WC Crystals Research Articles

Investigation of the Influence of Powder Fraction on Tribological and Corrosion Characteristics of 86WC-10Co-4Cr Coating Obtained by HVOF Method

Samples using powders of four different fractions, 15–20 μm, 20–30 μm, 30–40 μm and 40–45 μm, were fabricated to investigate the wear resistance, corrosion resistance and tribological properties of the 86WC-10Co-4Cr coating obtained using the HVOF method. The phase composition, microstructure and elemental distribution were analyzed using X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques. The hardness was measured on a Vickers microhardness tester, the friction coefficient and wear rate were investigated using a tribometer, and the corrosion resistance was evaluated on an electrochemical corrosion station. The results showed that the cross-sectional microstructure of the coating is mainly represented by multifaceted WC crystals embedded in the Co-Cr matrix and the presence of lower tungsten carbides, particularly W2C. The 15–20 μm fraction particles were subjected to superheating, contributing to the decarburization process. The 20–30 µm and 30–40 µm sized particles prevented overheating and had a more homogeneous structure. The 40–45 µm powder fractions did not reach sufficient temperature for complete melting, resulting in the formation of pores in the coating layers. The phase composition of the coatings included WC, W2C and CoO phases. According to the results of the study, it was found that the optimal powder fraction for coating the 86WC-10Co-4Cr composition with improved characteristics is the fraction of the 20–30 µm sized particles.

Substrate Engineering toward Selective Growth of Ultrathin WC Crystals and Heterostructures via Liquid Cu‐Zn Catalyst

AbstractTransition metal carbides (TMCs) grown by chemical vapor deposition (CVD) offer promise for numerous novel phenomena and applications in the 2D limit. Despite considerable efforts thus far, the flexible customization of TMCs and their heterostructures still remains challenging. Herein, a substrate engineering is developed to achieve customized manufacturing of ultrathin WC single crystals and WC/graphene (WC‐G) heterostructures by varying the concentration of Zn in Cu‐Zn alloy substrate. It is worth noting that Zn atoms can remarkably reduce the nucleation density of graphene and promote the nucleation of WC. Thus, an increasing Zn content is applied to synergistically modulate the growth of graphene and WC, enabling the controllable fabrication of WC and WC‐G heterostructures. The synthesized WC crystals exhibit an ultrathin nature down to 3 nm, as well as high crystalline, ultra‐clean surface, and superb chemical stability. Based on that, the typical metallic properties with a temperature‐dependent resistance (nearly 1.30 Ω at 300 K and nearly 0.08 Ω at 1.7 K) and low resistance as well as excellent nonlinear optical performance of WC are demonstrated. This work provides fresh insights into regulating the growth behavior of multiblock‐structured carbides and promotes the study of their optic and electronic properties.

Change of interfacial structure between the matrix and second phase of Y–Zr-modified WC-8Co cemented carbide

Based on wet chemical precipitation and spray drying technology, a Y–Zr-doped W powder was prepared as a precursor material for the preparation of Y–Zr-modified WC-8Co cemented carbides. The Y–Zr-modified WC-8Co hard alloy prepared by low pressure sintering method was used to study the change in the interface structure between the second phase and the substrate. The result showed that in the Y–Zr-doped W powder, the second phase was Y2O3 and ZrO2 particles. The Y2O3 particles had a coherent interface with the W matrix, which was the reason for the formation of intragranular nano second phases in the modified hard alloys. The interface between ZrO2 and W particles exhibited a semi coherent relationship, which was consistent with the interface structure of WC crystals and the second phase at grain boundaries. Some free second phase particles Y2O3 and ZrO2 in the doped powder formed a staggered interface structure in the modified alloy, generating Y–Zr–O solid solutions dispersed in the bonding phase. The performance of Y–Zr-modified WC-8Co hard alloy were superior to pure WC-8Co alloy prepared by the same process. The stable and compatible interface structure between WC grains and the second phase was an effective means to improve the performance of modified WC-8Co cemented carbide.

Effect of laser cladding power on electrochemical properties of WC–9Co coatings by DFT calculation

The temperature field distributions and decarburization of WC–9Co coatings fabricated at the different laser power were simulated using the COMSOL software, and the temperature–induced changes in the crystal structures and planes of WC–9Co coatings were obtained by the density functional theory calculation. Furthermore, the differential charge density analysis was conducted to elucidate the corrosion mechanism of WC crystals in 3.5 % NaCl solution, and the obtained results were validated by the CV curve calculations using the electrochemical module in the COMSOL. The results show that the decarburization degree of WC is increased with the increase of laser power. Simultaneously, the crystal plane (101) of WC is also transferred to (100); while the (111) plane of W2C underwent sequential transformations to (220) and (200), which sheds light on the thermal behaviors, crystal properties and corrosion kinetics of WC–9Co coating to provide valuable insights for the practical applications.

Nanomechanical response of tungsten carbide single crystals in extreme conditions: Temperature and strain rate dependence

The mechanical response of tungsten carbide (WC) single crystals under extreme conditions was investigated by means of pendulum-based nano-impact testing, from ambient to high temperatures. Dynamic hardness measurements and damage accumulation observations were conducted by single and multiple impact tests, respectively, at high strain rates (∼ 103-104 s-1). Large WC single crystals with different orientations, namely, (0001) basal and (101¯0) prismatic crystals, were studied. The results obtained from impact testing were compared to results acquired by high temperature nanoindentation in the quasi-static regime (10−2 s−1), at a similar length scale, up to 600 °C. Whilst basal WC crystals were shown to exhibit a higher dynamic hardness at high strain rates across the entire range of temperatures, the hardness of prismatic WC crystals at high strain rates were only found higher at temperatures below 200 °C and comparable at elevated temperatures. Post-mortem topographical scans revealed the generation of substantial plastic deformation during impact and large pile up formation in the vicinity of the impact imprints in prismatic crystals. Electron Channelling Contrast Imaging (ECCI) of the repetitive impacts suggested preferential planes for the accommodation of stress through plastic flow at high strain rates, as previously reported in the quasi-static regime. The results shed light on the mechanical behaviour of WC in extreme conditions, evidencing anisotropic mechanical behaviours.

Theoretical investigation of compressed hexagonal and rock-salt structures of molybdenum and chromium carbides

This research paper reports the structural, electronic, elastic anisotropy, and thermodynamic characteristics of Molybdenum and Chromium Carbides (MoC and CrC) at zero and under high pressures. We consider both hexagonal tungsten carbide phase (WC) and rock-salt (NaCl) structures. Our investigation is performed using the first-principles pseudo-potential plane waves method. The fundamental ground state is the hexagonal (WC) phase for MoC and CrC compounds. MoC is relatively more ionic than CrC. The investigated materials are brittle in the WC phase and ductile in the RS phase for pressures below 30 GPa. MoC and CrC are hard materials and anisotropic in sound velocity. Phonon spectra are reported for both compounds in a WC and NaCl at zero-pressure and high pressures. Ideal stoichiometric WC crystals of MoC and CrC are predicted to be dynamically stable. The pressure and temperature dependence of various macroscopic thermodynamic parameters of MoC and CrC compounds are also discussed.

High-temperature mechanical behavior of ultra-coarse cemented carbide with grain strengthening

Ultra-coarse grained cemented carbides are often used under conditions of concurrently applied stress and high temperature. Improvement of high-temperature mechanical performance of ultra-coarse grained cemented carbides is highly desirable but still a big challenge. In this study, it is proposed that the high-temperature compression strength of ultra-coarse cemented carbides can be enhanced by modulating hard matrix grains by activated TaC nanoparticles, through solid solution strengthening of Ta atoms. Based on the designed experiments and microstructural characterizations combined with finite element simulations, the grain morphology, stress distribution and dislocation configuration were studied in detail for ultra-coarse grained cemented carbides. The mechanisms of Ta dissolving in WC crystal and strengthening ultra-coarse grains through interaction with dislocations were disclosed from the atomic scale. This study opens a new perspective to modulate hard phases of cemented carbides for improving their high-temperature performance, which will be applicable to a variety of cermet and ceramic-based composite materials.

The influence of Ti plasma etching pre‐treatment on mechanical properties of DLC film on cemented carbide

The pre‐treatment of substrate surface had been a key part of DLC film preparation to improve mechanical and tribological properties. Ti plasma etching pre‐treatment was investigated in this paper as a new effective surface pre‐treatment method to substitute transition layer. This pre‐treatment used high‐energy Ti plasma to impact substrate surface. Ti plasma etched the substrate to a depth of 407 nm and increased the roughness from 1.36 to 40.39 nm. A trace layer of substrate, together with cobalt, oxides, and other impurities, was removed. Ti plasma broke some top WC crystals and combined with the free carbon ions separating from the substrate. A DLC film was deposited on the etched surface. Compared with DLC films deposited on the untreated substrate and Ti transition layer, the DLC film on the Ti plasma etched substrate had best adhesion strength of 34.14 N. The three DLC films had the same sp3 bonding carbon content, but Ti plasma etching treatment could promote the formation of sp3 bonds on the interface of substrate and DLC film. This DLC film had low friction coefficient of 0.12 and low wear rate of 5.11 × 10−7 mm3/m·N. In summary, Ti plasma etching pre‐treatment could significantly improve the adhesion of DLC film and keep its excellent tribological properties.

Abnormal WC crystal growth from liquid Co flux occurs via eta phase decomposition

The growth mechanism of large WC crystals from a liquid Co-based flux is identified. This is achieved by systematically varying the growth temperature and Co content from 1200 to 1400 °C and 70–83 at.% respectively. Crystal growth was characterised using metallography and X-ray diffraction. The WC grains were bimodally distributed, consisting of a smaller (10–20 μm) population of grains, which nucleated homogenously from the liquid, and a secondary population of abnormally large grains, several millimetres in size. The abnormal grains nucleated on the (W,Co)6C eta phase, and subsequently consumed it through a carburisation reaction. The size of abnormal grains was enhanced by adopting the eutectic composition, such that the first solid phase to form was the eta phase, whilst at the same time cooling slowly through the eta➔WC transformation temperature, at ~1300 °C. This growth mechanism could be exploited for a variety of metal carbides with similarly sluggish diffusion rates.

Fracture Energy Measurement of Prismatic Plane and \u03a32 Boundary in Cemented Carbide

The grain boundary network of WC in WC-Co is important, as cracks often travel intergranularly. This motivates the present work, where we experimentally measure the fracture energy of Σ2 twist grain boundaries between WC crystals using a double cantilever beam opened with a wedge under displacement control in a WC-10wt%Co sample. The fracture energy of this boundary type was compared with cleaving {10overline{1}0} prismatic planes in a WC single crystal. Fracture energies of 7.04 ± 0.36 Jm−2 and 3.57 ± 0.28 Jm−2 were measured for {10overline{1}0} plane and Σ2 twist boundaries, respectively.

Synthesis of Y2O3-doped WC-Co powders by wet chemical method and its effect on the properties of WC-Co cemented carbide alloy

Abstracts WC-8Co composite powders doped with Y2O3 were prepared by wet chemical method, and the densification of alloy proceeded by spark plasma sintering at 1200 °C, 1250 °C, and 1300 °C. The microstructure of the composite powders and alloys was analyzed by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. Result showed that the addition of Y2O3 could refine WC particles during carbonization and WC grains during sintering. In the microstructure of WC-Co alloys, Y2O3 solidly dissolved in the body of WC crystal with the form of the second-phase nanocrystal. Moreover, the effect of sintering temperature on WC-8Co cemented carbide doped with Y2O3 was studied. The comprehensive performance of WC-8Co cemented carbide doped with Y2O3 was best at the sintering temperature of 1250 °C with the hardness of 19.64 GPa and fracture toughness of 11.97 MPa m1/2.

Nano-Indentation Properties of Tungsten Carbide-Cobalt Composites as a Function of Tungsten Carbide Crystal Orientation.

Tungsten carbide-cobalt (WC-Co) composites are a class of advanced materials that have unique properties, such as wear resistance, hardness, strength, fracture-toughness and both high temperature and chemical stability. It is well known that the local indentation properties (i.e., nano- and micro-hardness) of the single crystal WC particles dispersed in such composite materials are highly anisotropic. In this paper, the nanoindentation response of the WC grains of a compact, full-density, sintered WC-10Co composite material has been investigated as a function of the crystal orientation. Our nanoindentation survey has shown that the nanohardness was distributed according to a bimodal function. This function was post-processed using the unique features of the finite mixture modelling theory. The combination of electron backscattered diffraction (EBSD) and statistical analysis has made it possible to identify the orientation of the WC crystal and the distinct association of the inherent nanoindentation properties, even for a small set (67) of nanoindentations. The proposed approach has proved to be faster than the already existing ones and just as reliable, and it has confirmed the previous findings concerning the relationship between crystal orientation and indentation properties, but with a significant reduction of the experimental data.

Effect of carbon content on microstructure and mechanical properties of WC-10Co cemented carbides with plate-like WC grain

Four sets of WC-10Co cemented carbides with different carbon content were prepared by adding the ultrafine WC powders as seeds during the in-situ sintering reaction among W, Co and C. The effect of carbon content on microstructure and mechanical properties were studied. The results show that the microstructure, phase composition and mechanical properties of WC-10Co cemented carbides with plate-like WC grains were seriously affected by the carbon content. The fast growth of WC grains with high carbon content could proceed the prismatic plane preferentially along the <1 0 1(−) 0> directions, resulting in the high content of plate-like WC grains. The density increased with the increment of the carbon content and reached the maximum value, then, followed by a decline. The hardness and the transverse rupture strength of the alloy in the two-phase zone with carbon content of 5.91 wt% reached the maximum value. The existence of plate-like WC grains could impede the propagation of the cracks due to the decrease of the weakest carbide regions and the increase of the basal facets of broken WC crystals. In this case, more fracture energy was required to crack propagation and further improved the transverse rupture strength. Additionally, the plate-like WC was benefit to reduce the wear volume and bring about a better wear resistance. Thus, the alloy with the appropriate proportion of carbon content can obtain higher mechanical properties and wear resistance.

Investigate on effect of cryogenic cooling on oxidation wear of WC-Co carbide tool milling titanium alloy

For titanium alloy at high cutting speed, the severe tool wear will ineluctably take place diminishing the available tool life because of the high instantaneous temperature rise. Especially the WC-Co material in carbide tool will be reacted with the oxygen element in the air and generate oxide, and the tool oxidation wear is inevitable. In milling Ti–6Al–4V alloy operations, this article presents the first comprehensive investigation on the oxidation wear effect of cryogenic cooling on carbide tool compared with the conventional cooling. Based on the Gibbs free energy of the chemical reaction, the machining characteristics of the oxidation reactions were analyzed in detail. A series of machining testes were executed adopting controllable cryogenic cooling milling system. The surface and cross-section morphology and phase composition characteristics of tool were measured by SEM and XRD measuring equipments, as well as the oxidation wear mechanism of tool in cryogenic cooling. The results show that the thermal oxidation degree of the elements of WC-Co is higher in the conventional cooling processing. After cryogenic cooling intervention, oxidation reactions in tool are reduced significantly. Even the most probable reaction is disappeared, and the oxidation degree is decreased. When the temperature drops to 180 K, it has the best cooling effect. In cryogenic, the effective binding of WC crystal particle to the adhesive phase Co is a major factor for inhabiting oxidation wear of tool. The investigations indicate that the cryogenic cooling method can effectively inhibit the oxidation effect of carbide tool in milling titanium alloy process, and it improves the resist oxidation ability of tool.

Transport Properties of Topological Semimetal Tungsten Carbide in the 2D Limit

AbstractRecent theoretical calculations and spectroscopy measurements have shown that several materials with the tungsten carbide‐type structure, such as molybdenum phosphide and tungsten carbide (WC), are a new type of topological semimetal hosting triply degenerate nodal points. So far, most of experimental studies are performed on large‐size bulk crystal. 2D nanostructures, with large surface‐to‐volume ratio, are expected to provide an attractive system for probing topological surface states and the development of device applications. Here the transport characterization of high‐quality ultrathin WC single crystals is reported. The magnetoresistance (MR) shows a strong anisotropic behavior, which may be attributed to the transport anomalies of three‐component fermions as theoretically proposed. The scaling analysis reveals the presence of the surface states in WC. The observation of the nonsaturating MR in strong magnetic fields, together with nonlinear and strong temperature dependent Hall effect, is consistent with the proposed multiple bands character of carriers in WC. In addition, a low‐temperature upturn in the resistance is observed, indicating that electron–electron interaction plays a prominent role in charge transport. The results demonstrate that the ultrathin WC crystals offer a potential platform for exploring exotic transport properties arising from the three‐component fermions in the 2D limit.

Microstructure and formation mechanism in a surface carburized tungsten heavy alloy

A 95W–3.5Ni–1.0Fe–0.5Co heavy alloy was surface modified by pack carburization. A new carburized layer consisting of outside surface layer and inner surface layer was formed. The double layered microstructure showed porous WC aggregates in the outside surface layer and W grains coated by shell-like (Ni, Co, Fe)6W6C carbides in the inner surface layer. There were two paths for C atoms diffusion and reaction through W grains and matrix phase. Co changed the formation orders of carbides and acted as a medium for promoting C atoms reaction with Ni, Fe, Co and W in the matrix to form shell-like (Ni, Co, Fe)6W6C carbides in the inner surface layer. Meanwhile, C atoms diffused in the W grains and reacted with W in an order of W→W2C→WC. WC nucleated and grew in carburized W grains and micropores produced in the growing WC crystals due to the lattice expansion from W→W2C→WC. As the carburization process continued, the shell-like (Ni, Co, Fe)6W6C carbide was also carburized to WC, and finally the whole W grain was carburized into porous WC aggregates in the outside surface layer.

Scale effect in mechanical characterization of WC-Co composites

The study of mechanical response of materials at small length scales has gained importance due to the recent advances in micro- and nano-fabrication as well as testing systems. However, most of the reported work has been dedicated to investigate single-crystals or boundary-containing metallic systems, while much less attention has been paid to composite materials, and in particular those combining soft and hard phases. In this work, a systematic procedure is followed for machining micropillars of diameters ranging from 1 to 4μm in a WC-Co composite with a WC mean grain size around 1μm, by means of focused ion beam milling. In-situ uniaxial compression of the micropillars and subsequent field emission scanning electron microscopy inspection were conducted. Clear size effects are evidenced. Smaller test specimens (probe size approaching the mean WC size) exhibit deformation/failure mechanisms observed for WC crystals; while for bigger sample sizes, the mechanical response involves several mechanisms directly linked with the microstructural characteristics of the bulk-like cemented carbide material. On the other hand, independent of micropillar size, carbide-carbide or carbide-binder interfaces are found to be preferential sites for nucleation of critical damage events.

2D WC single crystal embedded in graphene for enhancing hydrogen evolution reaction

Electrochemical water splitting is regarded as one of the most economical and eco-friendly approaches for hydrogen revolution. Developing a low-cost and earth-abundant non-noble-metal catalyst will be of the most significance. Tungsten carbide (WC) is highly promising due to its platinum (Pt) -like behavior in surface catalysis. Here we first report a liquid metal solvent based co-segregation (LMSCS) strategy to fabricate a high uniformity of 2D WC crystals embedded in graphene by chemical vapor deposition (CVD) in one step. The 2D in-plane WC–graphene heterostructures (i-WC–G) are remarkably stable under an electro-catalytic environment and ensure good interfacial synergy between the 2D WC crystallites and graphene to achieve a more effective hydrogen evolution. The overpotential is as low as 120mV and the Tafel slope is 38mV/dec, which indeed exhibits outstanding catalytic potential among the reported 2D material systems. Our elegant and versatile approach allows the fabrication of other high-quality 2D transition metal carbides (TMCs) and their in-plane heterostructures, which will further promote practical catalytic applications of metal carbides.

Thermal Cycling Behavior of Thin WC‐Co Sintered Pellets

Hexagonal WC crystal phase, with a and c axis slightly larger than expected is obtained for thin sintered (at 1 400 ºC under vacuum) WC‐20wt% Co samples having no extra carbon added. This effect can be associated with the elastoplastic hysteretic thermal expansion behavior detected in the thin disks and to the associated observed anisotropy showing a higher expansivity in the axial compared to the in‐plane direction. By using a two phase model where the matrix is WC and the second phase is Co, it can be computed (from expansion data) that tensile forces are acting at room temperature in both directions for the examined specimen. By heating to about 50–80 ºC the strains/stresses become compressive as expected for WC‐20wt% Co.

The effects of fine WC contents and temperature on the microstructure and mechanical properties of inhomogeneous WC-(fine WC-Co) cemented carbides

Inhomogeneous WC-(fine WC-Co) cemented carbides with improved hardness and toughness were successfully prepared through the addition of fine WC using planetary ball milling combined with sinter isostatic hot pressing (SHIP) technology. The inhomogeneous microstructure of the alloys consisted of coarsened WC grains and WC-Co consisting of fine WC dispersoids and Co binder phase. The increase of temperature and the addition of fine WC enhanced the sintering process. The morphologies of the coarsened WC and of the fine WC consisted of triangular and near-hexangular prisms, respectively. Due to crack path deflection and crack bridging, the prism-like coarsened WC crystals efficiently hindered cracks propagation. Intergranular fracture became predominant when adding fine WC. However, the excessively coarsened WC and some pores in alloys with 20wt% fine WC could decrease the mechanical properties. The inhomogeneous WC-(fine WC-Co) cemented carbides with 10wt% fine WC, sintered at 1430°C for 40min, could provide a combination of superior hardness and toughness.