Bulk WC Research Articles

Phase transformations in WC-Cr3C2-Ni thermal spray coatings under low, medium and high thermal input: Part 2 – DSC analysis and heat-treated coating characterisation

Conventionally, bulk WC and Cr3C2-based carbide compositions have been used independently of each other. However, recent investigations have begun to explore combining these carbides together within the same composite/hard metal coating system. This work explored the diversity of phases produced in the coatings from a 42 wt% WC-42 wt% Cr3C2–16 wt% Ni powder sprayed under “low”, “medium” and “high” thermal input conditions using high velocity oxygen-fuel (HVOF), an argon‑helium (ArHe) plasma and an argon‑hydrogen (ArH2) plasma respectively. The thermally driven solid-state transformations in the coatings were investigated as a function of carbide dissolution/decomposition, using differential scanning calorimetry (DSC). All coatings exhibited a well defined exothermic peak attributed to the transformation of the metastable supersaturated Ni-alloy and WC1-X phases in the as-sprayed coatings, into the equilibrium Ni alloy and a mix of the intermediate metastable carbide phases β-W2C and Cr3C2-Y. At higher temperatures the metastable compounds transformed to the equilibrium phases of ε-W2C and Cr3C2. A defining feature of the XRD analysis was the significant shift in lattice positions of all phases, indicating substitutional alloying. The mechanisms of phase formation are discussed.

First-principles investigation of hydrogen interaction with Cu/WC interface

The WC-Cu composite used as a thermal barrier interlayer between tungsten and CuCrZr heat sink can effectively alleviate the thermal strain mismatch. In this work, the interaction of hydrogen (H) and Cu/WC interface are investigated by performing the first-principles density functional theory calculations. Considering the H solubility in the WC bulk, we found that the most energetically favorable site is not the traditional octahedral interstitial site but is the projection site of octahedral interstitial on the W basal plane. The H diffusion barrier for octahedral interstitial sites and the diffusion path refers to the channel for H penetration and migration into the bulk WC is as high as 0.773 eV. According to the interface calculations, it indicated that the C-terminated interface with the atoms in WC followed the stacking sequence of the Cu (111) plane can yield the strong adhesion for (111)Cu/(0001)WC interface. Furthermore, the higher dissolution energy for H in this Cu/WC interface indicates no interstitial segregation or trapping site for H in the region due to the localization of interface electrons. Note that the H atom in WC also exhibits a high diffusion barrier. Both of these factors combine to determine that the interface can effectively block the penetration behavior of H into the WC, and the WC-Cu composite materials can use as the H permeation barrier. Besides, our ab initio molecular dynamics (AIMD) simulation also confirmed that H positioned at the interface can spontaneously migrate from the interface to the Cu matrix, and the interface can block the H penetration.

Phase transformations in WC-Cr3C2-Ni thermal spray coatings under low, medium and high thermal input: Part 1 – Feedstock and as-sprayed coating characterisation

Conventionally, bulk WC and Cr3C2-based carbide compositions have been used independently of each other. However, recent investigations have begun to explore combining these carbides together within the same composite/hard metal coating system. This work explored the diversity of phases produced in the coatings from a 42 wt% WC-42 wt% Cr3C2–16 wt% Ni powder sprayed under “low”, “medium” and “high” thermal input conditions using high velocity oxygen-fuel (HVOF), an argon‑helium (ArHe) plasma and an argon‑hydrogen (ArH2) plasma respectively. During spraying, the Ni binder melted, leading to variations in the extent of carbide dissolution and peritectic decomposition as a function of thermal input. Low thermal input, typified by the HVOF technique, led to preferential dissolution/decomposition of the Cr3C2 grains into the melt, while retaining more of the higher melting point WC grains. The coating was dominated by a supersaturated Ni phase, along with the retained carbides. High thermal input, typified by the ArH2 plasma, was postulated to exceed the peritectic decomposition temperature of WC. The coating consisted of a supersaturated Ni alloy, along with the metastable phases WC1-X and β-W2C. No metastable Cr-carbides formed in any of the coatings. The mechanism of coating phase formation is discussed.

Impact of laser process parameters in direct energy deposition on microstructure, layer characteristics, and microhardness of TC21 alloy

In the present study, layers consisting of 40% Stellite-6 and 60% WC were deposited on Ti-6Al-3Mo-2Sn-2Zr-2Nb-1.5Cr-0.1Si (TC21) alloy by means of direct energy deposition (DED) technology aiming to improve the microstructure and microhardness. Five powder feeding rates ranging from 20 to 100 ɡ min−1 were applied using CW fiber-coupled diode laser with 4 kW output power. The deposited layers were analyzed via scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and X-ray diffractometry (XRD). The results show that WC particles are dispersed in a heterogeneous manner in the deposition zone, especially at the rates 20, 40, and 60 ɡ min−1. In addition, microcracks appeared in the interface zone particularly at 100 ɡ min−1 due to the higher induced residual stresses caused by the difference in the coefficient of thermal expansion between Stellite-6, WC particles, and TC21 substrate alloy. Several complex carbides and intermetallic compounds such as W2C, TiC, Cr7C3, Co3W3C, and Co25Cr25W8C2 were detected in the deposited layers depending on the powder feeding rate. With further increase in the powder feeding rate, the fractions of W2C and the bulk (unmelted) WC particles were increased and that of the TiC particle was reduced correspondingly due to the thermal diffusion. The layer thickness increased from 1.3 to 2.7 mm when the powder feeding rate increased from 40 to 100 ɡ min−1, while the dilution ratio decreased from 23 to 5.3% as a result of the thermal diffusion of the laser energy. The microhardness of the composite was found to be three times higher than that recorded for the TC21 substrate (1020 vs. 340 HV0.05). The results revealed that the best homogeneous microstructure with the highest microhardness was achieved at the powder feeding rate of 100 ɡ min−1 whereas microcracks free layers were accomplished at 40 ɡ min−1.

Fabrication of novel ZnTe-based photocatalyst: Synergistic effect of dye sensitization and bridge engineering for boosting hydrogen evolution over ZnTe

Herein, a novel ZnTe-based photocatalyst is successfully synthesized via a facile combination of water-bath and hydrothermal processes. Morphology characterization and X-ray diffraction analysis reveal that ZnTe presents irregular granular shape and cubic crystal structure. Moreover, Mott-Schottky measurement shows that the conduction band potential of ZnTe is −0.84 V (vs NHE). With Eosin Y (EY) sensitization, ZnTe exhibits superior photocatalytic hydrogen evolution activity (223.5 μmol g−1 h−1). Meanwhile, WC-ZnTe heterojunction is constructed by depositing ZnTe nanoparticles on bulk WC and obtains the optimal H2 generation rate (559.1 μmol g−1 h−1) under EY sensitization. Electrochemical and photoluminescence results further prove that WC as electron bridge could reduce the interfacial resistance and suppress e−-h+ pairs recombination. This study explores the potential application of ZnTe as a newly active photocatalyst in photocatalytic water splitting, and emphasizes the synergistic effect of dye sensitization and bridge engineering.

Fabrication of novel noble-metal-free ZnIn2S4/WC Schottky junction heterojunction photocatalyst: Efficient charge separation, increased active sites and low hydrogen production overpotential for boosting visible-light H2 evolution

Herein, we report the synthesis of ZnIn2S4 nanoparticles on bulk WC by a facile hydrothermal process to construct novel and highly efficient noble metal-free Schottky junction heterojunction photocatalysts. Morphology characterization revealed that the ZnIn2S4 nanoparticles were deposited on the surface of the WC. Meanwhile, the combination of the ZnIn2S4 and the metal-like WC formed the Schottky energy barrier, which greatly promoted the migration and separation of photo-generated carriers. Especially, the optimal ZnIn2S4/WC photocatalysts have a lower overpotential for hydrogen evolution (−0.35 V) than pristine ZnIn2S4 (−0.49 V). The hydrogen production ability of the optimal ZnIn2S4/WC photocatalyst (2400.3 μmol·h−1·g−1) was approximately 9.2 times higher than that of ZnIn2S4-1% Pt (260.1 μmol·h−1·g−1). Photocatalytic experiments demonstrated that metal-like WC plays an important role in replacing precious metal to increase the active site of ZnIn2S4. Moreover, a feasible Schottky junction reaction mechanism of intensive photocatalytic activity was discussed. This study proves that it is a very fascinating strategy to combine the advantages of ZnIn2S4 and metal-like to construct Schottky heterojunction for photocatalytic hydrogen production.

Enhanced stability of tungsten carbide catalysts via superhydrophobic modification for one-pot conversion of biomass-derived fructose involving water and iodine

Developing cost-effective and high-performance catalysts in biomass chemistry is motivated by creating sustainable technologies dependent on inexpensive and abundant resources. The existing methods for the direct preparation of 5-methylfurfural (5-MF) from biomass-derived fructose mediated by hydrogen iodide (HI) mainly require precious metal catalysts. This research showed that low-cost tungsten carbide (WC) was capable to produce HI from iodine (I2) via liquid-phase hydrogenation, but a pivotal limitation was the poor stability under the combination effects of water and I2. To solve this problem, the bulk WC and the high dispersion supported WC were modified by hydrophobic organosilicones through simple liquid phase deposition. Both of the two superhydrophobic WC materials had better stability and their catalyticactivity were comparable to the expensive palladium-carbon (Pd/C) catalyst in the preparation of 5-MF, especially the superhydrophobic high dispersion supported WC. This work highlights the potential of the low-cost WC with simple superhydrophobic modification in assisting the direct preparation of 5-MF from fructose mediated by HI, which has high significance to the sustainable development of the low-cost biomass chemical engineering.

A novel noble-metal-free binary and ternary In2S3 photocatalyst with WC and “W-Mo auxiliary pairs” for highly-efficient visible-light hydrogen evolution

Herein, noble-metal-free In2S3-WC and In2S3-based photocatalysts with dual cocatalysts of flower-like MoS2 and bulk WC were synthesized through a facile solvothermal reaction. Electrochemical results indicated WC increased the interface conductance of photocatalyst and boosted the transference of photogenerated carriers. The optimal In2S3-WC show a hydrogen production rate of 128.11 μmol·h−1·g−1, which is around 6 times higher than In2S3-1%Pt (20.73 μmol·h−1·g−1). The above results demonstrate commercial WC has a crucial impact of replacing precious metal and separating carriers. Morphology characterization exhibited hydrangea In2S3 and flower-like MoS2 were attached to bulk WC. Gratifyingly, photocurrent, impedance and photoluminescence results demonstrated MoS2 further effectively separated the photogenerated carriers. Remarkably, the higher hydrogen generation rate of 390.52 μmol·h−1·g−1 obtained by the optimal ternary In2S3-WC-MoS2 composite was around 3 and 18 times higher than the optimal In2S3-WC and In2S3-1% Pt. The photocatalytic results demonstrated that "W-Mo auxiliary pairs" was a great efficient synergistic cocatalyst for In2S3 to increase active sites and improve e--h+ pairs separation for H2 generation. Furthermore, a probable reaction mechanism of the enhanced photocatalytic H2 generation was also proposed. This presented research provides an effective concept to design novel and efficient noble-metal-free photocatalyst with "W-Mo auxiliary pairs" for prominent H2 generation activity.

Fabrication of chrysanthemum-like CdSe/bulk WC: A novel Schottky-junction photocatalyst for improving photocatalytic hydrogen production

The developments of direct synthesis for shape-controlled and high-efficiency photocatalysts are exceedingly exigent for achieving clean hydrogen (H2) energy by solar light. In this work, we developed facile synthesis to construct a novel chrysanthemum-like cadmium selenide (CdSe)/bulk tungsten carbide (WC) Schottky-junction photocatalyst. The characteristic results for the component, microstructure and chemical state of photocatalyst revealed that chrysanthemum-like CdSe was intimately decorated onto the bulk WC surface using an ultrasonic-hydrothermal method. The photocatalytic results indicated that the optimal sample obtained the maximum H2 evolution amount (2711.9 μmol g-1 in 5 h) under visible light irradiation, which was 7.4 times higher than that of chrysanthemum-like CdSe. The observably improved H2 evolution activity was attributed to the ample active sites, the lower onset overpotential (−0.25 V) and the slower surface charge recommendation rate (0.011 s-1). Meanwhile, intimate contact at the CdSe/WC interface could engender synergistic effect and actuate the formation of Schottky barrier, which effectively facilitated the reduction of water to H2. This study uncovers an easily available synthesis for shape-controlled CdSe and in-depth explores the design of CdSe-based Schottky-junction photocatalyst for promoting photocatalytic H2 evolution performance.

ZnSe nanoparticles with bulk WC as cocatalyst: A novel and noble-metal-free heterojunction photocatalyst for enhancing photocatalytic hydrogen evolution under visible light irradiation

The exploration of inexpensive and stable semiconductor photocatalysts are extremely urgent for obtaining sustainable hydrogen (H2) energy by photocatalytic water splitting. Herein, a novel and noble-metal-free heterojunction photocatalyst, which is ZnSe nanoparticles (NPs) deposited on the surface of bulk WC, was successfully fabricated via one-pot solvothermal method. The photocatalytic results indicated that ZnSe is a great potential photocatalyst for the reduction of water to H2. Meanwhile, as a replacement of noble-metal cocatalyst, bulk WC could remarkably improve the photocatalytic H2 evolution activity of ZnSe NPs under visible light irradiation in the heterojunction system. The H2 evolution of the optimal sample achieved 2978.31 μmol g−1 in 5 h, which was 5.4 times higher than that of ZnSe NPs. The observably boosted H2 generation activity could be ascribed to the ample reactive sites and the broadened visible-light absorption. Moreover, intimate interfacial contact between ZnSe NPs and bulk WC engenders synergetic effect and Schottky junction. Electrochemical, steady-state and time-resolved PL measurements further confirmed that the novel heterojunction photocatalyst could effectively accelerate the separation of charge carriers, decrease the overpotential and prolong the lifetime of photoinduced electrons. This study provides a novel and cost-effective approach for designing efficient noble-metal-free photocatalysts and improving H2 evolution activity of selenides under visible-light-driven photocatalytic water splitting.

Hot Pressing of Tungsten Monocarbide Nanopowder Mixtures by Electroconsolidation

The paper presents theoretical and experimental results in the field of the manufacturing of cemented tungsten carbide materials. The important issue of avoiding any additional substances like plasticizers was challenged in order to reach the maximal possible density of sintered material while keeping its purity. To solve the problem, the electroconsolidation method of hot pressing supported by direct current was applied. The respective apparatus was constructed that enabled WC nanopowders to be sintered under pressure and high temperature during a very short time of ca. 3 minutes. In the experiments, because of the short heating time, grain size of the sintered bulk WC increased insignificantly, in general, remaining smaller than 1 μm. Similarly, sintering under hot pressing with direct current, a mixture of 3% by weight Y2O3 stabilized ZrO2 and 50% by weight WC, produced a fine structure with a uniformly distributed WC grains. The applied electric field led to the formation of a temperature gradient around the pores, with a favourable impact on the compaction of large pores and an increase in the final density of the bulk material. The experimental research confirmed that the main mechanism of the densification of nanodispersed powders of tungsten monocarbide was a locally inhomogeneous diffusion-viscous flow with intergranular slipping.

Structural tuning and catalysis of tungsten carbides for the regioselective cleavage of C[sbnd]O bonds

Tungsten carbides exhibit excellent performance in many heterogeneous processes because of their distinctive catalytic properties. Preparation of tungsten carbides with controllable phase composition relevant to their catalytic behavior is essential yet challenging. In this study, tungsten carbides embedded in carbon spheres (WxC@CS) were fabricated through carburization of organic–inorganic hybrid precursors. W1.25C@CS with rational structure-tuning properties exhibits promising regioselectivity (reaching 91.5%) toward aryl CO bond cleavage, specifically during hydrogenolysis of guaiacol to phenol. A structure reconstruction strategy was adopted to elucidate structure–performance relationship by transforming commercially available bulk WC from inert phase to composition-dependent active catalysts. Combined catalytic and characteristic analyses illustrate that the catalyst performance is dependent on the C-defect structure. The intimate connection between the phenol space time yield and the C/W atomic ratio on the exterior interface of the catalyst was verified. The C/W atomic ratio of 7.2 leads to the optimal catalytic performance. Density functional theory calculations were performed to define the catalytic mechanism at the atomic level. The theoretical analysis suggests an appropriate configuration of surface W and C atoms for activation of hydrogen and guaiacol molecules, rendering the intrinsic active sites for phenol production. This work provides insights into controlling the surface compositions of tungsten carbides to develop efficient CO bond cleavage catalysts, which verifies the importance of hydrogenolysis catalysis in lignin-derived compounds involving complex O-containing guaiacols and phenolics.

HRTEM and Nanoindentation Studies of Bulk WC Nanocrystalline Materials Prepared by Spark Plasma Sintering of Ball-Milled Powders

In the present work, mechanical milling technique using a high-energy ball mill was employed for preparing of nanoscaled WC grains powders with an average grain size of 7 nm in diameters of WC. The present study demonstrates a successful consolidation process achieved at 1250 °C for sintering of ball-milled WC powders into full dense bulk buttons (above 99.6%), using SPS technique. The as-consolidated WC bulk nanocrystalline buttons revealed high hardness value (~24 GPa) with low elastic modulus (~332 GPa). Moreover, they possessed a high fracture toughness (15 MPa m1/2) that has never been reported for pure WC.

Photocatalytic activity of electron-deficient and porous WO3 nanoparticles derived from thermal oxidation of bulk WC particles

Abstract Electron-deficient and porous WO 3 particles were prepared by the simple thermal oxidation of commercial bulk WC. The crystal structure of as-prepared WO 3 particles was monoclinic phases and its crystallinity and morphology were controlled by varying the calcination temperature. Small and interconnected WO 3 particles were divided and formed from bulk WC particles with increasing calcination temperature. The electronic structure of thermally oxidized particles was studied by W L III -edge XANES. Interestingly, as-prepared WO 3 samples were in an electron-deficient state (W +6−δ ). The photocatalytic performance of as-prepared samples was investigated in terms of oxygen production in AgNO 3 aqueous solution and decomposition of the organic dye, Rhodamine B. The photoelectrochemical and photocatalytic activity were correlated and the results were most favorable for WO 3 particles thermally oxidized at 700 °C for oxygen production and 600 °C for photocurrent production.

Tungsten carbide hollow microspheres as electrocatalyst and platinum support for hydrogen evolution reaction

Tungsten carbide with special spherical hollow microstructure was synthesized and tested as an electrocatalyst or electrocatalysts support for platinum. Both bulk WC and as-synthesized WC supported Pt were evaluated as hydrogen evolution catalysts. It was found that the synthesis temperature is crucial to the compositions and surface states of products, and has direct influence on HER activity. The WC possesses spherical micro-morphology with a high specific surface area, and its hollow and cracked structural features are considered to be beneficial for its electrocatalytic ability and potential as electrocatalysts support. The bulk WC hollow microspheres displayed excellent HER activity. The 20% Pt/WC also exhibited better electrocatalytic performance than the commercial 20% Pt/C: the current density has obviously increased, and the kinetic parameters such as Tafel slope and exchange current density have obvious improvement as well. The investigations indicated that the as-synthesized WC could be considered as a potential electrocatalyst and electrocatalyst support.

One-step synthesis of ultrafine WC–10Co hardmetals with VC/V2O5 addition by plasma assisted milling

WC–10Co cermets were directly produced from the elemental nanopowders W–C–Co with addition of VC or V2O5, which was prepared by innovative ball milling assisted by the dielectric barrier discharge plasma (denoted as DBDP-milling). By only 3h of DBDP-milling, the particle size of W, Co, and VC was greatly reduced, and the W flakelets were uniformly coated with graphite nano-sheets. It is the VC addition by DBDP-milling that dramatically refines the microstructure of bulk WC–Co cemented carbides, and some WC grains possess a truncated sector prism shape with both size and thickness less than 100nm. The ultrafine-grained microstructure is responsible for the improved mechanical properties of hardmetals. Further, the V2O5 addition by DBDP-milling has an identical inhibiting effect as the VC doping on the grain growth of WC during sintering.

Ab initio study of the structural, elastic, and thermodynamic properties of tungsten monocarbide at high pressure and high temperature

Tungsten monocarbide (WC) exhibits unique physical and chemical properties. It is an indispensable industrial material used as cutting tools and has many potential applications in catalyst, energy storage, and so on. We performed calculations of the electronic structure in the framework of the density functional theory (DFT) with generalized gradient approximation (GGA) to investigate structural and elastic properties of WC. Bulk WC is very incompressible, but its bulk modulus is still smaller than diamond though Lin et al. reported that nano-crystalline WC was as incompressible as diamond. WC undergoes different compressibilities along a and c directions: the a-axis is more compressible than the c-axis. The ratio of shear modulus to bulk modulus (G/B) was studied and it was found that WC translated from the brittle to ductile state at ∼63GPa. In order to compare with the EOS determined at finite temperature, the vibration effects of the crystal lattice are taken into account based on quasi-harmonic Debye model. The temperature effect on bulk modulus is discussed and the thermodynamic properties of WC, such as heat capacity CV, Debye temperature θD, and thermal expansion α, are calculated simultaneously.

Electronic properties of WC nano-compound: Compton spectroscopy and band structure calculations

We present the electronic properties of tungsten carbide (WC) nano-compound using the Compton scattering technique. We have measured the Compton profile (CP) of nano-powder using our 137Cs Compton spectrometer. To determine the theoretical CPs of WC nano-compound, we have employed linear combination of atomic orbitals (LCAO) with Hartree-Fock and density functional theory. Although all the LCAO-based calculations show similar agreement with the experiment, the second-order generalised gradient approximation gives a marginally better agreement with the measured CP data. Layered structures and slight over-lapping in energy bands show a small role of exchange and correlation potentials in case of WC nano-compound, which is in contrast to bulk WC.

Effect of Electrical Discharge Machining on strength and reliability of WC–30%Co composite

Tungsten carbide/Cobalt (WC–Co) composite is one of the important composite materials, which is used for manufacturing of cutting tools, dies and other special tools. It has very high hardness and excellent resistance to shock and wear. It is not possible to machine this material easily with conventional machining techniques. Due to the good electrical conductivity of WC–Co, it is usually machined by Electrical Discharge Machining (EDM). EDM process often results in the surface damage of bulk WC–Co, and the influence of the damage would affect the reliability. It is important to investigate the effect of electric discharge machining process on the properties of WC–Co cemented carbides before applying its engineering application. For these composites, maintenance of proper fracture strength is an important concern and is to be controlled. In this work, an attempt has been made to investigate the fracture strength and the reliability of EDMed WC–Co composite using the Weibull distribution analysis. The comparison of results between the machined composites and un-machined composites is carried out and presented in this study.

Catalytic Conversion of Syngas into Higher Alcohols over Carbide Catalysts

This work investigates the use of the bulk carbides Mo2C, WC, and NbC as catalysts for the conversion of syngas into higher alcohols. K2CO3/WC produces mainly CH3OH and CH4 with a low activity. NbC has a very low activity in CO hydrogenation. K2CO3/Mo2C produces mixed alcohols with a reasonable activity and selectivity. In a 94 h test the activity and the specific surface area of the K2CO3/Mo2C catalyst decreased significantly, but X-ray diffraction and transmission electron microscopy did not indicate a strong sintering of the carbide. A likely cause for the deactivation is the formation of carbonaceous deposits on the catalyst. At the same general activity level Li, K, and Cs provide similar promotional effects for Mo2C, although K at a loading level of alkali metal/Mo = 0.164 mol/mol provides the better behavior at equal conditions. The effect of further additives on the K2CO3/Mo2C system was evaluated, but only Cu yielded an improvement.