WC-Co Hardmetals Research Articles

Thermal conductivity loss in WC-Co hard metal due to high-temperature cyclic loading damage as a function of microstructure and temperature

Many structures made of WC-Co hard metal are subject to thermo-mechanical loading and have to conduct heat in order to function properly in industrial application. The current work provides results on a significant drop in thermal conductivity of WC-Co hard metals as a function of material volume damage that accumulates during cyclic high-temperature loading of the materials depending on material microstructure. Average WC grain size and Co binder metal content of the investigated grades ranged from submicron to medium and from 10 to 12 wt%, respectively. The hard metals were subjected to uniaxial cyclic loading in a vacuum for different numbers of load cycles at 700 °C and 800 °C. Damage features accumulated in the material volume were documented by means of scanning electron microscopy. Thermal conductivity properties of virgin and damaged materials were determined via laser flash analysis. The results indicated a significant decrease depending on the materials' microstructure, i.e. the defects' predominant location within the microstructure. The damage features that occurred mainly between WC grains in the coarser-grained grade led to larger drops in thermal conductivity with rising temperature compared to damage features that occurred within the Co binder metal in the finer-grained grade. The presented results are of high relevance to the thermo-mechanical load situation of e.g. milling tools since the heat conduction away from their cutting edges is hindered by the documented effect and deemed to lead to a self-acceleration of the damage accumulation.

Printing Direction Effects on the Sliding Contact Response of a Binder Jetting 3D-Printed WC-Co Hardmetal

Printing Direction Effects on the Sliding Contact Response of a Binder Jetting 3D-Printed WC-Co Hardmetal

Comparative Study of Multilayer Hard Coatings Deposited on WC-Co Hardmetals

This paper examines the impact of a multilayered gradient coating, applied via plasma-activated chemical vapor deposition (PACVD), on the structural and mechanical attributes of nanostructured WC-Co cemented carbides. WC-Co samples containing 5 and 15 wt.% Co were synthesized through a hot isostatic pressing (HIP) process using nanoparticle powders and coated with two distinct multilayer coatings: titanium nitride (TiN) and titanium carbonitride (TiCN). Nanosized grain formation without microstructural defects of the substrates, prior to coating, was confirmed by magnetic saturation and coercivity testing, microstructural analysis, and field emission scanning electron microscope (FESEM). Nanoindentation, fracture toughness and hardness testing were conducted for uncoated samples. After coatings deposition, characterizations including microscopy, surface roughness determination, adhesion testing, coating thickness measurement, and microhardness examination were conducted. The impact of deposited coatings on wear resistance of produced hardmetals was analyzed via scratch test and dry sliding wear test. Samples with higher Co content exhibited improved adhesion, facilitating surface cleaning and activation before coating. TiN and TiCN coatings demonstrated similar roughness on substrates of identical composition, suggesting Co content’s minimal influence on layer growth. Results of the mechanical tests showed higher microhardness, higher elastic modulus, better adhesion, and overall superior tribological properties of the TiCN coating.

On the fcc to hcp transformation in a Co-Ru alloy: Variant selection and intervariant boundary character

Ru is a common addition to the Co-binder in WCCo hardmetals for advanced cutting tool applications. Internal Co-Co interfaces control many properties such as hot-hardness, toughness, and creep resistance. Hence, phase transformations determining the internal interface character can be harnessed to achieve superior properties. We investigate the γ (face-centred cubic) to α (hexagonally closed packed) martensitic phase transformation of a model Co-Ru binder alloy. We describe the crystallography of γ to α phase transformations and the resulting α/γ and α/α interface character distributions. The stabilisation of the γ-phase results in the formation of low energy (0 0 0 1) α/γ interphase boundary planes. Preferred formation of α/α intervariant and twin-related boundaries with symmetrical tilt configurations are also observed. We discuss how crystallographic constraints of the transformation promote the formation of grain boundary planes other than those of the lowest energy configurations.

Property correlations of WC-Co with modified binders

In the last few decades, there has been an upward trend in incorporating alloying additives into WC-Co hardmetals for property enhancement. Alloying elements such as Ru, Re and Mo have been used effectively to improve some functional properties of WC-Co. These elements dissolve in the cobalt binder of WC-Co during sintering to form solid solution phases with different properties from the cobalt binder. Property correlations of binder modified WC-Co compositions are rarely published. In this paper, correlations between properties and microstructure of binder modified WC-Co compositions were established. Observed property correlations were explained based on solid solution formation and strengthening.

Correlation of Different Cemented Carbide Starting Powders with the Resulting Properties of Components Manufactured via Binder Jetting

For several years, researchers have been exploring the use of the binder jetting powder-based additive manufacturing process to produce WC-Co hardmetals. Compared to other additive manufacturing processes, binder jetting has the potential for high-volume production. However, due to the powder-based approach, the resulting green bodies typically have low green density, limiting the achievable hardness and requiring higher Co content. Choosing the appropriate starting powder and post-processing can extend previous limitations and allow the selection of a suitable powder based on the application. This investigation focuses on exploring and evaluating the correlation between varying morphologies of WC-Co starting powders, their processability using the BJT method, and the resultant mechanical properties of sintered components.

Influence of Alternative Hard and Binder Phase Compositions in Hardmetals on Thermophysical and Mechanical Properties

Due to the classification of Co as a CMR (carcinogenic, mutagenic, toxic to reproduction) as well as the classification of both Co and WC as CRM (critical raw materials) more and more research is being carried out to investigate possible substitutes for WC-Co hardmetals. To directly compare their microstructure as well as mechanical and thermophysical properties, five very different hardmetals were investigated. For this purpose, the compositions WC-Co, WC-FeNiMn, WC-HEA, NbC-Co and HEC-Co were selected in order to investigate alternative binders for cobalt as well as different alternative hard phases for WC. The results of the hardness measurements showed that for the hardmetals with alternative binders (WC-FeNiMn and WC-HEA) hardness values of 1327 HV10 and 1299 HV10 comparable to WC-Co with 1323 HV10 can be achieved. When WC is replaced by HEC as the hard phase, a significantly higher hardness of 1543 HV10 can be obtained, demonstrating the great potential of high-entropy carbides. Furthermore, the hot hardness measurements between RT and 900 °C showed significantly higher values (up to approx. 290 HV10) for the WC-HEA and HEC-Co hardmetals compared to those of WC-Co. However, the fracture toughness of the alternative hardmetals was lower compared to that of conventional WC-Co hardmetals. In terms of thermophysical properties, the results of the hardmetals with alternative binders were close to those of WC-Co. Thus, it can be shown that it is possible to produce alternative hardmetals with comparable properties to WC-Co and that with further optimization they show great potential to replace WC-Co in the near future.

Microstructural analysis and ASTM B611-21 wear testing of novel binder hardmetals for future percussive drilling applications

Since their discovery exactly one hundred years ago, WC-Co hardmetals (cemented carbides) have been a well-established composite material. They are extensively used in multiple fields of industry such as cutting applications, drilling, mining, and milling, due to the combination of high hardness and toughness. In recent years the partial or complete substitution of Co as the metallic binder has been a central topic of hardmetal research for economical, legal, ethical, and health-related reasons. The presented work focuses on the complete substitution of Co as the binder element in hardmetals that in the future could be used in percussive drilling applications. Other suitable metallic elements have been used combination with Ni as the binder basis to strengthen the binder via the well-known principle of solid solution strengthening. Four alternative binder grades were selected, and various sintering experiments were conducted to determine the influence of maximum sintering temperature (Tmax), atmosphere, and annealing treatment. HV50, KIc via indentation fracture toughness, microstructure, magnetic saturation as well as differential thermal analysis (DTA) and dilatometry (DIL) were investigated. To simulate the high-stress and abrasive conditions in percussive drilling to a certain degree, wear tests in the form of a modified ASTM B611–21 test were conducted. The results were compared with industrially available reference grades and show that completely Co-free, Ni-based alloys can compete with Co when used as the binder matrix in hardmetals and upon further optimisation may very well be viable candidates for Co replacement.

Wire-EDM cutting strategies of WC-Co hardmetals: Effect of number of EDM pass on surface integrity

In this study, the effect of the number of wire-EDM passes on the surface integrity of WC-Co hardmetals was investigated. As-received doughnut-shaped WC-25 wt%Co samples were wire EDM’ed with one, two, three and five passes. The resulting surface conditions were investigated in terms of macroscopic and microscopic examinations. Surface roughness and hardness measurements were carried out. Detailed SEM investigations were coupled with EDS analyses. Based on the experimental findings, a detailed cutting strategy was offered for WC-Co hardmetals depending on the expectations of surface conditions, production route, cost and productivity. If there is at least one subsequent process including material removal, wire-EDM with one pass is suggested to decrease cost and increase productivity. However, heat-affected zone (HAZ), recast layer and lower surface hardness region should be removed at subsequent production steps. If there are no production steps to remove HAZ, recast layer and lower surface hardness region, the number of wire-EDM passes should be maximised.

A novel express method for determining WC grain sizes and its use for updating dependencies of coercivity and hardness on WC mean grain size in hardmetals

A new express method for measurements of WC grain sizes in WC-Co hardmetals was developed. This method is as precise as the EBSD grain size measurement method but much faster and thereby less expensive. The new measurement method is based on a peculiar procedure of hardmetal samples' preparation, recording microstructure images and their segmentation followed by determining WC mean grain sizes by use of a specially developed software; the measurement error is less than ±3%. Dependencies of coercivity and hardness on the WC mean grain size of WC-10 wt% Co hardmetal samples were established by the new express measurement method. With the aid of this method the dependencies reported in literature were updated as a result of more precisely determined values of the WC mean grain size compared to those established in literature by conventional methods based on SEM measurements.

Surface treatments and substrate-coating interface effects on damage accumulation kinetics in hard-coated WC-Co hardmetals under cyclic multi-axial high-temperature loading

Coated hardmetals are commonly used tool materials for metal cutting applications. For these applications, the substrate- and the coating surface qualities are improved by grinding, polishing, or blasting. The focus of the current work was to study the influence of different combinations of substrate-coating surface processing techniques on the induced damage in Al2O3/TiCN hard-coated WC-12 wt% hardmetal substrates and the depth to which the damage reaches into the substrate. The substrate and coating surfaces were prepared by grinding, polishing, dry blasting, wet blasting, or remaining in the deposited state. Investigations were performed using cyclic indentation with a novel ball-in-cone test method in a vacuum at 700 °C, under conditions that imitate the combined shear-compression loads that occur at the cutting edges of metal cutting tools. Additionally, finite element simulations were performed to characterize the stress state arising during ball-in-cone testing. The kinetics of defect formation and accumulation in the hardmetal substrate as a function of the loading situation during the ball-in-cone test will also be discussed. The defects that formed in the substrate during cyclic loading were studied using scanning electron microscopy in cross-sections prepared by focused ion beam milling. A larger increase in defect frequency was observed for rougher coating surfaces and substrate-coating interfaces than for smoother ones. Furthermore, 6 μm thick coatings exhibited a higher defect frequency after cyclic loading than 15 μm thick coatings. Most of the observed defects were in the nm-size range close to the substrate-coating interfaces and tended to have increased in size after cyclic loading. The ball-in-cone test set-up enables the study of the damage mechanisms in coated hardmetal substrates with different treatments prior to and after deposition under tunable milling-like stress and temperature conditions.

Faceting/Roughening of WC/Binder Interfaces in Cemented Carbides: A Review.

Hardmetals (or cemented carbides) were invented a hundred years ago and became one of the most important materials in engineering. The unique conjunction of fracture toughness, abrasion resistance and hardness makes WC-Co cemented carbides irreplaceable for numerous applications. As a rule, the WC crystallites in the sintered WC-Co hardmetals are perfectly faceted and possess a truncated trigonal prism shape. However, the so-called faceting-roughening phase transition can force the flat (faceted) surfaces or interfaces to become curved. In this review, we analyze how different factors can influence the (faceted) shape of WC crystallites in the cemented carbides. Among these factors are the modification of fabrication parameters of usual WC-Co cemented carbides; alloying of conventional cobalt binder using various metals; alloying of cobalt binder using nitrides, borides, carbides, silicides, oxides; and substitution of cobalt with other binders, including high entropy alloys (HEAs). The faceting-roughening phase transition of WC/binder interfaces and its influence on the properties of cemented carbides is also discussed. In particular, the increase in the hardness and fracture toughness of cemented carbides correlates with transition of WC crystallites from a faceted to a rounded shape.

Strength determination for rough substrate-coating interfaces with three-dimensional defect structure

Quantitative information on the strength of interfaces between thin ceramic coatings and their substrates is crucial when aiming to understand their failure behavior. Many technically relevant substrate-coating interfaces are not ideally planar, but rather rough and contain stress concentrators forming three-dimensional defect structures. An example for such a realistic case, a polycrystalline diamond coating that partially penetrates voids and cavities in its Co binder-depleted WC-Co hard metal substrate, was investigated within the current work with respect to its unknown interface strength behavior. Special focus was laid on the influence of the applied load direction on the observed fracture behavior. Micromechanical specimens were produced via focused ion beam milling as geometry variants of a micro shear compression specimen with their loaded areas' relative inclination towards the substrate-coating interface varied from 0° to 88°. Specimen loading was performed until fracture with a flat punch indenter in a scanning electron microscope. The recorded fracture loads were associated with the spatial stress distributions at fracture via finite element-based analysis. A plateau of the determined maximum principal stress triggering fracture in the ceramic-ceramic interfaces was found for inclination angles ≥45°. This plateau value was identified as the interface strength by observation of the crack path at the substrate-coating interface via scanning electron microscopy and analysis of the effectively loaded interface area values. The presented novel material testing technique gives first and previously not accessible insight into the fracture behavior of rough substrate-coating interfaces with complex defect structure.

Development of WC-NiCrMo hardmetals

The demand for improved components with a wide range of properties is a reality in today's hardmetal market. Furthermore, supply shortages of raw materials such as W and especially Co resulted in rapid price fluctuations. Combined with stricter environmental legislation and health protection, these facts prompt the demand for alternative or modified binders. Therefore, in this work, WC-NiCrMo composites were developed as a potential alternative to conventional WC-Co hardmetals. To evaluate this new hardmetal composite, prototypes of submicrometric WC with approximately 15 vol% of NiCrMo binder were produced by conventional powder metallurgy route. Thermodynamic assessment, wettability testing and constant heating rate dilatometry were performed to design an adequate sintering route. A detailed characterization of the mechanical properties (Young's modulus, Vickers hardness and fracture toughness), together with corrosion resistance assessment (OCP, polarization curves, EIS) and abrasive wear (ball-cratering) resistance evaluation were undertaken. Good wettability of molten NiCrMo binder on WC surface was observed, and highly dense compacts could be successfully attained by gas pressure sintering. The new WC-NiCrMo composite has lower hardness but higher corrosion resistance and better wear resistance than the conventional WC-Co hardmetals.

Approaching the 100th anniversary of the Hardmetal invention: From first WC-Co samples towards modern advanced Hardmetal grades

Hardmetals on the basis of WC are the best and unique materials for applications characterized by intensive wear and abrasion, high impact loads, thermal shocks, high bending and compressive loads, elevated temperatures and severe fatigue. Therefore, their discovery in the beginning of the 20's resulted in revolutionary changes in different branches of industry including metal-cutting, mining, construction, wiredrawing, etc. The history of the invention of WC-Co hardmetals in the beginning of the 20's of the last century is described. Brief information about the early production and, research and development of hardmetals is given. A range of modern conventional industrial hardmetals are briefly described. Uncommon advanced industrial hardmetal grades including functionally graded hardmetals, nanostructured hardmetals and hardmetals with alloyed binder phases are reported. Other types of uncommon hardmetals that are presently under development or fabricated on a limited scale are described.

High-temperature wettability in hard materials: Comparison of systems with different binder/carbide phases and evaluation of C addition

Metal-ceramic wettability is a decisive parameter in the high-temperature sintering of hard materials. Wettability tests enable the study of this property with minimum material waste, especially useful in the search of alternative systems to WC-Co hardmetals. In this investigation, Fe-based binders – FeNiCr and FeCrAl – were tested on Ti(C,N) and WC substrates, aiming to assess: the high-temperature interactions, the dissolutive character of the liquid phase and the nature of the interphases generated, and the influence on sintering behaviour. As a result, FeNiCr led to excellent wetting scenarios for both ceramics, whereas FeCrAl alloys induced the formation of aluminium oxides. The effect of C addition on wettability was also evaluated, resulting in an improvement of this property by the inclusion of this element in the binder phase. Inspection of the microstructures resultant from powder metallurgy processing of the different configurations confirmed their excellent correlation with wettability results. As a consequence, the effectivity of this technique as a model of the sintering scenario could be asserted.

Nanoscale patterning of metallic surfaces with laser patterned tools using a nanoimprinting approach

In this study, we demonstrate that metallic substrates with a nanocrystalline grain size can be structured down to the micro- and nanometre range during a room temperature nanoimprinting process. WC-Co hard metal dies are patterned by Direct Laser Interference Patterning (DLIP), generating an array of separated asperities with a height of 4.7 µm and a spacing of 11.3 µm. Additionally, Laser Induced Periodic Surface Structures (LIPSS) with a spacing of 500 nm are formed as a hierarchical substructure. The patterned dies can be used to deform metallic substrates due to their high hardness. For structure replication, the WC-Co tool is pressed onto a nanocrystalline CuZn30 model alloy at room temperature. The pattern transfer process from the hard metallic tool to the nanocrystalline alloy is discussed in detail: At low contact pressures, the asperities on the tools act as single contact points, leading to the formation of separated dimples with a depth of up to 0.8 µm in the nanocrystalline alloy. At high contact pressures, the tool pattern, including the LIPSS is almost completely transferred to the substrate. The formed asperities protrude out of the surface, with a height to width aspect ratio of 0.4. It is thereby demonstrated, that plastic flow processes during the imprinting of nanocrystalline alloys allow for the replication of the DLIP tool pattern, even down to nano-scaled LIPSS. The imprinted array of surface asperities exhibits hydrophobic properties, due to a combined topographical and chemical influence on the wetting behaviour.

On the transition of failure control from material-intrinsic defects to defects forming during monotonically increasing and cyclic mechanical loading in WC-Co hard metal at elevated temperature

WC-Co hard metals are composite materials with extraordinary mechanical properties especially at elevated temperatures which make them common materials for metalworking tools. The current work investigates damage and fracture behavior of hourglass-shaped specimens made of a WC-Co hard metal with submicron-sized WC grains and 12 wt.% Co binder. All investigated specimens were isothermally heated inductively to 700°C in a servo-hydraulic testing machine equipped with a vacuum chamber to avoid surface oxidation. The nature of origins of fracture and the evolution of bulk material damage features was studied via scanning electron microscopy for experiments under monotonically increasing load as well as cyclic loading under a stress ratio R = σmin / σmax = -1. A formation of small cavities that formed in the material bulk during loading was observed for all investigated loading conditions. For monotonically increasing load, the coalescence of these cavities was identified as the dominant damage mechanism. For cyclic loading, the number and size of the mentioned cavities did rise with the applied number of load cycles and rising stress amplitude. The coalescence of the cavities was found to be a main mechanism controlling the fatigue crack propagation process at elevated temperature. The found results imply that a certain combination of stress amplitude and temperature exists, at which a transition of failure control occurs from: (i) the size of material-inherent defects present prior to loading, to (ii) the kinetics of the coalescence of cavities that form due to plastic deformation and creep in the Co matrix during loading.

Influence of different binder metals in high entropy carbide based hardmetals

ABSTRACT High-entropy carbides (HEC) are a class of promising new hard phases for a sustainable improvement of hardmetal properties. In this work, hardmetals of the HEC (Ta,Nb,Ti,V,W)C were studied with two typical binder volume fractions of 16 and 24 vol-% consisting of Co, Ni and FeNi. The sintering behaviour, microstructure, phase composition, magnetic and mechanical properties are discussed and are compared to a conventional WC-Co hardmetal. It was shown that the HEC has a high-phase stability and that dense hardmetals with promising mechanical properties were obtained.

Defect initiation and accumulation kinetics in hard-coated WC-Co hardmetal under multi-axial loads at elevated temperature in a novel ball-in-cone test setup

In the metalworking industry, hard-coated WC-Co hardmetal tools are often used in machining applications. Generally, tool life is limited by pre-existing defects in the hardmetal substrate and defects that form during tool application. The complex, multi-axial loading situations and the high temperatures present in the cutting-edge area of machining tools are among the main reasons for the formation and growth of defects in operation. At present, there is a lack in experimental setups that can replicate these conditions. In the current work a novel material testing method is presented, which uses a spherical indenter and inclined specimen surfaces to apply multi-axial loads at 700 °C. The local stress state in the specimen was calculated using finite element simulation implementing an experimentally parameterized material model considering ratchetting and creep of the substrate material. Stresses ranging from mainly compressive to tensile-compressive were predicted. Initiation and accumulation kinetics of defects in the nm- to μm size regime were studied quantitatively. The comparison of stress calculations and damage development shows that positions with tensile-compressive stresses exhibited significantly higher defect formation rates than those with mainly compressive stresses.