Properties of WC–8Co hardmetals with plate-like WC grains prepared by plasma-assisted milling

WC-8C0 hardmetals with different proportions of prismatic WC grains and plate-like WC grains were directly produced through sintering the W-C-8C0 elemental powder mixture which was fabricated by dielectric barrier discharge plasma(DBDP)-assisted milling.The morphology of prepared WC-8C0 hardmetals,geometry and the preferential orientation of plate-like WC were investigated by X-ray diffraction(XRD) and scanning electron microscopy(SEM) analysis.The results demonstrate that the microstructure and mechanical properties of the sintered hardmetals are related to the morphology of W grain which is dependent on DBDP-milling time.The DBDP for 1 h(DBDP-1 h)-milled W-C-Co powder contains granular W particles that tend to form prismatic WC grains,while the DBDP for 3 h(DBDP-3 h)-milled powder contains lamellar W particles that generate plate-like WC grains.By adjusting the weight ratio of DBDP-1 h powder and DBDP-3 h powder in W-C-8C0 mixture,the proportion of plate-like WC in the hardmetals can be controlled,and relatively high transverse rupture strength(TRS) is obtained as the proportion of plate-like WC grain in the hardmetals is about 35%in present experimental condition.     

Carbon content dependence of grain growth mode in VC-doped WC–Co hardmetals

Carbon content dependency of grain growth mechanism and grain growth inhibition mechanism in VC-doped WC–Co hardmetals is investigated. VC-doped WC–Co hardmetals with three different carbon contents were sintered with liquid phase and then rapidly quenched to freeze up the structure at the sintering temperature. In these samples, spatial distributions and atomic scale structures of V-rich phases are investigated using transmission electron microscopy (TEM) and related techniques. In these measurements, doped V is found in liquid phase as solute, in large (W,V)C_x precipitates and in interface segregations. Further detailed observations and discussions are carried out for the (W,V)C_x segregated at the WC grain/Co phase interfaces. These (W,V)C_x phases change their form from planar films to small islands depending on the carbon content. The WC grain/Co phase interfaces are fully covered by planar (W,V)C_x in the sample of low carbon content. On the other hand, the WC grain/Co phase interfaces are partially covered by (W,V)C_x islands in the material of high carbon content. During sintering, the WC grains in this sample grew much faster than those in the sample of low carbon content. These structural differences are discussed in terms of WC grain/(W,V)C_x interface energy.     

Influence of Ca ions and temperature on the corrosion behavior of WC–Co hardmetals in alkaline solutions

The effect of temperature and Ca ions on the corrosion behavior of hardmetals was investigated in 0.1 M NaOH and 0.05 M Ca(OH)₂ alkaline electrolytes using impedance spectroscopy, potentiodynamic polarization and surface analytical techniques. It was found that calcium containing alkaline solutions efficiently decrease the anodic currents up to 5 times by forming a calcium containing deposit on the top of the WC–Co hardmetal surface, which remains stable even at higher temperatures (40 °C and 60 °C). This positive influence of Ca ions is predominant under polarization in the range from 0 to +0.85 V (Ag/AgCl) but is not apparent under OCP conditions. In NaOH, however, the corrosion resistance strongly decreases at higher temperatures as compared with the room temperature. At the slightly elevated temperature in 0.1 M NaOH the Co binder phase loses its passivity and is almost completely washed out of the compound material. A WC skeleton remains on the surface and hence the ductility in the hardmetal is lost. In the end, the material could completely fail under such operating conditions. Also the Ni alloyed binder loses its strong passivation ability at the elevated temperatures.     

Near-nano WC–Co hardmetals: Will they substitute conventional coarse-grained mining grades?

Development of nanostructured hardmetals is a task of great importance. Nevertheless, in spite of some “euphoria” with respect to nanograined hardmetals, their potential application ranges are yet not clear. In some works, near-nano and nano hardmetals are believed can potentially substitute conventional medium- and coarse-grained WC–Co grades. In the present work near-nano hardmetals with WC mean grain size of nearly 200 nm and Co contents of 10–33 wt.% were produced and examined with respect to their hardness, fracture toughness, transverse rupture strength and wear-resistance. The near-nano hardmetal with 10% Co having a hardness of 20 GPa and fracture toughness of 9.5 MPa m^1/2 is characterised by exceptionally high wear-resistance obtained by use of the ASTM B611 test in comparison with an ultra-fine grade with 10%. The wear-resistance of the near-nano hardmetals in the ASTM B611 test significantly decreases with increasing the Co content and the wear rates of the difference between the wear rates of the grades with 10% and 33% Co is equal to nearly 44 times. The near-nano hardmetals with 25%, 28% and 33% Co having a moderate hardness and high fracture toughness corresponding to conventional coarse and ultra-coarse-grained mining grades have a very low wear-resistance in laboratory tests on concrete-cutting, granite-cutting and percussion drilling of quartzite. A number of grades with the very similar hardness of 13 ± 0.2 GPa, WC mean grain sizes varying from 0.2 to 4.8 μm and Co contents varying from 3% to 25% were produced and examined by use of the ASTM B611 test. The wear-resistance of the near-nano grade with 25% Co is found to be lower by more than three times compared to the coarsest grade with 3% Co at almost the same hardness. In this case, in spite of the very similar hardness of all the samples, the proportion of the soft binder phase on the surface subjected to abrasive particles when performing the test is significantly higher for the near-nano grade compared to the coarse- and ultra-coarse grained hardmetals. Thus, near-nano and presumably nano hardmetals are expected to never substitute conventional medium- and coarse-grained mining grades. The only application range, where near-nano and nano hardmetals can potentially substitute conventional grades, is an application range of hardnesses of above 18 GPa.     

Pulsed-laser fabrication of gas-filled hollow Co–Pt nanospheres

We report on nitrogen-filled hollow Co–Pt nanospheres produced via pulsed-laser ablation in ambient nitrogen gas. The resulting nanospheres are characterized by a single-crystalline face-centred cubic Co_55±3Pt_45±3 shell and a void filled with molecular nitrogen, typically occupying the sphere’s central region. The average diameter of the spheres and the voids is 35 ± 8 and 16 ± 2 nm, respectively. The calculated number density of nitrogen atoms, measured within these voids, is 1.58 ± 0.4 nm^?3. The resulting pressure in the voids near ambient temperature (300 K) and at the boiling temperature for the Co–Pt alloy (～3000 K) is estimated to be 1.9 ± 0.3 and 34.3 ± 9 MPa, respectively. The gas-filled Co–Pt hollow spheres are formed in only one step involving two physical processes. First, after each laser pulse, the vaporized, supersaturated Co–Pt ablated species are condensated in the plume under high pressure and temperature, resulting in nitrogen gas trapping. Between two laser pulses, the pressure and temperature in the plume drop rapidly, the nitrogen-rich liquid nanospheres become thermodynamically unstable and the nitrogen gas bubble starts to expand until the solidification of the nanospheres. The fast solidification of the solid shell prevents further outward diffusion of nitrogen and thus an amount of nitrogen gas is preserved in the void. These nanospheres have the potential in biomedical, magnetic and catalytic applications.     

Viscosity of WC–Co compacts during sintering

This paper describes experiments performed on WC–Co compacts in order to measure the viscosities of a Newtonian constitutive law commonly used to simulate sintering. An intermittent loading method is used during two series of experiments. The first series are dedicated to determining the axial viscosity and takes place in a loading-dilatometer. The second one takes advantage of a video-extensometry device and provides results about the viscous Poisson's ratio. The axial viscosity is obtained as a function of relative density and temperature. Viscosity shows strong exponential increase with increasing density during isothermal conditions but decreases from 10 to 1 GPa·s between 1100°C and 1325°C during a conventional sintering cycle. Viscous Poisson's ratio shows low values at low densities and increases to 0.5 at full density.     

A new approach to develop complicated superalloy castings

As a precision casting process, investment casting process has been used widely in casting industry because of its many advantages. For almost all alloys, complicated near-net-shaped components can be cast relative easily and both external and internal designed complex castings can be achieved using investment casting. Investment casting is usually required for making complex shape castings while other manufacturing processes are too costly and time-consuming [1]. Conventional investment casti…     

Chemical synthesis of WC–Co from water-soluble precursors: The effect of carbon and cobalt additions to WC synthesis

The chemical synthesis of WC–Co from water-soluble precursors and the effect of carbon content and cobalt addition were studied. Ammonium metatungstate AMT as tungsten source, glycine as a carbon source and cobalt acetate Co(C₂H₃O₂)₂ as a cobalt source was dissolved in water and spray-dried, and thermal synthesis in Ar atmosphere was performed. In order to understand the effects of carbon content and cobalt addition on synthesis steps, and the chemical and phase structure, thermogravimetry (TGA) with Differential Scanning Calorimetry (DCS) and mass spectrometry was used together with X-ray diffractometry and chemical analysis. The results reveal that carbon content mainly affected reduction temperatures and cobalt addition to reaction route and solid state synthesis temperature. This presented manufacturing route with water-soluble raw materials was a potential way of preparing nanostructural WC–Co composition with the correct phase structure and chemical composition.     

High temperature mechanical properties of WC–Co hard metals

Understanding of the load situation and consequently the lifetime of cutting tools made of WC–Co hard metal requires quantitative data for thermo-mechanical properties. For the elevated temperatures present in application, these data are currently rather rare. The present work does discuss elastic material properties up to 1100 °C and compressive yield strength up to 900 °C, both as a function of Co content. The fracture toughness was determined as a function of the WC grain size and Co content up to 800 °C. Young's modulus and yield strength decrease with increasing temperature. A significant rise in fracture toughness was observed at 800 °C with increasing Co content and decreasing WC grain size. A possible reason for this increase is an increase in the plastic zone size at elevated temperatures.     

Co含量对金刚石–WC–Co复合材料耐磨性的影响

随着采矿和城市基建等行业的发展,对矿用WC–Co采掘工具的耐磨性提出了更高的要求。通过添加金刚石增强WC–Co矿用工具的耐磨性是一种可行的新思路。在烧结制备金刚石–WC–Co复合材料的过程中钴相作为催化剂会加速金刚石向石墨转变。为研究Co对复合材料中金刚石石墨化程度的影响,采用放电等离子体烧结技术(SPS)制备金刚石–WC–Co复合材料,分析了复合材料中金刚石石墨化程度并采用砂轮法研究了复合材料的磨损性能和磨损机理。结果表明：金刚石–WC–Co复合材料中金刚石可以起到增韧效果;Co含量增加会促进复合材料致密化进程,同时也会降低复合材料的硬度;随着Co含量增加,复合材料材料耐磨性变差。磨损过程中 WC–Co基体率先被磨损去除,金刚石后被磨损,金刚石会增强材料的耐磨性能。     

Mechanical properties of WC–Co coatings with different decarburization levels

In this study, two kinds of WC–Co coatings with different decarburization levels were deposited by high-velocity oxy-fuel(HVOF) spraying using the ultrafine WC–Co composite powder and commercial micronsized powder, respectively. The hardness and elastic modulus were measured on the top surface and cross sections of the prepared coatings by the nanoindentation method. The results show that the ultrafine-structured coating has much higher density and inhibited decarburization than the conventional coating, which thus results in higher hardness and elastic modulus values than the micronsized coating. The wear resistance of thermal-sprayed cermet coatings greatly depends on the cross-sectional hardness and elastic modulus which reflects the bond strength between splats to some extent. Based on the analysis, a better understanding of the microstructure and properties in cermet coating materials was obtained.     

Mechanical deformation of WC–Co composite micropillars under uniaxial compression

In this work, WC–Co micropillars machined by focused ion beam have been tested under uniaxial compression to investigate the stress–strain behavior and associated deformation mechanisms. The results indicate that yielding phenomena is evidenced by multiple strain bursts. Experimental data is found to fall within the bounds defined by the mechanical responses expected for an unconstrained Co-binder like model alloy and a bulk-like constrained binder region in WC–Co composites; capturing then local phase assemblage and crystal orientation effects.     

Mechanical Properties and Microstructural Behaviour of Microwave Sintered WC–Co

Metals and Materials International - Cemented carbides have been of great interest within industrially manufacturable hard materials for their mechanical properties. Microwave sintering is known...     

Alloyed W–(Co,Ni,Fe)–C phases for reaction sintering of hardmetals

It has been shown that W–Co–C phases could dissolve a substantial amount of metals such as V, Cr and Ta, which are known to positively influence the microstructure of hardmetals with respect to uniform grain size distribution and fine grain size. This offers a tool to circumvent the conventional doping of hardmetals with individual carbides. In the present study we used double- and triple-alloyed κ-W₉Co₃C₄ (i.e. κ-(W,V,Cr)₉Co₃C₄ and κ-(W,V,Cr,Ta)₉Co₃C₄) and applied a variety of sintering experiments to obtain WC–Co, WC–(Ti,Ta,Nb)C–Co and WC–(Ti,Ta,Nb)(C,N)–Co hardmetals. We also prepared κ-W₉Fe₃C₄, alloyed κ-W₉Ni₃C₄, and κ-W₉(Fe/Ni)₃C₄, and used the latter for sintering.     

Quantitative characterization of the microstructure and properties of nanocrystalline WC–Co bulk

Microstructure of the nanocrystalline WC–Co cermet bulk was quantitatively described by transmission electron microscopy based precession electron diffraction technology. It is discovered that the fraction of the Σ2 grain boundaries increases with the decrease of WC grain size. The effect of microstructure on mechanical properties depends on Co distribution, Σ2 boundaries fraction and WC grain contiguity.

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Prediction of Coating Properties of Thermally Sprayed WC–Co on Complex Geometries

This work outlines the development of an analytical software tool that enables the prediction of various coating properties on any given sprayable geometry. The prediction is achieved by analyzing the input computer-aided design geometry and by correlating the resulting kinematic conditions with experimental measurements. The results of the developed tool have been validated experimentally using HVOF-sprayed WC-17Co coatings. Specifically, coating thickness, microhardness, WC vol.% and specific sliding wear rate are examined and their values are predicted for the case of the external spray of a rotor-like model.     

A computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides

Interfaces and surfaces often play a vital role for the properties of polycrystalline materials, such as cemented carbides, and the study of these planar defects is, therefore, of great importance. Cemented carbides (or hardmetals) is a unique class of materials where hard carbide (WC) grains, usually micrometer sized, are embedded in a more ductile metal binder phase (usually Co) in order to combine superb strength with high hardness, making them ideal as tool material in e.g. metal machining. In the manufacturing and industrial usage of cemented carbides temperatures reach high levels, especially in the former case where the material is sintered at temperatures where the binder phase is a liquid.This is a computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides upto and above the melting temperature of Co. We make use of an analytical bond order potential (ABOP) fitted to density functional theory (DFT) data in order to make the free energy calculations feasible. A variety of free energy methods are used: including quasi harmonic approximation, temperature and thermodynamic integration, and calculation of liquid surface tension and work of adhesion for phase boundaries. We present the temperature dependent interface and surface energies for some typical cases, data which should be useful as a supplement to other studies limited to 0 K.     

A new approach to zinc–nickel separation using solution alkalinization method:application to a zinc plant residue

The present investigation involves the separation of zinc and nickel from a sulfate solution using the acidic leaching of zinc plant residue after cadmium removal step as precursor(42.88 wt% Zn,8.50 wt% Cd and 2.33 wt% Ni).Separation of nickel from the solution was done by pouring it into a strong alkaline sodium hydroxide solution due to precipitation of nickel hydroxide and conversion of zinc to the soluble Zn(OH)₄^2-complex.Higher degrees of separation were reached by pou...     

Microwave sintering of nanocrystalline WC–12Co: Challenges and perspectives

Microwave sintering of microcrystalline and nanocrystalline WC–12Co powder compacts was carried out employing different time–temperature schedules. The microcrystalline powder compacts were made from powders with particle sizes ranging from 5 to 45 μm by using methyl cellulose as the lubricant. The nanocrystalline powder compacts were made from powders having a mean WC grain size of 38 nm, without employing any lubricant. The sintered samples were characterized with respect to their densities, Vickers hardness, fracture toughness and microstructures and the challenges encountered during microwave sintering of the WC–12Co powders are discussed.     

Variation of WC grain shape with carbon content in the WC–Co alloys during liquid-phase sintering

WC–Co hard metals have faceted WC grains dispersed in a ductile cobalt-rich matrix. The effect of carbon (C) content on the shape of WC grain in the WC–Co metals during liquid-phase sintering is investigated in this work. The WC grain shape varies with the C content and, more importantly, the shape change occurs reversibly with the C content.