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.     

Wear damage of cemented carbides with different combinations of WC mean grain size and Co content. Part I: ASTM wear tests

In the present work we made and examined cemented carbides characterized by very different WC grain sizes varying from near-nano with a WC mean grain size of about 200 nm to coarse-grain with a WC mean grain size of about 4.5 μm and Co contents varying from 3 to 24 wt.%. The major objective of the present work was to examine the wear damage, wear behavior and wear mechanisms of cemented carbides having nearly the same hardness but greatly varying with respect to their WC grain size and Co content in the high-load ASTM B611 test and low-load G65 test.Both the hardness and resistance to fracture and micro-fatigue of cemented carbides play an important role in the wear damage by use of the high-stress ASTM B611 test when the carbide surface is subjected to alumina particles at high loads. In this case, the wear-resistance increases with increasing the WC mean grain size and decreasing the Co content at nearly the same hardness of the different cemented carbides. The submicron and near-nano cemented carbides are characterized by lower wear-resistance in comparison with the coarse-grain grade due to their reduced fracture toughness, fracture resistance and resistance to micro-fatigue.The Co mean free path in the carbide microstructure plays an important role with respect to wear-resistance in the low-stress ASTM G65 test when the carbide surface is subjected to gentle scratching by abrasive silica particles. The predominant wear of the thick Co interlayers leaving unsupported WC grains plays the decisive role in the wear behavior of the coarse-grain grade resulting in its low wear-resistance. In contrast to the ASTM B611 test the wear rate decreases with decreasing the WC mean grain size and increasing the Co content due to the corresponding reduction of Co mean free path in the carbide microstructure. As a result, the wear-resistance of the near-nano grade in the ASTM G65 test is the best of all in spite of its reduced fracture toughness.Phenomena of micro-fatigue, micro-fracturing and micro-chipping are found to play a decisive role in the wear damage of cemented carbides if they are subjected to abrasion wear, high loads and severe fatigue.     

Characterisation of wear properties of ultrafine-grained hardmetals using a special abrasive wheel test

Three-body abrasive wear tests were carried out on super ultrafine-grained hardmetals (WC intercept length about 0.2 μm) with different Fe, Ni, Co binder systems, on hardmetals with Co binder and on standard materials with coarser WC grains. Apart from the standard materials, the hardmetals were all produced in laboratory scale from commercial nanocrystalline WC powders from different companies. The practical application of these hardmetals is in the field of cutting dry wood. The main purposes of this work were to characterise the wear properties of the produced grades and to derive predictions from laboratory wear tests regarding practical applications. Therefore a new wear test apparatus was built. The wear results showed a logarithmic correlation with hardness and a threshold to the low wear region, which was exceeded by some grades, at binder mean free paths of less than 40 nm. Moreover the laboratory wear results correlate well with results from field testing. A tribofilm was found on the worn surface and characterised by scanning electron microscope (SEM), AFM and TEM. In an analysis of the wear mechanisms the role of this tribofilm must be considered.     

In-situ observation of hardmetal deformation processes by transmission electron microscopy. Part I: Deformation caused by bending loads

WC-based hardmetals have a unique combination of different properties including high hardness, fracture toughness, abrasion resistance, strength, fatigue resistance, etc. Such properties are achieved due to extraordinary features of tungsten carbide, which is characterized by a certain degree of plastic deformation before failure when loading. The major objective of this work is to examine hardmetal deformation processes leading to the formation and movement of crystallographic defects in a hardmetal lamella as a result of its in-situ bending directly in a transmission electron microscope. The deformation is found to result in the formation of different crystal lattice defects in the Co-based binder and WC grains. Mainly dislocations form in the tungsten carbide grains. The dislocations start moving when increasing the applied bending load. The deformation of the binder phase is found to result in the formation of mainly lamellae and stacking faults in the Co crystal lattice. As a result of the formation, interaction and movement of the crystal lattice defects in the WC phase and binder phase a significant rate of plastic deformation of the hardmetal lamella was achieved under bending loads without its breakage.     

Influence of microstructure on hardness and thermal conductivity of hardmetals

Hardmetals or cemented carbides are used in a wide range of applications due to their excellent mechanical properties. WC-Co hardmetals with the same room temperature hardness can be obtained by different combinations of the WC grain size and cobalt content. However, the thermal conductivity of such hardmetal grades is not equal. Applications such as cutting may require a certain combination of hardness and thermal conductivity, which means that a targeted adjustment is desirable. In this study a wide range of hardmetal grades was studied in respect of microstructure, hardness and thermal conductivity in the temperature range between 20 °C and 1000 °C. Results show that thermal conductivity is considerably influenced by Co content, WC grain size and Cr₃C₂ content. Furthermore, hardmetal grades with the same hardness at room temperature retain hardness very differently at elevated temperatures. For the selection of hardmetal grades for high temperature applications these findings help to choose the right composition in regard to Co content and WC grain size.     

纳米晶WC-Co硬质合金中WC晶粒度的评价方法

在改进的传统金相样品制备和Murakami侵蚀剂化学腐蚀条件下,场发射扫描电镜在10万倍放大倍率下可获得衬度良好、WC晶粒轮廓清晰的二次电子图像。用截线法定量测定了纳米晶WC-Co硬质合金中WC晶粒度,结果表明:在适当的工艺条件下,添加微量新型VC基复合晶粒生长抑制剂可获得纳米晶硬质合金:WC平均晶粒尺寸50nm,标准偏差14nm,最大晶粒尺寸110nm。     

Analytical characterization of the corrosion mechanisms of WC-Co by electrochemical methods and inductively coupled plasma mass spectroscopy

WC-Co hardmetals are used for their combined high hardness and toughness. However, their poor corrosion resistance in aqueous solutions reduces the spectrum of their application. The goal of this work was a systematic investigation of the corrosion mechanisms of the WC-Co composite with electrochemical methods and analytical chemistry solution analysis characterization. WC-Co, Co and WC samples were investigated by electrochemical impedance spectroscopy, potentiodynamic polarization and inductively coupled plasma mass spectroscopy (ICP-MS). Concerning the corrosion susceptibility, the solution pH dominates the effect of specific ions. In neutral and acidic solution, the corrosion process of WC-Co consists mainly of Co dissolution. WC dissolution becomes more significant at alkaline pH. Degradation is mainly the result of selective uniform dissolution of the phases (Co or WC) not of localized corrosion because of the poor passivating ability of Co (except in alkaline pH). Synergistic effects due to galvanic coupling between the Co binder and WC are accelerating Co dissolution and hindering WC dissolution in the hardmetals compared to the pure compounds. This although the Co binder phase contains W and C, making it more corrosion resistant than pure Co. A further influence of the locally separated anodic and cathodic reactions is that the cathodic reduction on WC induces local pH increase which causes chemical dissolution of WC detected only by ICP-MS.     

Nitridation sintering of WC–Ti(C,N)–(Ta,Nb)C–Co hardmetals

Gradient sintering of WC–Ti(C,N)–(Ta,Nb)C–Co hardmetals in vacuum and nitrogen atmosphere was investigated via interrupted sintering experiments and evolving gas analysis. Reduction of surface oxides with corresponding gas evolution was followed by spreading of the binder, shown with coercivity measurements. With low carbon balance, η phases were formed during the intermediate stages of sintering. By introducing nitrogen gas at a later stage all of the η phases could be decomposed again and a graded near-surface microstructure could be formed. If nitrogen is introduced at an early stage when porosity is still open, the TiC powder particles are nitrided, leading to a considerable uptake of nitrogen. Carbon released by this reaction must be balanced by addition of W metal. W and Co binder promote the reaction of TiC with N, whereas TaC and NbC do not. The concept of reaction sintering hardmetal by bulk nitridation was applied on two hardmetal grades and was found an effective way of introducing the nitrogen into such hardmetals.     

Microstructure and mechanical properties of ultrafine-grained hardmetals

The interest in ultrafine-grained hardmetals as woodcutting tool materials derives from their excellent mechanical properties compared with those of conventional hardmetals. The aim of this work was to determine the mechanical properties of ultrafine-grained hardmetals and to correlate the measured effects with microstructural parameters. The ultrafine-grained hardmetals (WC grain size 0.3 μm) investigated consisted of different WC powders and different binder systems: Co and complex binder systems. The mechanical properties of ultrafine-grained hardmetals were tested under two different loading conditions: monotonically increasing and cyclic alternating bending loads. It could be shown that the binder systems of different compositions show different behaviours under cyclic loads. Ultrafine-grained hardmetals with Co binder exhibit high bending strength values, but high fatigue sensitivity. Ultrafine-grained hardmetals with complex binders show lower bending strength values but their sensitivity to fatigue is lower. This implies that different damaging mechanisms exist for ultrafine-grained hardmetals with Co and complex binders.     

Wear resistance of nanostructured Cr-based WC hardmetals sintered by spark plasma sintering

Nanostructured Cr-based WC hardmetals are successfully sintered by spark plasma sintering. The wear behaviour of these Cr-based WC hardmetals with different C contents ranging from 5.57 wt% to 6.91 wt%, is evaluated performing sliding wear tests under two different wear conditions. This work analyses the influence of the C content on the wear performance through the study of the phase formation and WC grain size. The Cr-based hardmetal with 5.57 wt% C content exhibits a lower wear rate than Co-based WC hardmetals tested under similar dry ball-on-plate wear conditions, even considering that these Co-based WC hardmetals have higher WC content (90 wt%) than Cr-based WC hardmetals (83.2 wt%). The combination of a nanosized WC grain and the avoidance of brittle (Cr,Fe)₇C₃ or soft graphite phases leads to a superior wear performance. Thus, the use of Cr-based binders in the hardmetal industry, alternatively to Co-based binders, is promising in applications in which high wear resistance is needed.     

纳米材料在硬质合金中的应用

WC晶粒不断细化是硬质合金发展的一个重要特征。从硬质合金的纳米原料、纳米硬质合金、纳米材料助长或增强超粗晶硬质合金以及硬质合金的纳米涂层材料等4个方面论述了纳米材料在硬质合金中的应用,着重报道了中国在这些方面的优势。纳米粒径原料的制备是首要难题,1997年发明的“紫钨原位还原”技术利用传统工艺制备纳米、超细碳化钨粉末,碳化钨粉的粒径可小于20 nm。纳米硬质合金技术利用低压热等静压或热等静压,克服了烧结过程中 WC异常长大的难题,制备100～200 nm纳米硬质合金,抗弯强度在5000 MPa以上,使用性能优于亚微或超细晶硬质合金,已用于生产。利用“纳米颗粒溶解法”制备的超粗晶硬质合金晶粒度可达12μm;而含有纳米Co2 W4 C增强相的超粗晶硬质合金产品,使用寿命比普通合金产品提高了2～3倍。涂层材料纳米化,是硬质合金工具的一个发展方向,在耐磨性、硬度和抗裂纹扩展方面有明显优势,加工工件表面质量更好,工具使用寿命更长。     

In-situ observation of hardmetal deformation processes by transmission electron microscopy. Part II: Deformation caused by tensile loads

In the 1st part of this article, hardmetal deformation processes caused by bending loads were examined in-situ by transmission electron microscopy. The major objective of this work is to examine hardmetal deformation processes in special thin hardmetal samples as a result of applying tensile loads in-situ directly in a transmission electron microscope with the aid of the push-to-pull method. Applying tensile loads to the samples results in the plastic deformation of the Co-based binder phase leading to the formation of different crystal lattice defects in the binder. Force-time and displacement-time curves recorded when loading the samples and maintaining the loads provide evidence for continuous processes of the formation and movement of crystal lattice defects, presumably dislocations, in the WC phase and Co-based binder leading to a high rate of the binder plastic deformation. After increasing the tensile loads up to a certain level leading to the severe plastic deformation of the binder phase, the samples suddenly fail as a result of the crack initiation and propagation at WC-Co interfaces. Presence of cobalt on the WC surface after the cracking suggests that the cracks propagate through the binder region adjacent to the interface rather than through the interface itself.     

Diffusion parameters of grain-growth inhibitors in WC based hardmetals with Co,Fe/Ni and Fe/Co/Ni binder alloys

The diffusion behaviour of the grain-growth inhibitors (GGI) Cr and V during early sintering stages from 950 to 1150 °C was investigated by means of diffusion couples of the type WC-GGI-binder/WC-binder. Besides Co, also alternative Fe/Ni and Fe/Co/Ni binder alloys were investigated. It was found that the diffusion in green bodies differs significantly from sintered hardmetals. Diffusivities in the binder phase were determined from diffusion couples prepared from model alloys and were found to be almost equal for Co and alternative binder alloys. The diffusion parameters determined from green bodies allowed to estimate the GGI distribution in a hardmetal during heat up. This was subsequently used to estimate an appropriate grain size of VC and Cr₃C₂ in hardmetals, which is required to ensure a sufficient GGI distribution during sintering before WC grain-growth initiates.     

Micro-abrasion investigations of conventional and experimental supercoarse WC-(Ni,Co, Mo) composites

Typically the wear resistance of hardmetals is assessed employing ASTM G65 and ASTM B611 tests. Both these methods employs abrasives of grain size larger than 200 μm. In numerous applications (e.g. effluent pumps) sintered carbides are exposed to wear by fine hard particles and micro-abrasion test is an appropriate method for materials selection and assessment. The relevant standard (BS EN 1071–6) recommends 4.5 μm SiC abrasives. It was found that in the case of supercoarse sintered carbides (6–10 μm WC grain size) this micro-scale test produces ill-defined craters difficult to measure accurately. Therefore, through an optimization procedure, the threshold grain size of hardmetals to be tested with 4.5 μm SiC as well as the abrasive medium for micro-abrasion testing of coarse and supercoarse grades were established. Diamond slurry (1 μm average grain size) produces well-defined craters through the grooving mechanism. Using these optimised abrasives, conventional WC-Co and WC-(Ni-Mo, Ni-Co-Mo) supercoarse hardmetals were tested. The supercoarse grades, regardless of the type of metal matrix, proved to be more wear resistant than finer grained sinters. Larger WC grains, within the supercoarse group, are also conducive to lower abrasion rate.     

A comprehensive review on synergy effect between corrosion and wear of cemented tungsten carbide tool bits: A mechanistic approach

Cemented tungsten carbide tool bits are widely used in cutting, oil and gas, tunneling, and mining industries there these materials exposed to extremely harsh environments that cause early stages of failure. Tungsten carbide hardmetals are suffering from tribocorrosion, which is a material degradation process that occurs due to the combined action of wear and corrosion. On the one hand, tungsten carbide materials composed of hard ceramics (WC grains) phase in a soft metal matrix binder (Co, Fe, and Ni) phase; hence, it leads to different tribological mechanisms. On the other hand, the different electrode potential of hard WC grains (act as a cathode) and soft binder phase (act as an anode) leads to the formation of the micro galvanic couples between these phases in many aqueous environments causing corrosion. Since tribocorrosion mechanisms are significantly influenced by the varying composition and microstructure of tungsten carbide hardmetals, the effect of size and morphology of the WC grains on tribocorrosion has also been reviewed. This review particularly highlighting the various tribological and electrochemical aspects and their possible degradation mechanisms that are generally encountered by the drill bits during operating conditions. Hence, the understanding of these mechanisms is, therefore, very essential for the selection, improvement, and development of high-performance hardmetals.     

Model single point abrasion experiments on WC/Co hardmetals

WC based hardmetals are widely used in applications where abrasion resistance is important.This paper describes the utility of scratch tests as model single point abrasion tests to evaluate the response of WC/Co hardmetals to the type of conditions that will occur in abrasion. The different types of experiments were single and multiple pass tests, and multiple pass tests in corrosive media.The resultant scratches were observed by stepwise SEM imaging, stereographic image correlation and 3D reconstruction, and EBSD analysis.It was found that many of the features observed in the model tests paralleled those seen in other abrasive tests on WC/Co materials such as build up of plastic damage in WC grains, fracture and fragmentation of WC grains, deformation of the Co binder phase and re-embedment of WC grains in the surface layers of the material. For tests in the presence of corrosive media, the binder phase was removed early in the sequence of scratches, leading to breakdown of the structure of the material.Stereo reconstruction was shown to be a valuable way of visualising and measuring the physical dimensions of scratches, providing a future route for the quantification of damage in these model experiments.     

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.     

Novel hardmetals with nano-grain reinforced binder for hard-facings

Ordinary hard-facings obtained by plasma spraying or thermal spraying are characterized by a combination of hardness, wear-resistance and fracture toughness inferior to that of conventional WC-Co hardmetals. Therefore, in the majority of applications they are incomparable with WC-Co materials and cannot substitute them. A fundamentally new approach to the fabrication of novel WC-based hardmetals in form of hard-facing obtained by PTA (plasma transferred arc) welding was elaborated. The microstructure of the novel hardmetals of the W-C-Fe-Cr-Si system depends on the plasma welding parameters and consists of carbide grains embedded in a Fe-based binder. The binder comprises hard nano-precipitates in form of nanoparticles of the η-type phases and mixed Cr-Fe carbides embedded in a Fe-based binder matrix. The wear-resistance of the novel hardmetals obtained in form of hard-facings is comparable with that of conventional WC-Co hardmetals fabricated by the powder metallurgy technology.     

WC-base cemented carbides with partial and total substitution of Co as binder: Evaluation of mechanical response by means of uniaxial compression of micropillars

The influence of the chemical nature of the metallic binder on the plastic deformation of cemented carbides was studied. Three different cemented carbide grades - WC-Co, WC-CoNi and WC-NiMo - with similar microstructural characteristics (binder content and carbide grain size) were investigated. Mechanical response was evaluated by means of uniaxial compression of micropillars, and tests were carried out in-situ in a FESEM with a nanoindenter equipped with a flat-diamond punch. After uniaxial compression, inspection of deformation phenomena was done at both surface and bulk of micropillars through scanning and transmission electron microscopy, respectively. It is found that yielding phenomena and strain hardening increase as Co is totally substituted by a NiMo alloy, while contrary effect results from partial replacement of Co with Ni. Relative differences are directly linked to intrinsic ductility of the metallic phase and operative plastic deformation mechanisms. Moreover, for the three materials studied, stress-strain responses show pronounced yielding events related to glide at WC/WC interfaces. Although they are discerned at different stress levels, estimated values of sliding resistance of WC/WC boundaries are found to be alike for the three grades studied.