WC-Co Composites Research Articles

Deformation and fracture of WC grains and grain boundaries in a WC-Co hardmetal during microcantilever bending tests

The deformation and fracture behaviour of constituents of a WC-Co hardmetal were investigated by microcantilever bending technique. The compositions of FIB fabricated microcantilevers were: I) single grains of WC, II) WC grains of different orientations and III) the mixture of WC grains and Co phase. The crystallographic orientation of WC grains and the fracture surface of beams were studied by EBSD and SEM analyses, respectively. It was revealed that the elastic deformation depends mainly on the composition of the beams and the orientation of the WC grains. The Young's modulus of WC grains showed an orientation dependence with decreasing values from the basal (E~800 GPa) towards the prismatic orientations (E~500 GPa), which is in agreement with the theoretical predictions. The deformation behaviour of WC grains exhibited plasticity before their fracture with an average fracture strength of σ = 12.3 ± 3.8 GPa. It was found that the effect of dislocations and nanometre-sized defects (e.g. pores) plays an important role in the bending test of WC grains. Most of the WC/WC boundaries showed brittle failure with an average fracture strength of σ = 4.1 ± 2.5 GPa. It was concluded that the majority of the boundaries in the WC-Co composite are high energy WC/WC boundaries and their fracture strength is generally much lower than that of the WC grains.

Influence of compaction and degassing on the properties of submicron WCCo produced by the PPS method

Influence of compaction and degassing on the properties of submicron WCCo produced by the PPS method. The present study is concerned with the effect of the parameters of the degassing operation (temperature, load and heating rate) conducted at the initial stage of the Pulse Plasma Sintering (PPS) process and the sintering temperature at the final stage of the process, on the properties and microstructure of WCCo with a 6wt% cobalt content sintered by this method. The results of the study have shown that when the heating rate is too high, the material obtained is porous. In most experiments, the sintering temperature of 1050°C appeared to be too low to obtain WCCo composites with density close to the theoretical value (GT). Sintering at the temperature increased to 1070°C yielded sinters with density above 99%GT, with hardness of about 1900 HV30 and fracture toughness KIC=9.3 MNm-3/2.

The influence of WC grain size on the durability of WCCo cutting edges in the machining of wood-based materials

WCCo cemented carbides are one of the basic materials used for tools. They consist of tungsten carbide in 70–96% and a binding warp – cobalt. High hardness of these materials determines their high resistance to abrasive wear. These properties predispose them to be used as a material for cutting tools. This study presents the results of tests on the durability of cutting edges made of WCCo composite of different WC grain in the machining of wood-based materials. The tests showed a several-fold increase of the durability of edges made of WCCo composite of grain size of 0.2÷0.5 μm compared to WCCo blades of WC grain size of 0.5÷0.8 μm and 0.8÷1.3 μm.

Evaluation of the reliability on WC-40%Co composites through Weibull analysis

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 materials for high temperature application areas. It has superior hardness and high resistance to shock and wear. Conventional machining techniques are not suitable for machining of this material. Due to the excellent electrical and thermal conductivity of WC–Co, it is usually machined by Electrical Discharge Machining (EDM). EDM process of WC-Co composites often results in the damage of the machined surface would affect the reliability. It is important to investigate the effect of EDM process on the properties of WC–Co composites before applying its engineering application. For these composites, it is very important for maintenance of proper fracture strength and is to be controlled. In this work, an attempt has been made to investigate the material removal rate (MRR), the factors influencing MRR, the surface defects induced by EDM, fracture strength and the reliability of EDMed WC-Co composite using the Weibull distribution analysis. The comparison of results between the machined and un-machined composites is carried out and presented in this study.

Very-small angle neutron scattering study on grain coarsening inhibition by V-doping of WC-Co composites

The mechanical properties of cemented carbides can be tuned by controlling WC grain coarsening and the simultaneous growth of the binder pocket size during the sintering. So far, bulk studies considering this phenomenon are scarce, but here, we report the first very-small angle neutron scattering (VSANS) study on cemented carbides. VSANS is supplemented with electron backscatter diffraction (EBSD) and the microstructural refinement by increasing V-doping (0, 0.02, 0.22, and 0.76 wt%) is quantified. The capability of VSANS as a non-destructive bulk probe for cemented carbides is shown, paving way for forthcoming in-situ studies.

Complex taper profile machining of WC-Co composite using wire electric discharge process: analysis of geometrical accuracy, cutting rate, and surface quality

Complex profiles, comprising curved features and inclined surfaces, are an inherent attribute of tungsten carbide (WC-Co) tooling applications like dies, molds, and cutting tools. Such profiles are difficult to machine by conventional processes due to high hardness of WC-Co. Wire electric discharge machining (WEDM) is a frequently employed non-traditional machining process for producing intricate shapes and complex geometries in difficult-to-cut materials. However, this process offers its own challenges regarding machining accuracy of arced profiles such as undercutting or overcutting of material. Machining task becomes more complicated if arced profiles are to be machined at a certain taper angle. Keeping in view the stringent requirements of productivity and geometrical accuracy for WC-Co tooling, current research is focused on comprehensively evaluating the performance of WEDM process for producing complex tapered profiles in WC-Co composite. Furthermore, instead of using simple brass wire, a zinc-coated wire electrode is employed to enhance cutting rate. The impact of WEDM control variables namely, pulse-on/off time, servo voltage, and wire tension, in conjunction with workpiece taper, is studied on geometrical accuracy, cutting rate, and surface quality of complex tapered profile. Results of experimentation are analyzed using various statistical techniques. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis are carried out for in-depth evaluation of machining results. Experimental results show that a complex tapered profile can be generated with as small as 0.16° angular error and 6-μm radial error using optimal parameter settings. Optimal settings of process parameters result into 33.3% reduction in angular error, 14.3% reduction in radial error, 12% increase in cutting speed, and 14.4% reduction in surface roughness of workpiece.

Microstructure analysis of high density WC-Co composite prepared by one step selective laser melting

WC-Co composite, also well known as cemented carbide, is one of the most challenging materials for one step additive manufacture process, such as selective laser melting (SLM). Up until now, defect free cemented carbide has never been successfully synthesized yet using one-step SLM. In this study, the critical effect of the morphology of feedstock carbide granules on the microstructure was initially investigated for SLM processed carbides. Crack free WC-20Co cemented carbide with high density has been successfully synthesized using one-step SLM without further heat treatment. The density is significantly high for SLM processed carbides although still unsatisfactory yet compared to conventional liquid phase sintered carbides. Spherical granules are more favorable than non-spherical granules in obtaining a higher final density. The SLM process results in inhomogeneous and rapid WC grain growth, which is attributed to the non-uniform temperature distribution and varying amount of time experienced by the material in the liquid state during the SLM process.

High Energy Milling of WС-FeСr Cemented Carbide

During production of cemented carbides hard and brittle tungsten carbide (WC) and ductile metal powders (mainly from Fe-group) are milled together. Complete milling results in a Gaussian distribution and narrow particle size range of the milled powder which promote the homogeneity and improve the properties of sintered composites. Cobalt, conventional metal employed in cemented carbides, possesses good comminution characteristics with WC powder. However, its toxicity and fluctuating price pushes researchers to find suitable alternatives and Fe-based alloys have shown most promising results. Cemented carbides with the Fe-Cr system as metal binder phase have potential to perform better than regular WC-Co composites in corrosive and oxidative environments. The goal of this paper was to prepare uniform cemented carbides powders with relatively high fraction of stainless Fe-Cr steel. To achieve a uniform powder mixture is a challenge at high ductile steel fraction. High energy milling (HEM) is a powerful technique for achieving (ultra) fine powder mixtures with narrow powder size range. HEM was carried out in a novel high energy ball mill RETSCH Emax. Milling in tumbling ball mill, which is the most widely used method, was employed for reference. Prepared powder mixtures were characterised in terms of particle size, size distribution and shape. In addition, powder mixtures were consolidated via spark plasma sintering to evaluate the effect of the milling method and the duration on the microstructure of final cemented carbide.

Infiltration studies of additive manufacture of WC with Co using binder jetting and pressureless melt method

Additive manufacturing (AM) of tungsten carbide-cobalt (WC-Co) is explored starting with WC preforms shaped with binder jet additive manufacturing (BJAM) followed by melt infiltration of Co. The research objective is to demonstrate the ability to net-shape WC-Co composites through BJAM of a WC preform followed by backfilling with cobalt via pressureless infiltration. This method also has the potential to minimize shrinkage and grain growth compared to other AM techniques. The effects of sintering, Co content, and infiltration time on the net shaping and properties of processed composites are shown. The best shaped material had an average grain size of 5.1 μm, 32 vol.% Co, density of 98.54% theoretical, fracture toughness of 23.2 MPa m1/2, and hardness of 9.0 GPa. Data presented illustrates that the proposed approach results in favorable ceramic-metal (cermet) properties and is viable for fabricating cermets of other material combinations. Successful AM of cermets provides complex geometries, high throughout, and low costs.

Nano-scale hardness and elastic modulus of WC-Co composites and their relationship to the tools life during particleboard milling

Nano-scale hardness and elastic modulus of WC-Co composites and their relationship to the tool life during particleboards milling. The paper summarizes the results of the technique of hardness and elastic modulus determination using a load and displacement sensing indentation experiments. During the tests these properties were measured in nano-scale for different types of WC-Co composites using the Anton Paar TriTec Ultra Nano Hardness Tester (UNHT). In addition, durability examination of WC-Co tools during milling of particleboard was carried out. In the final stage, the correlation of the studied properties with the lifetime of tools was analyzed. The studies have revealed a relationship between micro-hardness and the total cutting length of the WC-Co tools.

Corrosion behaviour of WC hardmetals with nickel-based binders

Cobalt is the standard binder in tungsten carbide (WC) hardmetals due to its compatibility with the WC phase, resulting in composites with exceptional hardness and wear resistance. However, their corrosion resistance is not satisfactory in many applications, leading to the early deterioration and failure of tools and equipment.In this work, the corrosion of WC hardmetals with three alternative binders (FeCoNi, NiCrCoMo and NiCrMo) is compared with a benchmark WC-Co composite, using electrochemical techniques such as open circuit potential (OCP) monitoring, polarisation curves and electrochemical impedance spectroscopy (EIS), assisted by scanning electron microscopy (SEM).

Micromechanical properties of WC-(W,Ti,Ta,Nb)C-Co composites

A deep knowledge of micromechanical properties of each constitutive phase of cemented carbides is crucial to improve their performance on the basis of microstructural design optimization. In the present work, a systematic experimental procedure has been followed to determine the intrinsic hardness of individual WC and (W,Ti,Ta,Nb)C particles, as well as that of the metallic binder, within a WC-Co composite containing a solid solution of mixed carbide as a third phase. In doing so, massive nanoindentation and statistical analysis of the gathered data are combined. Results showed that (W,Ti,Ta,Nb)C particles are significantly harder than WC ones, independent of the hardness anisotropy exhibited by the latter. Hardness assessment for the metallic binder required further analysis, including data deconvolution using thin film models and consideration of substrate effects. The attained hardness values are then used for estimation of effective flow stress of the metallic phase by means of Tabor's equation, yielding values ranging between 1.3 and 2.0 GPa. These high constraining-related values are finally validated by experimental assessment of stress levels at which strain bursts are identified in stress-strain curves obtained by uniaxial compression of micropillars.

High-temperature oxidation behaviour of WC-FeAl composite fabricated by spark plasma sintering

In this study, the WC-FeAl composite was synthesized by mechanical milling and spark plasma sintering. The isothermal oxidation behaviour of the WC-FeAl composite was evaluated and compared with a commercial WC-Co composite at the temperature range of 700–900 ℃. The results showed that the oxide layer formed in the WC-FeAl composite contained FeWO4, FeAlO3, and WO3 phases at all tested temperatures. The XRD patterns indicated that the intensity of FeWO4 and FeAlO3 phases enhanced with increasing the oxidation temperature. The formation of these two phases was related to the diffusion of Fe and Al elements from the binder phase to the oxide layers. As a result of the presence of FeWO4 and FeAlO3 phases, more compact oxide scales were formed, and the volume of voids and cracks was decreased as compared to the WC-Co composite. The plot of specific mass variations as a function of time illustrated that the mass gain for WC-FeAl composite was almost half of WC-Co composite.

Application of FIB in the preparation of TEM specimens of experimental supercoarse WC-Co(Ni) composites

Advances in TEM's performance as well as in materials being investigated require the relevant improvement in sample preparation technology. For a long period of time TEM investigations of metals have been successfully performed using the electrochemical jet polishing method for the sample preparation, i.e. thinning. This classical approach has proved to be unsatisfactory in the case of electrically not conductive and/or multiphase materials – for instance various kinds of ceramics and composites. A milestone in solving this problem has been an elaboration of ion milling. Overcoming certain disadvantages of this technology, mainly sample damage, as well as providing site specificity of the sampling procedure, has been achieved owing to the onset and development of the focused ion beam (FIB) method. The current paper presents the authors' own experience in the preparation of TEM specimens of conventional WC-Co type and experimental WC-Ni based supercoarse sintered carbides. These metal matrix composites consist of large (ca. 10 μm) tungsten carbide particles and a binding metal phase (cobalt or nickel based). Because of the pronounced microstructural, mechanical and electrochemical inhomogeneity, as well as the size of hard WC particles, traditional (mechanical, jet polishing, electrochemical) procedures proved to be totally inadequate. Therefore multi stage lift-out procedure employing a focused beam of gallium ions of various energy (in situ lift-out of the sample plus H-bar geometry final thinning) and argon ion polishing aimed at the removal of the damaged (defected and amorphous) layer has been elaborated and successfully applied. A large number of defects in the WC particles (dislocations, stacking faults) and within the matrix areas (dislocations, microtwins) as well as high quality of the WC-metal matrix interfaces were found.

Fabrication of WC-Co cemented carbides with gradient distribution of WC grain size and Co composition by lamination pressing and microwave sintering

WC-Co composite powders with different particle sizes and Co contents were prepared by ball milling WC and Co powder mixtures for different durations. Functionally graded WC-Co cemented carbides with both Co content and WC grain size gradient were prepared by lamination pressing different WC-xCo (x = 10, 15, 20) powder mixtures and microwave sintering the layered compacts. The WC-xCo powder mixtures were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), and the phase composition and microstructure of the functionally graded cemented carbides (FGCCs) were investigated by XRD, and FE-SEM coupled with energy dispersive spectroscopy (EDS). Mechanical behaviors of the layered WC-Co materials were measured and compared with those of WC-Co cemented carbides with single composition. The results showed that increasing the milling time from 6 to 24 h, results in the decrease of the particle size of WC-Co composite powders from 0.31 to 0.11 µm. After lamination pressing and microwave sintering, the WC-Co samples show nearly complete densification with a relative density higher than 99.7% and no ƞ-phase was detected in the FGCCs. The Co content and WC grain size in FGCCs decrease from the core to the surface. Homogenization of Co has hardly occurred and no cracks have formed between the layers in the sintered samples. In the inner layer, the mean WC grain size is 529 nm, while in the outer layer it is only 274 nm. Because of the difference in Co content and WC grain size, FGCCs have a Rockwell hardness of 90.75 HRA at the surface, which decreases to 86.75 HRA in the core. However the fracture toughness increases from 11.53 at the surface to 18.12 MPa m−1/2 in the core. The present results show that FGCCs with high outer layer hardness and high inner layer toughness were successfully prepared.

Multi-layer graphene reinforced nano-laminated WC-Co composites

The current study applies towards investigate the sintering behavior, mechanical properties as well as stability of predesigned Co gradient of nano-laminated multi-layer graphene (MLG) reinforced WC-Co composites, highlighting the critical roles of MLG as exceptional toughening phase and superior cobalt gradient stability enhancement phase. During liquid phase sintering, the stability of predesigned cobalt gradient can be significantly enhanced via controlling MLG content. The three crucial cobalt gradient stability enhancing mechanisms are MLG preferential orientation, MLG inducing the generation of the liquid cobalt phase and MLG wrapping matrix grains. MLG intrinsic toughening mechanism i.e. MLG bending together with extrinsic toughening mechanisms such as MLG wrapping, interface debonding, MLG wall and network, MLG induced weak interface, MLG pullout, crack deflection, crack bridging and crack stopping is the major toughening mechanisms. Besides, stabilization of the predesigned Co-gradient and grain reinforcement are also the important contribution to the improved hardness to toughness relationship.

Characterization and analysis of the aspect ratio of carbide grains in WC-Co composites.

To study the aspect ratio distribution of carbide grains in WC–Co composites, two novel approaches, namely grain shape ellipse calculation and five parameter analysis (FPA) methods, are reported in this work. According to grain shape ellipse calculation, carbide grains that have a grain shape aspect ratio of 0.62 demarcate the more anisotropic grains with a larger size and the less anisotropic grains with a smaller size. Moreover, these grains remained predominantly populous under different cases of structural parameters and processing factors. According to five parameter analysis, the interface area aspect ratio can be derived from the relative area of prevalent (0001) basal planes and (10−10) prismatic planes, and can then be set relevance with the measured mechanical properties. The present work therefore offers new alternatives to establish the connection between the aspect ratio measurement and the property optimization of WC–Co composites.

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.

Comparison of the wear behavior of WC/(FeAl-B) and WC-Co composites at high temperatures

In this research, the sliding wear behavior of the hot pressed WC/40vol%(FeAl-B) composites was investigated at temperatures ranging from the ambient one to those as high as 600°C. The composites were then compared with hot pressed WC-40vol%Co and commercial WC-16vol%Co (H10F) in terms of their mechanical properties and high temperature wear behavior. It was found that the WC/(FeAl-B) composite recorded its maximum wear resistance at all the experimental temperatures, which was higher than that of WC-40vol%Co at these same temperatures due to the higher hardness of the FeAl-B than that of the Co matrix. Also, WC/(FeAl-B) exhibited a higher wear resistance at lower temperatures and a more proper behavior at higher temperatures than did the commercial WC-16vol%Co; this was attributed to the higher strength of the FeAl-B matrix at high temperatures. Examination of the wear surfaces revealed that abrasion was the wear mechanism in the commercial WC-16vol%Co and WC/(FeAl-B) composites at both ambient temperature and 300°C. At 400°C, however, the wear mechanism was more of an adhesive one, while binder oxidation was observed at 600°C.

Deformation and plastic coordination in WC-Co composite — Molecular dynamics simulation of nanoindentation

The deformation characteristics and plasticity mechanisms of the WC-Co composite were demonstrated by the molecular dynamics simulations on the atomic scale. It was found that the nucleation, expansion and interaction of dislocations are distinctly dependent on the orientation where the load is applied. At the same indentation depth, the dislocation density in WC is lower by an order of magnitude than in Co. When the indentation is applied on the WC basal plane, the dislocations form firstly on the pyramidal planes and are inclined to slip on the prismatic planes. At the incoherent WC/Co interface in the WC-Co composite, the distortion occurs with loading. In contrast, at the semi-coherent WC/Co interface containing dislocation network, the stress concentration is coordinated, and the nucleation of dislocations in Co is triggered. This is favorable to enhance the resistance against intergranular fracture of the WC-Co composite.