WC-Co Composites Research Articles

Dynamic constitutive relation and macro/micro failure mechanism of fine-grained WC-Co composite

As the core of an armour-piercing projectile, it is crucial to provide a microscopic perspective to elucidate the macroscopic mechanical responses of fine-grained WC-Co composite under dynamic loadings. This study investigated the dynamic constitutive relation and macro/micro failure mechanisms of fine-grained WC-Co composite. Results showed that the compressive and tensile strengths increase with the strain rate influenced by the inertia effect of crack propagation and the presence of the Co phase. The strain rate dependency mechanism of the composite was obtained by comparing it with typical ceramic materials. Macro-crack propagation and distribution were discussed under different stress states in dynamic compression and Brazilian splitting tests. Microscopic failure mechanisms were obtained by SEM technique, revealing that the Co phase creates numerous localized dimple fractures and improves the plastic deformation of the WC-Co composite.

Effect of yttrium oxide addition on the microstructure and mechanical properties of WC–ni composites fabricated from recycled WC and Ni

ABSTRACT WC-Co composites are widely used in cutting tools but are expensive and harmful due to Co in the composites. Alternatives, such as WC-Ni composites have inferior mechanical properties, as a result, WC-Co composites are used in most cutting tools. This study investigated whether adding yttrium oxide (Y2O3) to WC-Ni composites, fabricated from recycled tungsten carbide (WC) and Ni, improved their mechanical properties. Changes in the composites’ microstructure and mechanical properties depended on the amount of Y2O3 added. Y2O3 reacted with WC during sintering, promoting the formation of W and reducing the proportion of C in the composite. The added Y2O3 accelerated the dissolution of W in the Ni matrix, and increased the fractions of the eta phase and solid solution phase. Ni is strengthened by the dissolution of W in Ni, thereby supplementing the low toughness of the eta phase and ultimately yielding WC – Ni with improved hardness and fracture toughness (HV: max. 15.4 GPa, KIC: max. 13.4 MPa∙m1/2) compared to WC – Ni without Y2O3 (HV: 13.5 GPa, KIC: 12.7 MPa∙m1/2). The method for improving the properties of WC – Ni composites by the addition of Y2O3 has a high possibility of commercialization because it does not necessitate process changes.

Evaluation of the Surface Integrity and Recast Layer in Electrical Discharge Turning of WC-Co Composites.

Tungsten carbide (WC) and its composites are typically associated with high hardness and high wear resistance, posing challenges in conventional machining processes like turning. To address the machining difficulties of WC-Co, electrical discharge turning (EDT) was proposed. The rotational speed in EDT is a key factor influencing the machining results; however, conflicting reports exist about its impact on the EDT process. Therefore, the effect of rotational speed on three different machining regimes, including roughing, semi-finishing, and finishing, was investigated using energy-dispersive X-ray spectroscopy (EDX), SEM, and roughness tests. Additionally, elemental mapping was applied to illustrate the element distribution on the machined surface. The results indicated that increasing the rotational speed led to a 10% to 17% decrease in the recast layer thickness and a 14% to 54% reduction in the surface roughness (Ra).

Effects of NbC addition on mechanical and tribological properties of AlCrFeNi medium-entropy alloy

AlCrFeNi medium-entropy alloy (MEA) without expensive Co has demonstrated superior properties over the well-known AlCoCrFeNi high-entropy alloy. We added NbC particles to the MEA to make NbC-reinforced MEA alloys or MEA-matrix composites, and evaluated their wear resistances and mechanical properties. The materials showed considerably improved performance. To well judge the NbC-MEA for realistic applications, the wear resistance of the NbC-MEA samples was compared with those of a few industrial wear-resistant materials, e.g., high-Cr cast iron (HCCI) and WC-Co composites. It was demonstrated that the MEA samples with 40–60 vol% NbC exhibited markedly higher wear resistance than the reference materials, and the MEA demonstrated high superiority over the widely used Co as the metal-matrix for developing wear-resistant metal-matrix composites (MMCs).

Computational extreme value statistics of strength for WC-Co composites at small specimens

The mechanical behavior of ceramic–metal (WC-Co) composites at small specimen level is subject to significant uncertainty due to the random assembly of (1) its constituent phases, (2) the crystal orientation of the WC grains, (3) the intrinsic residual stresses, and (4) the variable level of confinement of the metallic binder, within the contiguous network of much stiffer WC grains. In the existing literature, a Weibull distribution of strength is typically assumed at large specimen level. However, it is unknown whether this assumption can be carried over to small samples. Thus, the aim of this study is to shed light into the randomness in the mechanical strength of WC-Co composites, by using Monte Carlo simulations employing Latin Hypercube Sampling. The marginal distribution of the model parameters is assumed to be a grafted zero-threshold Weibull-Gaussian probability distribution, applied individually to the ceramic particles and the metallic binder using autocorrelation fields. The deterministic models are the recently calibrated microplane models M7WC and MPJ2 for the ceramic phase (WC) and the metallic matrix (Co) respectively. Test data in the form of strength histograms, gathered from literature on tensile tests conducted on nanowires of WC-Co composites, are used to calibrate the Weibull and Gaussian parameters, the grafting point locations and the autocorrelation lengths. The deterministic models M7WC and MPJ2 are assumed to be predictive in this study, as they were already calibrated in a previously published work by the authors. The finite element meshes used in the Monte Carlo simulations are generated using real tomography images, which easily captures the Gaussian part of the probability density function (pdf) without any random variables. This approach is shown to account successfully for the randomness observed in the strength of these ceramic–metal composites. Predictions are made for the strength distribution of differently sized nanowire specimens in tension.

Spark Plasma Sintering of Fine-Grained WC-Co Composites.

Cemented carbides WC-Co are one of the basic tool materials. They constitute over half of the currently used tools intended for machining. The main advantages of WC-Co cemented carbides are high hardness and abrasion resistance. The properties of WC-Co sinters depend mainly on the content of the binding phase, the sintering method and the grain size of the powder from which the sinters were made. The aim of this study was to produce fine-grained WC-Co composites using SPS (spark plasma sintering) technology, as well as examine the effect of the applied technology on the basic properties of WC-Co sinters: microstructure, hardness, phase composition, compaction degree and tribological properties. In the processes carried out, no additives affecting the reduction in grain growth were used. Sintering was conducted at a temperature of 1200 °C with a holding time of 10 min. The process occurred under a load of 100 MPa. Finally, the samples were cooled in a vacuum of 10-6 mbar. We measured the hardness using a Vickers hardness tester. We took hardness measurements along the diameter of the sintered samples. In order to ascertain the fracture toughness (KIC), we measured the radial crack length around the Vickers indentation and applied Shetty's formula. The tribological tests were carried out with a tribotester using the T-01 ball-on-disc method. The obtained data enabled the characterization of the wear process of the tested materials.

Effect of SPS frequency on the transformation of diamond to graphite in WC-Co composite

(WC–6wt%Co)/5Vol% diamond composites were successfully fabricated at 1150–1300 °C and 40 MPa for 5 minutes under the thermodynamically unstable conditions of the Spark Plasma Sintering method. The influence of on-off time, and frequency of the DC pulse of SPS on diamond stability, microstructure, and mechanical properties of WC–Co/diamond composites were investigated. The diamond particles are strongly bounded with the cemented carbide matrix by a transition layer composed of a solid solution of tungsten and carbon in cobalt. The amount of graphitization was low in the sintered composites at 1300 °C due to the low temperature and short sintering time. Reduction of SPS frequency effectively inhibited graphite preformation, and no graphite was detected in the sintered composites at 1150 °C. Diamond morphology was not affected during SPS as examined by SEM, and Raman scattering spectra. The uniform dispersion of the super-hard diamond phase in matrix which good bonded, caused the high hardness, fracture toughness and flexural strength of this composite. The reduction of pulse frequency of SPS leads to a significant enhancement in the hardness, fracture toughness, and flexural strength of the composites, overcoming the graphitization of diamond typically observed in SPS. The sample sintered with 4 Hz showed simultaneously the highest hardness (23.6 GPa), the highest fracture toughness (24.6 MPa.m½), and the highest flexural strength (2050 MPa).

Testing for Abrasion Resistance of WC-Co Composites for Blades Used in Wood-Based Material Processing.

Commonly used tool materials for machining wood-based materials are WC-Co carbides. Although they have been known for a long time, there is still much development in the field of sintered tool materials, especially WC-Co carbides and superhard materials. The use of new manufacturing methods (such as FAST-field-assisted sintering technology), which use pulses of electric current for heating, can improve the properties of the materials used for cutting tools, thereby increasing the cost-effectiveness of machining. The ability to increase tool life without the downtime associated with tool wear allows significant cost savings, particularly in mass production. This paper presents the results of a study of the effect of grain size and cobalt content of carbide tool sinters on the tribological properties of the materials studied. The powders used for consolidation were characterised by irregular shape and formed agglomerates of different sizes. Tribological tests were carried out using the T-01 (ball-on-disc) method. In order to determine the wear kinetics, the entire friction path was divided into 15 cycles of 200 m and the weight loss was measured after each stage. In order to determine the mechanism and intensity of wear of the tested materials under technically dry friction conditions, the surface of the tested sinters was observed before the test and after 5, 10, and 15 cycles. The conclusions of the study indicate that the predominant effect of surface cooperation at the friction node is abrasion due to the material chipping that occurs during the process. The results confirm the influence of sintered grain size and cobalt content on durability. In the context of the application of the materials in question for cutting tools, it can be pointed out that sintered WC(0.4)_4 has the highest potential for use in the manufacture of cutting tools.

Increase of specific interfacial coherence in nanocrystalline ceramic-metal composites

A new strategy to establish a large number of interfacial coherence in nanocrystalline multi-phase composites was proposed. The route consisting of amorphization, crystallization and in-situ reaction was demonstrated using WC-Co composites as a typical example. Characteristics of crystal nucleation, growth crystallography, and phase transformation were studied in detail on the atomic scale for the ceramic grains. Mechanisms for the formation of specific interfacial coherence in the prepared nanocrystalline ceramic-metal composites were disclosed. Assisted by the interfacial coherence, the continuity of local deformation is maintained from metal to ceramic phase, resulting in a uniform strain extension instead of stress concentration at the interfaces. The present nanocrystalline composite achieved outstanding mechanical properties with simultaneously high hardness and fracture toughness. The findings in this work provide new insights to tailor grain structure and interface relationship in cermet composites for superior comprehensive mechanical performance.

The Assessment of the Transversal Rupture Strength (TRS) and Hardness of WC-Co Specimens Made via Additive Manufacturing and Sinter-HIP

This study is focused on the mechanical properties of WC-Co composites obtained via Selective Laser Sintering (SLS) using PA12 as a binder. The as-printed samples were thermally debonded and sintered, first in a vacuum, and then sinter-HIP (Hot Isostatic Pressure) at 1400 °C, using 50 bar Ar, which has led to relative densities up to 66%. Optical metallographic images show a microstructure consisting of WC, with an average grain size in the range of 1.4–2.0 µm, with isolated large grains, in a well-distributed Co matrix. The shrinkage of the samples was 43%, with no significant shape distortion. The printing direction of the samples significantly impacts the transversal rupture strength (TRS). Nevertheless, the mechanical strength was low, with a maximum of 612 MPa. SEM images of the fracture surface of TRS samples show the presence of defects that constitute the cause of the low measured values. The hardness values position the obtained composites in the medium coarse classical cemented carbides range. The results were also related to the amount of free Co after sintering, close to the initial one, as assessed by magnetic measurements, indicating a low degree of interaction with PA12 decomposition products.

Influence of cutting parameters on the tool life for WC-Co composites in the machining of wood-based materials

ABSTRACT WC-Co cemented carbides are the most commonly used material for machining wood-based materials. They owe their popularity to good properties, such as hardness and, at the same time, high fracture toughness. A significant impact on the properties of cemented carbides can have the method of their preparation. The durability of WC-Co tools can be improved by selecting the appropriate machining parameters. The study attempted to determine the optimum machining parameters for particleboard. The tools used in the study were both commercial blades and blades manufactured in laboratory conditions by sparkplasma sintering (SPS), using a mixture of WC6%Co powders. As part of this work, observations of the microstructure and durability tests of the blades during particleboardmilling processes were carried out. Wear of the tool was measured until loss on the contact surface VBmax, was 0.2 mm. Based on the tests, it can be concluded that when the lowest feed per tooth (Δz = 0.15 mm) was used, the tools had the longest tool life. In addition, the commercial tools covered a significantly longer cutting distance than the tools obtained using the SPS method.

WC-Co composite made with doped binder: The effect of binder proportion on microstructure and mechanical properties

Apart from hardness and density, the durability of cutting tools and other wear parts made of WC-Co composites depends on the homogeneity of the material. A homogeneous composite powder is obtained by mixing the carbide and the binder for long periods.To shorten the homogenization time and improve the sustainability of these products, a new type of binder was created by incorporating the grain growth inhibitor into the cobalt binder. The presence of the inhibitor in the binder is supposed to improve the latter's distribution in the mixture and make the binder harder, therefore more easily incorporable into the mass of the tungsten carbide particles, reducing the homogenization time.A WC powder with a particle size of about 0.5 μm was mixed with 6, 9 and 12% by weight of cobalt powder doped with Cr3C2 (doped binder). Classic mixtures (WC + Co + Cr3C2) were also prepared to assess the effect of the doped binder.The mixtures were prepared by planetary ball milling for 10 h, with a speed of 600 rpm, using a balls/powder ratio of 4: 1, and 20 ml of ethanol as an anti-caking agent.Samples were shaped and sintered by SPS technique at 1150 °C with a heating rate of 50 °C/min and a pressure of 50 MPa.As expected, a more homogenous microstructure is obtained in the samples made with the doped binder. The composites with 6 and 9 wt% Co has superior densification thanks to the doped binder. The best hardness/toughness compromise is registered for composites with 9% Co (2100 HV30 and 9.1 MPa*m1/2). Increasing the cobalt content to 12% decreases the hardness but does not improve toughness (9.5 MPa*m1/2).

Microstructure Evolution and Hardness Improvement of WC-Co Composites Sintered with Fe Substituting Part of Co Binder

WC-13Co (wt.%) composite with Fe added was prepared by pressureless sintering, and its microstructure and mechanical properties were analyzed by X-ray Diffraction (XRD), Scanning Electronic Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and Vickers hardness tester. The effect of Fe and C content on the microstructure of the eta phase, binder phase, grain size and porosity, as well as mechanical properties in the W-Co-Fe-C system, was discussed quantitatively. The position of added Fe in the lattice of the composites is clarified. When 2%–4% Fe was added, Fe existes in both the binder phase and the low-carbon eta phase. In the lattice of the eta phase, Fe occupies the position of Co and Fe uniformly existes in the binder with the structure of simple cubic when the composites is in the eutectic phase region. Differently from the previous report that the growth of WC in liquid Fe was severely limited, the size of WC in the W-Co-Fe-C system increases from 1.14 μm to 1.21 μm when the content of Fe increases from 0 to 4%, which indicates that the growth behavior of WC in liquid CoFe was different from that in liquid Fe, but closer to that in liquid Co. The sample added both 2% Fe and 1% C has the optimum matching of hardness and toughness. Compared with the hardness of 979 HV30 in the sample without adding Fe and C, the hardness of the sample with both 2% Fe and 1% C added achieved 1071 HV30, which is increased by 9.4% under the conditions of a slight increase in fracture toughness.

Dependence of residual thermal stress in cermets on the sintering technique

The effects of representative sintering techniques on the residual thermal stress (RTS) in the as-prepared cermets were investigated using the WC-Co composites as an example. The real processing parameters of the spark plasma sintering (SPS) and sinter-hot isostatic pressing (Sinter-HIP) techniques were introduced to quantify RTS and its mechanisms. The real microstructures of cermets were used in the models for evaluation of RTS distributions in different phases. The macro-mesoscale coupled calculations indicated that during the cooling of both the SPS and Sinter-HIP processes, the WC phase of the sintered cermets has a state of compressive stress, while Co phase has a tensile stress state. The internal stress magnitude of both phases increases linearly with the decrease of temperature. There are larger temperature and displacement fields in the SPSed cermets compared to those prepared by Sinter-HIP. The stress accumulation in the SPS process is faster than that in the Sinter-HIP by 30%. The RTS in the SPSed cermets is highly concentrated at the acute dihedral angles of WC/Co interfaces, which tends to promote the face-centred cubic (fcc) to hexagonal close-packed (hcp) martensitic transformation of the Co phase.

A computational framework for micromechanical modelling of WC-Co composites

In this study, the mechanical behavior under monotonic loads of tungsten carbide‑cobalt (WC-Co) composites is investigated extensively by analyzing (1) nanoindentation tests on WC particles and Co matrix, (2) nanowires made of WC-Co composites tested in tension and (3) micropillars made of WC-Co composites tested in compression. To this end, a novel computational framework consisting of two different microplane constitutive models developed for WC and Co phases are proposed. For the Co matrix, the microplane J2-plasticity, called the model MPJ2, and for the WC particles a modified version of the microplane model M7, called the model M7WC, are employed. Furthermore, finite element meshes in 3D obtained from experimental tomography reconstructions of WC-Co composites are employed to rule out any spurious geometric features that is likely to be encountered in artificially generated meshes. After calibrating the aforementioned models, it is shown that the finite element predictions not only confirm the extensive experimental observations but also shed further light into the mechanical behavior of these composites.

Achieving exceptional wear resistance in cemented carbides using B2 intermetallic binders

Conventional strategy for designing wear-resistant composites relies on the immersion of a carbide skeleton into ductile binders to impart necessary strength and deformability but sometimes overlooks the deformation compatibility, rendering the composites vulnerable to premature wear failures. In this work, extensive binder extrusion was specifically noted on traditional cemented carbides during sliding in absence of oxides protection. To alleviate this situation, we designed the WC-CoFeAl composites with a novel B2_CoFe binder phase structure, as per the calculated phase diagrams, in comparison to the WC-CoAl and WC-Co composites that respectively incorporate a single α-Co phase and a mixture of α-Co and ε-Co phases. The WC-CoFeAl reveals an exceptional binder extrusion resistance and realizes a ≥50% wear reduction within a temperature range of 25–400 °C. Originating from the intrinsically high strength and the dynamically generated gradient nano-structures, consisting of a nano-grained surface tribolayer and an ultra-fine grained subsurface layer, the B2 intermetallic binders promote an improved deformation compatibility with the WC skeleton, which contributes to the maintenance of robust contact surfaces. The current work thus highlights the important role of deformation compatibility on the durability of cemented carbides, which offers a new thread to future development of wear-resistant composite materials.

Influence of process parameters on the particle–matrix interaction of WC-Co metal matrix composites produced by laser-directed energy deposition

The prediction of the in-service behaviour of metal-matrix composites produced by laser-directed energy deposition is a fundamental challenge in additive manufacturing. The interaction between the reinforcement phase and the matrix has a major impact on the micro and macroscopic properties of these materials. This interaction is fostered by the exposition of the materials to high temperatures. Hence, it is highly influenced by the thermal cycle of the manufacturing process. In this work, an experimental approach is adopted to determine the influence of the main process parameters on the properties of metal-matrix composites. Statistical regression models are employed to consider the role of the most relevant parameters, from exploration to exploitation. The obtained trends are further corroborated by the corresponding microstructural, SEM, and EDS analyses. In terms of surface hardness, the DOE reveals different trends of the response depending on the composition of the feedstock employed. It is concluded that the strengthening behaviour of the material varies throughout the experimental domain studied. When high WC% feedstocks are employed, the main strengthening mechanism responsible for the increase of hardness is the solid-solution of tungsten and carbide precipitation. On the contrary, when low WC%s are employed, grain refinement becomes the main strengthening mechanism.

Neutron diffraction characterizations of NbC-Ni cemented carbides thermal residual stresses

Cemented carbide materials develop very large micro-stresses due to the coefficient of thermal expansion (CTE) mismatch between the metallic carbide phase and the binder phase. While micro-stresses developed in WC-Co cemented carbides have been investigated, there is very little to no information on the micro-stresses developed in NbC-Ni materials.In this study the thermal stresses developed in NbC-Ni materials as a function of the carbide grain size (from 2.5 μm to 12.8 μm) and the binder content (from 8 vol% to 12 vol%) were characterized by neutron diffraction. It was found that the residual stresses decreased with the increase of the binder content and the increase of the carbides grain size. For both binder and carbide phases, stresses were higher for smaller NbC particles, reaching 4 GPa in Ni at the lowest binder content. In general, stresses were higher than the ones measured in WC-Co or WC-Ni composites. This unexpected finding was attributed to the very different shapes presented by the carbide particles; NbC particles are almost spherical, and WC particles are triangular plates with truncated corners. These different shapes create different strain/stress distributions on the microscopic level around the carbide particles.

Effects of TiC0.4 on microstructure and properties of WC composites

WC composites with 5 wt% TiC0.4 were fabricated by spark plasma sintering (SPS) at sintering temperature of 1400–1700 °C for 10 min under an external pressure of 50 MPa. It can be observed that the WC-5 wt% TiC0.4 composites mainly consisted of WC phase and non-stoichiometric (W1−x, Tix)Cn solid solution phase and the amount of WC dissolving in TiC0.4 increased with sintering temperature. The (W1−x, Tix)Cn solid solutions uniformly distributed around WC grains and formed a semi-coherency interface with WC, which restricted the growth of WC grains and enhanced mechanical properties of the WC-5 wt% TiC0.4 composites. The average grain size of WC (251.5 ± 201.6 nm) in WC-5 wt% TiC0.4 composite sintered at 1600 ℃ was considerably smaller compared to most of previously reported WC-based composites. The existence of high content of C vacancies in non-stoichiometric TiC0.4 raised remarkably the diffusion rate of W and Ti atoms in the sintering process of WC-5 wt% TiC0.4 composites in contrast with stoichiometric TiC. The WC-5 wt% TiC0.4 composite sintered at 1600 ℃ possessed the optimum properties, with the hardness of 21.9 GPa, fracture toughness of 7.6 MPa·m1/2, flexural strength of 612 MPa and compressive strength of 2.7 GPa. Moreover, it can be confirmed that the oxidation resistance of WC-5 wt% TiC0.4 composite at higher temperature outperformed that below 1218 ℃. In addition, the WC-5 wt% TiC0.4 composite sintered at 1600 ℃ showed higher high-temperature hardness, lower thermal conductivity and better resistance to oxidation than those of conventional WC-Co composites.

Revealing the interface characteristic of the semi-coherent Co(111)/WC(0001) interface: a first principles investigation

ABSTRACT In the present work, based on the first-principles calculations, we systematically discuss the electronic structure, adhesion work, interface energy, chemical bonding strength and interfacial mechanical failure forms of semi-coherent Co(111)/WC(0001) interface. After relaxation, the interface configurations of the middle top (MT) sites migrate toward the hexagonal close-packed hollow top sites (HCP), indicating that the MT occupancy is unstable. The atoms at the interface are more inclined to the HCP sites. The calculated results indicate that the C-terminated HCP1 site interface configuration has the largest adhesion work and the smallest interface energy, which depends on the formation of strong C-Co covalent bonds at the interface. Finally, the strong Co/WC interface configuration formed in the WC-Co composites can effectively carry out load transfer and reduce stress concentration, improving the strength and plasticity of the composites.