-related tribopairs for space exploration

PS400 is a multi-component alloy developed by NASA and intended for high temperature bearing applications. This study reports on four mating surfaces, namely bulk 4130 steel, diamond-like carbon (DLC), polycrystalline diamond, and molybdenum boride cobalt chrome coatings sliding against PS400 coating, and the best performing tribopair at 25 and 500 °C was identified. The temperature range and sliding speeds simulate extreme conditions for low-speed space-related applications, such as bearings on landing probes for future missions to Venus. PS400 vs. DLC showed low friction and excellent wear resistance up to 500 °C. DLC provided good lubrication at 25 °C, whereas at 500 °C oxide glazes transferred on DLC, maintaining low friction. The worn surfaces and the transfer films were analyzed using different analytical techniques.

In situ formation of molybdenum borides at hardfacing by arc welding with flux-cored wires containing a reaction mixture of B4C/Mo

This paper reports a study into the formation of the phase composition, structure, and properties of arc welding coatings by the flux-cored electrode materials from the Fe-Mo-B-C system. The welding alloys were applied using the flux-cored arc welding (FCAW) electrodes, which consisted of a shell made from the low-carbon steel filled with a reaction powder mixture that contained boron carbide and molybdenum in a ratio of 1:1. The calculation of the phase composition of alloys that correspond to the surfaced layers by a CALPHAD method using the Thermo-Calc OpenCalphad software shows that under the equilibrium conditions the boride phases of molybdenum and ferrite cannot co-exist. The main phase of such alloys is a FeMo 2 B 2 compound, which forms the eutectics with austenite. Given that the eutectic structures with borides are characterized by high brittleness, the introduction of components was conducted in the form of a reaction mixture in order to obtain the in situ formed boride phases in the form of separate structural components. Analysis of the results of studying the microstructure and phase composition of coatings reveals that they consist of three main structural components: the eutectic (FeMo 2 B 2 +ferrite) and the grains of molybdenum tetraboride MoB 4 . Thus, under the conditions of arc welding using the reaction mixture, an irregular structure is formed, which is favorable in terms of ensuring wear resistance due to the high microhardness of MoB 4 >27 GPa. The hardness of the coatings obtained is at the level of 63–65 HRC, and the wear resistance is higher compared to standard high-chromium alloys (grades Т620 and Т590) by 2‒2.5 times. This makes it possible to recommend the coating of a given system for hardfacing the working surfaces of equipment in the coal, processing, woodworking industries, etc., where abrasive wear is the dominant type of surface wear

Molybdenum Boride as an Efficient Catalyst for Polysulfide Redox to Enable High-Energy-Density Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries, despite having high theoretical specific energy, possess many practical challenges, including lithium polysulfide (LiPS) shuttling. To address the issues, here, hydrophilic molybdenum boride (MoB) nanoparticles are presented as an efficient catalytic additive for sulfur cathodes. The high conductivity and rich catalytically active sites of MoB nanoparticles allow for a fast kinetics of LiPS redox in high-sulfur-loading electrodes (6.1 mg cm-2 ). Besides, the hydrophilic properties and good wettability toward electrolyte of MoB can facilitate electrolyte penetration and LiPS redox, guaranteeing a high utilization of sulfur under a lean-electrolyte condition. Therefore, the cells with MoB achieve impressive electrochemical performance, including a high capacity (1253 mA h g-1 ) and ultralong lifespan (1000 cycles) with a low capacity fade rate of 0.03% per cycle. Also, pouch cells fabricated with the MoB additive deliver an ultrahigh discharge capacity of 947 mA h g-1 , corresponding to a low electrolyte-to-capacity ratio of about 4.8 µL (mA h)-1 , and remain stable over 55 cycles under practically necessary conditions with a low electrolyte-to-sulfur ratio of 4.5 µL mg-1 .

Study of Dry Sliding Wear and Immersion Corrosion on Al 5083 Reinforced With MWCNT, MoB, and Ni

Dry sliding wear and immersion corrosion behaviors of Al 5083-based hybrid composite reinforced with multiwalled carbon nanotube (MWCNT), Molybdenum boride (MoB) and nickel (Ni) are studied with different weight percentages. The reinforcement weight percentages of MWCNT ranges from 0 to 1.5, that of MoB from 1 to 4 and for Ni from 2 to 8. Dry sliding wear behavior at room temperature is studied using Pin on Disc by varying the sliding distances from 500 to 2000m, load from 10 to 40 N and sliding velocity from 0.25 to 1.75m/s. The wear studies revealed that there is a considerable decrease in wear rate for composites than the alloy material with increase in %wt of reinforcements for all test parameters. The worn surface analysis revealed that there are two types of wear mechanisms namely abrasive and adhesive. The uniform immersion corrosion tests also showed decreasing rate with increase in reinforcements.

A review and perspective on molybdenum-based electrocatalysts for hydrogen evolution reaction

Water electrolysis has been considered as a sustainable way for producing renewable energy of hydrogen. However, this process requires a low-cost and high-efficient hydrogen evolution reaction (HER) catalyst to improve the overall reaction efficiency. Molybdenum (Mo)-based electrocatalysts are regarded as the promising candidates to replace the benchmark but expensive Pt-based HER catalysts, due to their high activity and stability in a wide pH range. In this review, we present a comprehensive and critical summary on the recent progress in the Mo-based electrodes for HER, including molybdenum alloys, molybdenum sulfides, molybdenum selenides, molybdenum carbides, molybdenum phosphides, molybdenum borides, molybdenum nitrides, and molybdenum oxides. Particular attention is mainly focused on the synthetic methods of Mo-based materials, the strategies for increasing the catalytic activity, and the relationship between structure/composition and electrocatalytic performance. Finally, the future development and perspectives of Mo-based electrocatalysts toward high HER performance are proposed.

Structure, Stability, and Mechanical Properties of Boron-Rich Mo-B Phases: A Computational Study.

Molybdenum borides were studied theoretically using first-principles calculations, parameterized lattice model, and global optimization techniques to determine stable crystal structures. Our calculations reveal the structures of known Mo-B phases, attaining close agreement with experiment. Following our developed lattice model, we describe in detail the crystal structure of boron-rich MoBx phases with 3 ≤ x ≤ 9 as the hexagonal P63/mmc-MoB3 structure with Mo atoms partially replaced by triangular boron units. The most energetically stable arrangement of these B3 units corresponds to their uniform distribution in the bulk, which leads to the formation of a disordered nonstoichiometric phase, with ordering arising at compositions close to x = 5 because of a strong repulsive interaction between neighboring B3 units. The most energetically favorable structures of MoBx correspond to the compositions 4 ≲ x ≤ 5, with MoB5 being the boron-richest stable phase. The estimated hardness of MoB5 is 37-39 GPa, suggesting that the boron-rich phases are potentially superhard.

Effect of the Surface Oxidation of Molybdenum Boride on the Adsorption and Interaction of Carbon Oxide and Oxygen Molecules

Surface analysis conducted under ultrahigh vacuum conditions shows that the oxidation of a B‒Mo(110) alloy with a submonolayer concentration of boron atoms dramatically improves the efficiency of conversion of co-adsorbed carbon monoxide and oxygen molecules to carbon dioxide. It is established that this effect is associated with differences between the properties of the adsorbed particles before and after molybdenum boride oxidation, particularly an increase in the slope of the CO molecular axis to the plane of the adsorbent surface and the weakening of the chemisorptive bonds of oxygen with the substrate on the surface of the MoxByOz ternary compound. It is concluded that this substrate can be considered a model system and an alternative to the currently available catalysts for CO oxidation based on noble metals.

A facile pathway to prepare molybdenum boride powder from molybdenum and boron carbide

AbstractMolybdenum boride is an ideal hard and wear‐resistant material. In this study, a new method is proposed for preparing molybdenum boride, by which Mo first reacts with B4C to generate the mixture of molybdenum boride and C, and then the product is decarburized by molten Ca to generate CaC2. Pure molybdenum boride could be obtained after acid leaching to remove the by‐product CaC2. According to the experimental and thermodynamic calculation results, it is concluded that the single‐phase MoB could be successfully prepared, while Mo2B, Mo2B5, and MoB4 could not be synthesized by this method. Moreover, it was found that the particle size of finally prepared MoB is determined by particle size of raw Mo powder. The residual carbon content of the product could be decreased to 0.10 wt% after first reaction at 1673 K for 6 hours and then decarburization reaction at 1673 K for 6 hours.

Chemical and structural engineering of transition metal boride towards excellent and sustainable hydrogen evolution reaction

Herein, holey, thin, conductive nickel substituted cobalt molybdenum boride (Ni-CMB) nanosheets have been designed to obtain superior electrochemical HER performance with small overpotential of 69 mV at 10 mA cm-2 current density and lower Tafel slope of 76.3 mV dec-1 in alkaline medium. Incorporation of Ni leads to improved conductivity and favorable hydrogen adsorption on Mo sites, which collectively yield efficient electrocatalytic H2 production from Ni-CMB catalyst. The ultrathin nature (thickness = 5.0 nm) of the designed material expectedly helps to attain high exposure of active sites and facile charge transportation through the nanosheets. Additionally, the decorated mesopores (average size = 3.86 nm) on nanosheets have benefitted towards faster electrolyte diffusion, easy gas escape from catalyst surface to support high electrocatalytic performance. Finally, well-maintained morphology of the sample and evolution of HER active sites in the material have guaranteed long-term, sustainable hydrogen production even at high current densities, which clearly demonstrate its superiority over an expensive electrolyzer (Pt-C) in alkaline water.

Study of solidification pathway of a MoSiBTiC alloy by optical thermal analysis and in-situ observation with electromagnetic levitation

MoSiBTiC alloys are promising candidates for next-generation ultrahigh-temperature materials. However, the phase diagram of these alloys has been unknown. We have developed an ultrahigh-temperature thermal analyser based on blackbody radiation that can be used to analyse the melting and solidification of the alloy 67.5Mo–5Si–10B–8.75Ti–8.75 C (mol%). Furthermore, electromagnetic levitation (EML) was used for in-situ observation of solidification and microstructural study of the alloy. On the basis of the results, the following solidification pathway is proposed: Mo solid solution (Moss) begins to crystallize out as a primary phase at 1955 °C (2228 K) from a liquid state, which is followed by a (Moss+TiC) eutectic reaction starting at 1900 °C (2173 K). Molybdenum boride (Mo2B) phase precipitates from the liquid after the eutectic reaction; however, the Mo2B phase may react with the remaining liquid to form Moss and Mo5SiB2 (T2) as solidification proceeds. In addition, T2 also precipitates as a single phase from the liquid. The remaining liquid reaches the (Moss + T2 + TiC) ternary eutectic point at 1880 °C (2153 K), and the (Moss + T2 + Mo2C) eutectic reaction finally occurs at 1720 °C (1993 K). This completes the solidification of the MoSiBTiC alloy.

Bipolar Electrochemistry as a Simple Synthetic Route toward Nanoscale Transition of Mo2B5 and W2B5 for Enhanced Hydrogen Evolution Reaction

The development of non-noble metal and earth-abundant-based electrocatalysts for electrochemical hydrogen production is of great significance for large scale application of water splitting technology. Particularly, molybdenum- and tungsten-based materials, such as MoS2, WS2, MoP, and WP, are widely studied for their promising performance in catalyzing the hydrogen evolution reaction (HER), especially in their nanostructured forms. Here, we investigate the HER activity of nanostructured molybdenum and tungsten borides, which are highly understudied, and report the use of bipolar electrochemistry (BP) to enhance their performances. BP treatment induces the size reduction of commercial Mo2B5 and W2B5 from the micrometer range down to the nanometer range. The smaller Mo2B5 and W2B5 particles have greater exposure of active sites for HER catalysis and result in better HER performance. To illustrate, we found that the electrocatalytic performance of BP-treated W2B5 is close to the best HER electrocatalyst, Pt/C. Electrochemical impedance spectroscopy measurements reveal lower charge transfer resistance (Rct) of the materials after bipolar electrochemical treatment, thereby suggesting another effect of the BP method in enhancing HER catalysis. Overall, we demonstrate bipolar electrochemistry as a simple method to improve the catalytic activity of Mo2B5 and W2B5.

Heretophase Ceramics in the Hf–Si–Mo–B System Fabricated by the Combination of SHS and Hot Pressing Methods

This work is devoted to the fabrication of heterophase powder and consolidated ceramics based on hafnium and molybdenum borides and silicides by combining self-propagating high-temperature synthesis (SHS) and hot pressing (HP). Composite ceramic HfB2–HfSi2–MoSi2 SHS powders are fabricated according to the magnesium-thermal reduction flowsheet from oxide raw materials, in which the combustion wave has temperatures of 1750–2119 K and rather high mss combustion rates of 8.4–9.3 g/s. The structure of synthesized SHS powders consists of relative coarse MoSi2 grains up to 10 μm in size, submicron elongated HfB2 grains mainly located inside the MoSi2 grains, and rounded Si precipitates. The composition with a lower boron concentration contains numerous polyhedral HfSi2 grains smaller than 10 μm in size. The resulting powders have an average particle size of ~6 μm with a maximal size up to 26 μm. The phase compositions of the ceramics consolidated by the HP method and synthesized SHS powders are identical. The microstructure of compact samples consists of faceted elongated HfB2 grains 0.5–10.0 μm in size, polyhedral HfSi2 and MoSi2 grains up to 8–10 μm in size, and silicon interlayers. The consolidated ceramics have a high structural and chemical homogeneity, low residual porosity of 1.1–1.7%, high hardness of 11.7–12.6 GPa, and thermal conductivity of 62–87 W/(m K).

Prediction of phonon-mediated superconductivity in two-dimensional Mo2B2

We predict two new molybdenum boride monolayers as phonon-mediated superconductors with superconducting transition temperatures of 3.9 and 0.2 K.

Effect of the Phase Composition of Molybdenum Boride on its Catalytic Properties in the Epoxidation of 1-Octene by tert-Butyl Hydroperoxide

It was shown that the epoxidation of 1-octene by tert-butyl hydroperoxide in the presence of Mo2B is accompanied by the formation of an amorphous oxygen-containing phase on its surface. It was established that this is the phase that catalyzes the reaction and determines its speed and selectivity.

A novel approach to boriding of TZM by spark plasma sintering method

Boriding can be candidate method to increase mechanical properties and oxidation resistance of molybdenum based alloys. Rapid oxidation of TZM at elevated temperature and low wear resistance are considered to be main drawbacks for possible applications. In this study molybdenum boride layer was formed on the surface of TZM by spark plasma sintering (SPS) method. Pre-mixed TZM was used as a raw material in powder form. Both sintering and boriding process were performed in a single step at constant temperature of 1420 °C in various pressures (40–60 MPa) and holding times (5–15 min) under vacuum. The effects of molybdenum boride layer on density, hardness and oxidation behavior were investigated. Thickness, microstructure and hardness of Mo-B based protective layer showed a variety with different process parameters. The boride layer thickness was reached to approximately 200 μm by increasing pressure and holding time. Formation of this layer provided a decrease in mass loss about 73.4% during oxidation test at 1000 °C for 60 min. The maximum surface hardness value was obtained as 21.4 GPa while hardness of base metal was 2.3 GPa.

Synthesis, Characterization and Electrocatalytic Activity for Hydrogen Evolution Reaction of Ternary Transition Metal Borides

Recently, as the concerns on the global warming and the depletion of fossil fuels, the most immediate scientific challenge is developing clean and renewable energy. Hydrogen has focused as one of the promising alternatives to carbon-based fuels because hydrogen is eco-friendly energy. There are lots of methods to produce hydrogen. One easy and simple way to produce extremely pure hydrogen is water electrolysis, through hydrogen evolution reaction (HER). Currently, noble metals such as platinum are generally regarded as the state-of-the-art electrocatalysts for improving the kinetics and efficiency of HER. However, their prohibitive price and scarcity limit their application industrially. It is therefore highly desirable to discover highly active and stable HER electrocatalysts composed of inexpensive and earth abundant elements. In the last few years, boron based materials have been proposed as low-cost alternative electrocatalysts for HER. Recently, binary molybdenum borides (MoB, Mo2B, MoB2 and Mo2B4), iron boride (FeB2) and amorphous cobalt boride (Co2B) were reported to have outstanding catalytic activities for the HER. In this work, we have synthesized molybdenum-tungsten diborides using solid solution synthesis. We substituted tungsten for molybdenum. The synthesized materials were characterized by X-ray power diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray analysis (EDX) and scanning electron microscope (SEM). Linear sweep voltammetry (LSV) was performed to determine the electrochemical properties by using a three electrodes system, and cyclic voltammetry (CV) was conducted to measure stability.

PARAMETERS OF LASER PROCESSING AND THEIR INFLUENCE ON TRIBOLOGICAL CHARACTERISTICS OF IRON-BASED COATINGS

The paper considers improvement of physic-mechanical and operational properties of adhesive coatings after laser infusion with additional alloying В4С, ТаВ and МоВ. Influence of the laser infusion with additional alloying on structure, microhardness and wear-resistance of adhesive coatings of the Fe–Cr–B –Si system has been studied in the paper. While increasing a laser beam velocity microstructure is changed from equilibrium to quasi-eutectic. Presence of molybdenum boride and tantalum increases sensitivity of the coating to specific features of laser remelting. In both cases heat exchange conditions have been changed, a number of iron and chromium borides has been increased and due to this molybdenum and tantalum have been partially passing to free state that contributes to a disintegration of structural components. While introducing solid particles B4C into a coating they are dissolved in an iron matrix while being heated by a laser beam and under cooling they are isolated in the form of separated Fe an Cr boride inclusions. Laser infusion and alloying increase coating wear-resistance. Load increase from 30 to 70 Н improves coating wear resistance averagely by 15–26 % and wear resistance of non-alloyed coatings is improved by 26–43 %. An increase of melting rate and laser spot diameter does not exert significant influence on wear but an increase in overlapping coefficient leads to reduction of coating wear. Presence of solid particles in a coating and an increase in rate of melting by laser beam reduce coating wear resistance. Such rather complicated dependence of coating wear rate on conditions of laser melting and wearing process is due to a complex of structural and phase transformations which have contributed to formation of secondary solid inclusions and increased microhardness.

Preparation and Characterization of MoB Coating on Mo Substrate

A molybdenum boride (MoB) coating was prepared on molybdenum (Mo) by an in-situ chemical vapor deposition method. To develop a fuller understanding of the effect of depositional conditions on the growth of MoB coating formed on Mo substrate, this paper studied the phase composition and microstructure of the coating at varied conditions. At the same time, the relationships between the growth of the coating in thickness and depositional conditions were also investigated. The phase composition of the coating, which was prepared using 0.2 wt % boron (B), was composed of MoB and Mo2B phases. The single MoB phase was obtained using a high B content (above 0.2 wt %). The microstructure of the coating showed that MoB coating could be prepared at temperatures over 900 °C. With the increase of temperature and time, the thickness of the coating increased. The growth of the coating was also influenced by the content of NaF. With increasing content, the thickness of the coating first increased, and then decreased. In addition, the prepared coating had a high hardness (3130.85 HV), and the mass change of the coated sample was only 4.92 mg/cm2 after isothermal oxidation at 600 °C for 100 h. The MoB coating could provide a good oxidation resistance for Mo at 600 °C.

Binary Molybdenum Borides Nanostructures As Novel Electrocatalysts for Hydrogen Evolution Reaction

Molybdenum based materials have recently emerged as highly active non-noble metal electrocatalysts for hydrogen evolution reaction (HER) [1, 2]. Molybdenum borides in contrary have not been studied for their HER activity at the nanoscale, however, they were recently shown to be already efficient HER catalysts in the bulk [3]. In this study, we have successfully synthesized nanocrystalline molybdenum boride nanowires (MoB) and nanospheres (MoB2) using a simple, one-step, solid state reaction. The structural and morphological characterizations were studied using XRD, XPS and electron microscopy. The obtained molybdenum borides nanostructures were tested for their electrocatalytic activity for HER in acidic conditions. We found that, nanoscale MoB2 exhibits a low onset overpotential and low Tafel slope value compared to nanoscale MoB indicating a boron- and structural dependency of these nanoborides for the HER. Also, the onset overpotential and Tafel slope values obtained represent a significant improvement from those observed for the already highly active bulk borides [3], thus opening a new avenue for inexpensive nanoscale metal boride nanomaterials as highly efficient non-noble metal HER catalysts.

Molybdenum Borides for Hydrogen Evolution Reaction: Theoretical Study and Experimental Proof

In recent years’ molybdenum-based materials have been considered as alternative catalysts to noble metals, such as platinum, for the hydrogen evolution reaction (HER) [1, 2]. We have synthesized bulk binary metal borides (Mo2B, MoB and MoB2) by arc-melting as well as MoB and MoB2 at the nanoscale by solid-state reaction and study their electrocatalytic activity towards HER in acid conditions for the first time. Among all the binary molybdenum borides, MoB2 shows excellent HER activity in both micro- and nanoscale. However, the nanoscale MoB2 shows a lower onset overpotential of about 150 mV compared to microscale MoB2 under same conditions. The electrochemical studies clearly indicating that the HER activity was greatly dependent on their crystallographic structures (boron-dependency) as well as particle size [2]. Also, our recent density functional theory (DFT) calculations on MoB2 suggest that boron is playing a prominent role in the catalytic activity of this compound. Detailed synthetic procedures, electrochemical properties and DFT calculations for these binary molybdenum borides will be discussed.