Electrodeposition of coatings of double carbides of tungsten and molybdenum from tungstate–molybdate–carbonate solutions

The electrochemical deposition of coatings of double tungsten and molybdenum carbides from tungstate–molybdate–carbonate melts is investigated. The composition of formed coatings is investigated by X-ray fluorescent and X-ray phase analyses. The crystal size and coating thickness are determined using scanning electron microscopy. Optimal deposition parameters of W₂C · Mo₂C coatings are as follows: the melt composition is Na₂WO₄–(1.0–4.0 mol %) Li₂WO₄–(1.0–4.0 mol %) Li₂MO₄–(1.0–5.0 mol %) Li₂CO₃, the cathode current density is 750–1500 A/m², the process temperature is 1123–1173 K, and the electrolysis duration is 4 h.     

Microstructure and sliding-wear behavior of tungsten-reinforced W-Ni-Si metal-silicide <Emphasis Type="Italic">in-situ</Emphasis> composites

W-Ni-Si metal-silicide-matrix in-situ composites reinforced by tungsten primary grains were fabricated using a water-cooled copper-mold laser-melting furnace by the LASMELT process. Main constitutional phases of the W/W-Ni-Si in-situ composites are the tungsten primary phase, peritectic W₂Ni₃Si, and the remaining W₂Ni₃Si/Ni₃₁Si₁₂ eutectics, depending on the alloy compositions. The sliding-wear resistance of the W/W-Ni-Si intermetallic composites was evaluated at room temperature and 600 °C. Wear mechanisms of the W/W₂Ni₃Si in-situ composites were discussed based on morphology observations of the worn surface and wear debris. Results show that the W/W-Ni-Si composites have excellent wear resistance under both room- and high-temperature sliding-wear-test conditions, because of the high yield strength and toughness of the tungsten-reinforcing phase and the high hardness and the covalent-dominated intermetallic atomic bonds of the W₂Ni₃Si and Ni₃₁Si₁₂ metal silicides. Tungsten-reinforcing grains played the dominant role in resisting abrasive-wear attacks of microcutting, plowing, and brittle spalling during the sliding-wear process, while the W₂Ni₃Si and Ni₃₁Si₁₂ metal silicides are responsible for the excellent adhesive wear resistance.     

Carbothermal reduction of a highly dispersed mixture of chromium and silicon oxides

Conclusions The process of carbon reduction of a highly dispersed homogeneous composition from chromium and silicon oxides obtained by the solution method is accompanied by formation first of chromium carbides with a transition from lower to higher (Cr₃C₂) and then of Cr₅Si₃ and SiC. In the 1273–1673 K range the content of these phases is determined by the heating conditions and the ratio of the original components, sucrose, chromium acetate (or Cr₂O₃), and H₂SiO₃. Cr₅Si₃ phase is formed basically as the result of interaction of chromium carbides with SiO₂. An increase in the share of silica and carbon in the composition leads to broadening of the phase composition of the silicides and a decrease in SiC content.Translated from Poroshkovaya Metallurgiya, No. 1(337), pp. 59–64, January, 1991.The authors thank O. T. Khorpyakov, N. T. Bugaichuk, and N. G. Khotynenko for conducting the series of X-ray diffraction and infrared measurements.     

Intrinsic Kinetics of Chlorination of WO<Subscript>3</Subscript> Particles With Cl<Subscript>2</Subscript> Gas Between 973 K and 1223 K (700 °C and 950 °C)

Chlorination is one of the methods applied in extractive metallurgy for the treatment of minerals to obtain valuable metals, such as titanium and zirconium. The possibility of applying chlorination metallurgy to other metals such as tungsten was the major aim of this study. The kinetics of the chlorination of tungsten oxide (WO₃) particles has been investigated by thermogravimetry between 973 K and 1223 K (700 °C and 950 °C) and for partial pressures of chlorine ranging from 15 to 70 kPa. The starting temperature for the reaction of WO₃ with chlorine is determined to be about 920 K (647 °C). The influence of chlorine diffusion through the bulk gas phase and through the particle interstices in the overall rate was analyzed. In the absence of these two mass-transfer steps, a reaction order of 0.5 with respect to chlorine partial pressure, and an activation energy of 183 kJ/mol were determined. For tungsten oxide particles of less than 50-μm size, a complete rate expression has been obtained.     

Physicochemical interaction and structure development during the formation of metal gas-transfer coatings on diamond (review). 1. Kinetics

Work devoted to studying the phase composition and thickness of chromium, titanium, molybdenum, vanadium, and tungsten coatings which form on diamond powder during vacuum annealing of this powder mixed with chromium powder or oxidized powders of Ti, Mo, V, and W is analyzed. Coatings consist of metal (Cr, Ti, Mo, V, W) and carbide (Cr₇C₃, Cr₃C₂, TiC, -Mo₂C, V₂C, VC, W₂C, WC) phases. Diffusion of carbon during coating growth with increased metallizing time and temperature causes carbidization of chromium, titanium, and vanadium (it causes a reduction in the content of metal phase and an increase in carbide phases is coatings), growth of higher carbides (Cr₃C₂, VC, WC) at the expense of lower carbides (Cr₇C₃, V₂C, W₂C), and filling of carbon vacancies in the lattices of TiC and VC. Saturation of coatings with carbides correlates with the temperature-time range in which a further increase in coating weight slows down.Translated from Poroshkovaya Metallurgiya, No. 7(355), pp. 34–40, July, 1992.     

Phase formation during the carbothermic reduction of eudialyte concentrate

The phase transformations of eudialyte concentrate during the carbothermic reduction in the temperature range 25–2000°C are studied by thermodynamic simulation, differential thermal analysis, and X-ray diffraction. As the temperature increases to 1500°C, the following phases are found to form sequentially: iron and manganese carbides, free iron, niobium carbide, iron silicides, silicon and titanium carbides, and free silicon. Strontium, yttrium, and uranium in the temperature range under study are not reduced and are retained in an oxide form, and insignificant reduction of zirconium oxides with the formation of carbide ZrC is possible only at temperatures above 1500°C.     

A Morphological Study of the Carburization/Reduction of Tungsten Oxides with Carbon Monoxide

Thermogravimetry, X-ray diffraction analysis and scanning electron microscopy have been used to study the gas phase carburization/reduction by CO of WO₃, W₁₈O₄₉ and WO₂ over the temperature range 600 to 1100°C. The lower oxides W₁₈O₄₉ and WO₂ were produced by controlled reduction of WO₃ in H₂-H₂O mixtures. A wide range of particle morphologies and sizes have been obtained for the WC product and these have been related to the roles played by the lower oxides. The total carbon content of the carburized products is shown to be dependent upon both the temperature and time of processing.     

Physicochemical interaction and structure development during the formation of metal gas-transport coatings on diamond (review) II. Mechanism

Formation of chromium, titanium, molybdenum, vanadium, and tungsten coatings on diamond under various metallizing conditions and with different phase compositions of the metal sprays is studied. In the initial stages this process is controlled by the interaction of the diamond (carbon) surface with metal oxides through the gas phase as well as by diffusion of the metal spray. After formation of a continuous layer further coating growth is controlled by carbon diffusion. A reduction in the rate of mass transfer to the reaction surface and the rate of carbon yield from it are factors which limit coating growth. The former is connected with depletion of the metal spray; with a decrease in volatile phase flow, and reduction of MoO₃, V₂O₅, and WO₃, as well as with their removal during pumping; a reduction of the dissociation and disproportioning rate of the oxides; retardation of metal ion diffusion in the metal spray and appearance of oxides (V₂O₃, VO, Ti₂O₃, TiO, etc.) in it which interact weakly with carbon. Emergence of carbon at the surface is specified by its inhibition under diffusion in the growing layer of the coating, filling of carbon vacancies in carbide (TiC, VC) lattices, carbidization of metal, and transformation of lower carbides into higher forms.Translated from Poroshkovaya Metallurgiya, No. 8 (356), pp. 57–63, August, 1992.     

Reduction-Carburization of NiO-WO<Subscript>3</Subscript> Under Isothermal Conditions Using H<Subscript>2</Subscript>-CH<Subscript>4</Subscript> Gas Mixture

Ni-W-C ternary carbides were synthesized by simultaneous reduction–carburization of NiO-WO₃ oxide precursors using H₂-CH₄ gas mixtures in the temperature range of 973 to 1273 K. The kinetics of the gas–solid reaction were followed closely by monitoring the mass changes using the thermogravimetric method (TGA). As a thin bed of the precursors were used, each particle was in direct contact with the gas mixture. The results showed that the hydrogen reduction of the oxide mixture was complete before the carburization took place. The nascent particles of the metals formed by reduction could react with the gas mixture with well-defined carbon potential to form a uniform product of Ni-W-C. Consequently, the reaction rate could be conceived as being controlled by the chemical reaction. From the reaction rate, Arrhenius activation energies for reduction and carburization were evaluated. Characterization of the carbides produced was carried out using X-ray diffraction and a scanning electron microscope (SEM) combined with electron dispersion spectroscopy (SEM-EDS) analyses. The grain sizes also were determined. The process parameters, such as the temperature of the reduction–carburization reaction and the composition of the gas mixture, had a strong impact on the carbide composition as well as on the grain size. The results are discussed in light of the reduction kinetics of the oxides and the thermodynamic constraints.     

Protection of carbon materials from high-temperature gas corrosion

A study is made of composite carbon -borosilicide on carbon materials prepared by gas phase deposition, diffusion impregnation, liquid-phase impregnation, and fusion. It is shown that muldlayer coatings prepared from carbon-borosilicide materials, particularly silicon and titanium carbides, and molybdenum, tungsten, and hafnium borides and silicides have greater heat resistance in the temperature range 1500–2000°C in air. Their protective properties are strongly dependent on composition, coating structure, and preparation methods as well of the grade of the original carbon materials.NNTs Kharkov Physicotechnical Institute. Translated from Poroshkovaya Metallurgiya, Nos. 3/4(384), pp. 47–50, March–April, 1996. Original article submitted October 17, 1994.     

Interaction of W<Subscript>2</Subscript>B<Subscript>5</Subscript> with Me<Superscript>IV,V</Superscript>C carbides

It is established that the W₂B₅-Me^IV,VC join in the W-B-C-Me^IV,V quaternary systems is quasibinary, while polythermal joins are described by eutectic phase diagrams. It is shown that the eutectic composition and temperature correlate with the melting points of carbides and the Me ^d C content varies from 50 mol % in the W₂B₅-VC system to 4–6 mol % in the W2B5-TaC system. The existence of Me ^d C-Me ^d B₂-W₂B₅ ternary eutectic systems promising for the development of new ceramics is proved.     

Study of an Energy Storage and Recovery Concept Based on the W/WO3 Redox Reaction: Part I. Kinetic Study and Modeling of the WO3 Reduction Process for Energy Storage

Energy storage and recovery using the redox reaction of tungsten/tungsten-oxide is proposed. The system will store energy as tungsten metal by reducing the tungsten oxide with hydrogen. Thereafter, steam will be used to reoxidize the metal and recover the hydrogen. The volumetric energy density of W for storing hydrogen by this process is 21 kWh/L based on the lower heating value (LHV) of hydrogen. The main objective of this investigation was to study the kinetics of the reduction process of tungsten oxide (WO₃) and determine the optimum parameters for rapid and complete reduction. Theoretical treatment of isothermal kinetics has been extended in the current work to the reduction of tungsten oxide in powder beds. Experiments were carried out using a thermogravimetric technique under isothermal conditions at different temperatures. The reaction at 1073 K (800 °C) was found to take place in the following sequence: WO₃ → WO_2.9 → WO_2.72 → WO₂ → W. Expressions for the last three reaction rate constants and activation energies have been calculated based on the fact that the intermediate reactions proceed as a front moving at a certain velocity while the first reaction occurs in the entire bulk of the oxide. The gas–solid reaction kinetics were modeled mathematically in terms of the process parameters. This model of the reduction has been found to be accurate for bed heights above 1.5 mm and hydrogen partial pressures greater than 3 pct, which is ideal for implementing the energy storage concept.     

Simultaneous reduction nitridation for the synthesis of tungsten nitrides from Ni–W–O precursors

Abstract

Tungsten nitrides were synthesised from NiO–WO₃ and NiWO₄ precursors at 973–1273 K in a flow of H₂–N₂ gas mixture. The reduction–nitridation reactions were carried out isothermally in fluidised bed reactor, and the off-gas from the reactions was continuously analysed by gas chromatography. The effect of reaction temperature and precursor composition on the rate of formation of Ni–W nitrides was studied. The different phases developed during the reduction–nitridation reactions were identified by X-ray diffraction analysis technique. The morphology and the grain structure of the precursors were examined by SEM, and the elemental composition in the structure was analysed by electron dispersive spectrometry. The results showed that the reduction of Ni–W–O precursors proceeded in a stepwise manner (NiWO₄→Ni–WO₃→Ni–WO₂→Ni–W). Tungsten nitrides (WN and WN₂) were formed from the reaction of the freshly reduced W metal with N₂ gas and WN was the predominant phase detected at higher temperatures. The reaction mechanisms were elucidated from the apparent activation energy values and the application of different formulations derived from the gas–solid reaction model at early and later stages of reactions. It was concluded that the interfacial chemical reaction is the rate determining step at initial stages, while a combined effect of gaseous diffusion and interfacial chemical reaction controlled the reaction at later stages. At final stages, the nitridation reactions contributed to the reaction mechanism leading to produce tungsten nitrides.     

Diffusion bonding of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>-TiN composite with nickel-based interlayers

The diffusion bonding of a Si₃N₄-TiN composite with Ni, INVAR (Fe-Ni alloy), and IN600 (Ni-Cr-Fe alloy) interlayers has been investigated between 1100 °C and 1350 °C, under argon or nitrogen atmosphere. For the chosen bonding conditions, the Si₃N₄ phase of the composite reacts with the interlayer phase, leading to the release of silicon and nitrogen, whereas the TiN phase remains stable. The bonding mechanisms with nickel and INVAR (Ni-Fe alloy) interlayers are rather similar. Released silicon diffuses into the reaction layer and into the interlayer, forming a solid solution, whereas the released nitrogen remains gaseous. The bonding rate depends then on the elimination rate of nitrogen from the reaction interface. The thermal stability of these joints is very high up to 1100 °C. However, the interfacial porosity and the internal stresses created by the high nitrogen pressure are pernicious for the mechanical strength. The bonding mechanism with IN600 (Ni-Fe-Cr alloy) interlayer is rather different. The released nitrogen can form nitrides with interlayer elements (Cr, Al). Released silicon diffuses into the reaction layer and forms silicides. The joint porosity is less significant for the IN600 interlayer, which suggests a good mechanical strength. However, the formation of silicide is pernicious, because of the brittleness of these phases.     

Investigation of Cobalt (II)-Tungsten (VI)-Oxide Reduction in Hydrogen: Part I

CoWO₄ reduction in dry hydrogen, at temperatures lower than 550°C brings about an intermediate unstable phase of cubic structure, (a = 10.846Å) and composition Co₄W_2.5O; the tungsten which is not retained in this phase appears in the form ofβ- W (A-15 structure,a = 5.039Å). Another phase (noted ?@#@) which does not give any X-ray diffraction pattern forms by further reduction of Co₄W_2.5O. The whole set of these transformations is characterized by a kinetic global law described by a Johnson-Mehl equationξ = 1- exp(-kt) ^3/2 and an activation energy, 24 kcal/Mole, determined byβ-W continuous growth. The rate of C03W crystallization from? becomes substantial, only from around 700°C, temperature at whichβ-W is also converted into α-W. The final alloy Co₇W₆ is formed from about 800°C on, through reaction of Co₃W and α-W. In wet hydrogen, WO₂ forms instead ofβ- W and Co₄W_2.5O—which may be stabilized under certain conditions-is directly reduced into Co₃W after WO₂ reduction into α-W.     

Desiliconization and decarburization behavior of molten Fe-C-Si(-S) alloy with CO<Subscript>2</Subscript> and O<Subscript>2</Subscript>

One of the most important problems in the steelmaking process is an increase of the disposal slag mainly discharged from the dephosphorization process. In order to reduce the quantity of the disposal slag, the complete removal of silicon from molten pig iron is considered very effective before the dephosphorization in the pretreatment process. From this point of view, the desiliconization and the decarburization behavior of Fe-C-Si alloy with CO₂ and O₂ has been investigated in the present work. It is thermodynamically calculated that silicon should be oxidized in preference to carbon over 0.60 mass pct Si under the condition of s_SiO2=a _C=1 at 1573 K and is experimentally confirmed that silicon is only oxidized under the condition in actual. Even under the competitive region of desiliconizing and decarbonizing, under 0.60 mass pct Si, silicon is found to be oxidized down to about 0.1 mass pct Si in preference. The overall rate constants for the desiliconization and the decarburization are derived, and the value for the desiliconization is one order of magnitude larger than that for the decarburization. The influence of sulfur is also examined, and the retarding effect is not observed on the oxidation reactions.     

Synthesis of Fine Vanadium-Carbide (VC<Subscript>0.88</Subscript>) Powder Using Carbon Nanofiber

The synthesis of fine vanadium-carbide (VC_0.88) powder is considered. To produce the vanadium carbide, vanadium(III) oxide is reduced by means of carbon nanofiber in an induction furnace with an argon atmosphere. The carbon nanofiber is produced by the catalytic decomposition of light hydrocarbons. The specific surface of the carbon nanofiber is very high: ~150000 m²/kg, as against ~50000 m²/kg for soot. The impurity content in the carbon nanofiber is 1 wt %. By analysis of the phase diagram of the V–C system, the batch composition and the upper temperature limit in carbide formation may be determined such that vanadium carbide is formed as powder. Thermodynamic analysis yields the initial temperature at which the vanadium( III) oxide is reduced in a furnace with different CO pressures. The characteristics of the vanadium carbide are determined by the following methods: X-ray phase and elementary analysis; pycnometric analysis; scanning electron microscopy with local energy dispersion X-ray microanalysis (EDX); low-temperature nitrogen adsorption with subsequent determination of the specific surface by the BET method; sedimentation analysis; and synchronous thermogravimetric analysis and differential scanning calorimetry (TG/DSC). The material obtained with optimal reduction parameters consists of a single phase: vanadium carbide VC_0.88. The powder particles are predominantly clumped together in aggregates. The mean size of the particles and aggregates is 9.2–9.4 μm, with a broad size distribution. The specific surface of the samples is 1800–2400 m²/kg. Oxidation of vanadium carbide begins at about 430°C and is practically over at 830°C. Optimal synthesis requires stoichiometric proportions of the reagents in the production of vanadium carbide VC_0.88 at 1500–1600°C, with 20-min holding. In this process, carbon nanofiber effectively produces the carbide as the reduction product. The vanadium(III) oxide is reduced practically completely to VC_0.88.     

Study of the Reduction of Industrial Grade MoO<Subscript>3</Subscript> Powders with CO or CO-CO<Subscript>2</Subscript> Gases to Prepare MoO<Subscript>2</Subscript>

Industrial grade MoO₂ powders have a plenty of advantages relative to MoO₃ in the direct alloying steelmaking processes. In this work, the reduction of industrial grade MoO₃ powder with CO gas or the mixed gases of CO and CO₂ has been investigated in detail in order to prepare industrial grade MoO₂ powder. It is found that reaction temperature has a significant effect on the product composition. Using pure CO as the reducing gas, for temperatures below 868 K (595 °C), the main product is MoO₂ with some whisker carbon; for temperatures above 868 K (595 °C) the main reaction products are MoC and amorphous carbon; as the reaction temperature further increased, the final reaction product is Mo₂C. In addition, Mo₄O₁₁ is always formed as an intermediate product during the reaction processes both at lower and higher temperatures, which is similar to that observed on reduction of MoO₃ by H₂. It is found that adding CO₂ to the reducing gases eliminated carbon formation but still allows the formation of MoO₂ during the reaction process. This method may be applied to produce industrial grade MoO₂.     

WC-Co by Direct Reduction Carburisation Reactions

Abstract

Thermogravimetry, XRD and SEM have been used to study the direct reduction/carburisation of non-stoichiometric CoW0₄ with hydrogen/methane mixtures over the temperature range 900°C to 1100°C. The evolution of phases during reduction/carburisation has been related to differences in starting materials, temperature and gas phase composition. The thermochemistry of the reduction/carburisation process is detailed and actual results of processing are related to thermodynamic predictions. It is shown that the first stage of reaction is reduction of the tungstate to give the intermetallic compounds Co₃W and Co₇W₆ as well as tungsten metal. During this stage no carburisation occurs. The subsequent carburisation stage of the process yields a sequence of phases passing through carbides of the general formulae M₁₂C and M₆C before giving a mixture of WC and Co. In W-rich mixtures a transitory existence of W₂C is shown to exist.