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Recent progress in development of high-performance tungsten carbide-based composites
Abstract
Tungsten carbide has kindled a spark of interest because of its versatile properties for manufacturing of cutting tools. It has a broad range of applications for making the balls of ball point pen, bearings, nozzles, jewelry, ammunition, cutting and drilling tools, and electronic and mining equipment. Due to its high hardness, more than half of the WC produced annually is used for manufacturing of cutting tools. Not only the products made of this material have high hardness but also possess the advantages of high speed, wear resistance, and moderate fracture toughness. Although WC has very high hardness without binders, at room temperature, it is brittle and exhibits little or no plastic deformation. This limitation leads to the initiation of cracks and sometimes total failure. To overcome this and for the endurance of external forces, improved hardness at elevated temperatures, better fracture toughness, hindering WC tools abrasions, adhesion, oxidation, and corrosion, it is combined with some cementing material to hold its grains like cements hold the gravels in concrete. These cementing materials give strength. Thus various WC-based composites have been developed. A comprehensive review of WC-based composites has been made here, including the categories of composites, the strategy to enhance their properties, and their typical applications, besides the well-known cutting tools. The relationship between the microstructure and the properties are discussed in detail, which shows that supreme-quality WC-based composites could be achieved for advanced applications in the future. However, numerous research areas, such as densification of supercoarse WC-based composites, even distribution of WC particles, and its surface modification are still in progress.
... • Cobalt (Co): cobalt is the most important and widely used metal in tungsten carbide composites due to its suitable melting temperature and proper wettability. The use of cobalt results not only in excellent wettability and adhesion to tungsten carbide, but also in increased strength at room [73,74] and elevated temperatures [75,76]. Moreover, according to Basu and Sarin [15], to a certain extent, as the weight percentage of cobalt binder increases, the oxidation resistance of tungsten carbide improves. ...
... The previous investigations showed that nickel can be a proper alternative to cobalt in the production of tungsten carbide cermets in terms of cost and availability [7]. According to Zhang et al. [73], using nickel results in creating a strong bond between tungsten carbide powder particles, proper resistance to oxidation at high temperatures, and resistance to wear in crucial condition. Therefore, in the oxidation research field, some articles worked on Ni-binder-containing cermets [13,16]. ...
... In this regard, increasing the amount of binder can reduce the hardness and increase the fracture toughness. In addition, WC-FeAl cermet has better behavior with respect to carbon trioxide, with a 30% longer lifetime than WC-Co [73]. The lower cost of FeAl than Ni 3 Al, low density, high melting point, high thermal conductivity, and protective layer of alumina against high-temperature oxidation have increased the use of this type of binder [6]. ...
This study presents a conceptual classification scheme to review the literature on improving the oxidation resistance of tungsten carbide by modifying the binder. The first parts of the article are dedicated to the specification of the databases, the search method, and the description of the criteria chosen to classify the articles. Then, the data collected are presented in statistical graphs according to the proposed classification scheme. The data analyzed show that most of the significant improvements in oxidation resistance are achieved with advanced production processes, especially HIP and SPS, which eliminate porosity to a very high degree. In addition, statistical studies showed that the use of new replacement binders, Ni3Al, Fe–based alloys, FeAl, and Al2O3, improved the oxidation properties in 75–100% of cases. Meanwhile, the use of high–entropy alloys (HEAs) as cermet binders may be the subject of future research for oxidation, given the recently published results of good mechanical properties.
... WC-based alloys including ones doped with Co are the most abundant materials for cutting, pressing, drilling tools and etc. [1]. The common problem of these alloys is their high cost caused by expenses on preparation of the original WC and Co powders. ...
... Highly expensive conventional technologies of WC powder production and low rates of solid-phase reaction are the main reasons for that. WC powder is obtained by a number of methods [1,2], e.g., by tungsten and carbon interaction in hydrogen, methane, and in vacuum at temperatures of 1400e2000 C or by heating tungsten-carbon mixture in the carbon tube at 2800 C in microwave oven and etc. Solid-phase interaction in the reaction mixture is promoted usually by mechanochemical activation using high-temperature mechanochemical synthesis (HTMCS). Additionally, disperse powders of required compositions are obtained with the mechanical stimulation of thermal explosion being performed [3e5]. ...
... Good wetting ability of Ni and Fe within the WC powder provide efficient liquid-phase sintering at rather low temperature yielding in the dense low-porous alloy [15] that is characterized by nearly the same hardness as WC-Co system [16]. Also, WC-Ni-Fe constituents are mutually inert with no chemical interaction involved during sintering that allows preserving the shape and high viscosity of the alloy [1,16,17]. The most prospective are WC-Ni-Fe alloys with high content of alloying additive (to 20 wt%) that are widely used in production of drilling tool [18]. ...
The paper presents fabrication of WC-Ni-Fe hard metal alloy with the density up to 99.9% from theoretical, hardness ∼1303 HV, fracture toughness ∼12.4 MPa m0.5, compressive strength ∼1365 MPa and without open porosity. High quality of the materials is provided by the original synthesis route based on powder preparation and sintering. WC powder (particle size 0.1–14 μm) was obtained via mechanochemical synthesis using available precursors and polymethyl methacrylate to enhance the grinding. Rapid SPS consolidation was performed at 1200 °C and below achieving high density with the holding time being less than 18 min. Increased amount of Ni–Fe (8 wt% of each component) was used to enhance SPS heating due to deep wetting of the particles and, therefore, improved conductivity of the whole system. Earlier unknown data on densification dynamics and phase composition are presented for WC-8Ni-8Fe alloy under SPS sintering in the temperature range 1100–1200 °C. The results are proved by the means of XRD, SEM and EDX.
... A multitude of processes is utilized to make SiC ceramics, depending on the production cost, size, and shape. Hot pressing, Chemical Vapor Infiltration (CVI), Chemical Liquid-Vapor Deposition (CLVD), Liquid Silicon Infiltration (LSI), and Polymer Infiltration and Pyrolysis (PIP) are examples of these techniques [5][6][7]. ...
This review paper aims to look at silicon-based ceramic matrix composites and infiltration-based approaches for them. There are many different types of infiltration-based manufacturing processes, each with its own set of features. The best technique is chosen depending on the needs and desired attributes. With these considerations in mind, any type of infiltration might be selected to meet the requirements. Silicon-based ceramics has been highly used in the fields of aerospace, medical, automobile, electronics, and other various industries so it is important to study about their applications as well. This review outlines the evolution of composites from early 7000 BCE to composites today and discussed about various infiltration techniques for manufacturing silicon based ceramic matrix composites. This article also gives the comprehensive review of general characteristics and mechanical properties of silicon-based composites used in a variety of engineering sectors. The application section entails the wide range of engineering fields with consideration of infiltration techniques, which would be helpful for researchers to study and correlate the different infiltration techniques for end applications.
... Traditional routes for obtaining WC and TiC powders are characterized by rigid technological conditions and a low rate of solid-phase reaction [21,22]. The speed of solid-phase interaction of reaction mixture components is often increased through mechanochemical activation using high-temperature mechanochemical synthesis (HMS) [23][24][25]. ...
This paper presents a method for producing a hard alloy WC-5TiC-10Co with a density equal to 100% of the theoretical value, a hardness of 1484 HV, and a bending strength limit of 1924 MPa. The high quality of the product is ensured by original research method based on the mechanochemical synthesis of WC and TiC powders using available precursors. and the implementation of high-speed SPS consolidation of the obtained powder while achieving a high degree of packaging with a minimum grain growth at 1200 °C and processing time no more than 13 min. Experimental data on seal dynamics, phase composition, morphology and mechanical properties of WC-5TiC-10Co using SPS in the range 1000 to1200 °C are presented.
... In particular, it has excellent wear resistance and is often used in some areas with high wear resistance [9,10]. However, WC-based cemented carbides have low toughness and sinterability [11]. This is because the WC material has a hexagonal crystal structure, and carbon atoms exist in the gap of the tungsten metal lattice to form gap solid solution. ...
High-density WC–Ni composite ceramics were prepared by cold isostatic pressing–vacuum pressureless sintering–hot isostatic pressing with tungsten carbide (WC) powder and NiCl2·6H2O as a binder. Results show that with an increase in the contents of Ni in the metal binder phase, the relative density of WC–Ni composite ceramics is improved, and the formation of the carbon-deficient W2C phase is reduced. There is no W2C generated in the WC–1 wt% Ni material. At high temperatures, the Ni phase changes into the liquid phase and enters between the WC particles, thereby promoting the close alignment of the WC particles. Moreover, the WC particles will be more closely aligned under their own surface tension and capillary action, thereby promoting the densification of WC–Ni composite ceramics. The WC–0.5 wt% Ni composite ceramics are fully dense and show the best comprehensive performance with a microhardness of 23.0 GPa, a fracture toughness of 5.28 MPa m1/2, and a flexural strength of 1,396.58 MPa. WC–Ni composite ceramics are mainly composed of elongated triangular prism WC particles and Ni phase. Transgranular fracture was the main fracture mode of WC–Ni multiphase ceramic materials with a small amount of intergranular fracture due to the existence of the Ni phase. Such a fracture mode can increase the flexural strength of the composite material.
In this work, theoretically dense (> 99%) composites of Ag and WC have been prepared by press–sintering–infiltration for making electrical contacts used as an arc-resistant material in a model switching device. Composites with varying silver content and WC particle size were investigated to get an insight on their electrical contact resistance (Rc) and their ability to withstand enormous thermal stresses during switching. A break-only model switching sequence was used, where the evolution of Rc was measured over 50 cycles and the post-switching microstructures were investigated for thermal stress induced crack formation. A well-established 2D computational microstructure based model, object-oriented finite element analysis version 2 (OOF2), was used to determine the composite thermal conductivity (k) for various grades as a function of temperature. Rc was observed to be consistently low for the coarser WC containing composite and higher silver content composites. This response was attributed to the ductility of the surface layers formed during switching. Crack formation after switching was found to be a direct consequence of large thermal gradients during 50 cycles, which was minimal for coarser WC grained and higher silver content composites which have a higher thermal shock resistance.
This study examines the performance of a new class of wear-resistant but economical cutting tools produced by varying the binder composition of standard cemented carbide composites. By replacing some or all of the cobalt binder with rhenium and nickel-based superalloy, a stronger composite tool results, potentially capable of machining heat-resistant superalloys at significantly higher cutting speeds. Sample tools with alternative binder were produced and compared to standard tools bound with cobalt only. Turning experiments on Inconel 718 were run to evaluate wear resistance and tool life for several grades. The experimentation also examined the effects of varying the relative proportions of each binder constituent as well as the overall binder percentage in the composite. Results show a clear advantage of the alternative binder tools as evidenced by a 150% increase in tool life or the equivalent of an 18% increase in cutting speed. Although increasing amounts of rhenium in the binder show a positive effect on performance, the effects of superalloy and overall binder % are inconclusive.
The present study investigates the efficiency of spark plasma sintering (SPS) technique on the tribological behavior of tungsten carbide composites, and aims to develop the wear resistance of these composites by addition of cubic boron nitride (cBN). Wear tests of spark plasma sintered 6(wt.%)Co/WC, 25(vol%)cBN/6Co/WC and conventionally fabricated 6(wt.%)Co/WC as a reference sample were performed by ball-on-disk contact with dry and rotational sliding at room temperature in order to determine the friction coefficient and wear rate. Wear mechanisms were explained by using SEM observations and the wear rates were computed by a surface profilometer. In all cases, the major wear mechanisms were observed gradually in the form of microcraking, material removal by grain pull out, and generation and spalling of a tribochemical layer. Based on the experimental results, the addition of cBN considerably enhanced the wear resistance of tungsten carbides. In addition, these results revealed that SPS process has outstanding potential for the fabrication of tungsten carbides with high wear properties for tribological applications.
A combined experimental/numerical methodology is developed to fully consolidate pure ultrafine WC powder under a current-control mode. Three applied currents, 1900, 2100 and 2700 A, and a constant pressure of 20 MPa were employed as process conditions. The developed spark plasma sintering (SPS) finite-element model includes a moving-mesh technique to account for the contact resistance change due to sintering shrinkage and punch sliding. The effects of the heating rate on the microstructure and hardness were investigated in detail along the sample radius from both experimental and modeling points of view. The maximum hardness (2700 HV10) was achieved for a current of 1900 A at the core sample, while the maximum densification was achieved for 2100 and 2700 A. A direct relationship between the compact microstructure and both the sintering temperature and the heating rate was established.
This paper analyses the changes induced by HIP after sintering on the microstructure and mechanical properties of WC–7wt.%Co ultrafine hardmetal grade. The well known correlation between porosity reduction and fracture strength improvement is confirmed in these compositions, this being more effective at HIP temperatures above the eutectic point of the alloy. In absence of GGI, hardness decreases continuously as the HIP temperature increases. However, for specimens containing VC and Cr3C2 additions, hardness increases as the HIP temperature increases from 1200°C to 1400°C. This anomalous trend, confirmed by WC grain size observations, could be related to the activation of coalescence mechanisms during solid state HIPing, which are inhibited by the presence of a liquid phase.
WC-25vol%FeAl compacts have been prepared using wet milling and the subsequent pulse current sintering. Powder mixtures consisting of (1) WC, Fe-40at%Al and (2) WC, Fe, Al powders have been mixed to give the desired composition of WC-25vol%FeAl. They have been milled using a cylindrical ball mill with ethanol. Milled powders have been dried at room temperature and consolidated using pulse current sintering method at 1273 K in vacuum. The compacts thus obtained have had fine and homogeneous structure. Vickers hardness of both compacts has increased with increasing milling time. Transverse rupture strength of the former compact has increased with increasing milling time, but that of the latter one abruptly decreased. This is due to the formation of Fe3W3C and Al-O phase.
Cemented carbides, owing to their excellent mechanical properties, have been of immense interest in the field of hard materials for the past few decades. Although a number of processing techniques have,been developed to obtain nanostructured cemented carbide powders with grain size of less than 100 nm, they are expensive, as such, the challenge remains for producing nano-sized powders economically. The lack of understanding and control of grain growth and its effect on deformation and fracture of the resulting carbide materials is another area that requires proper attention. In addition, the effect of binder materials and their content on the mechanical properties of cemented carbides is not clearly understood yet. This review aims to address some of the key issues and challenges faced in the research and development of cemented carbides, especially on powder refinement and consolidation, and their effect on mechanical properties.
The effect of oxygen content in WC-FeAl powder is examined on mechanical properties of the sintered composites. The oxygen content is varied by controlling the milling and/or drying processes of the WC-FeAl mixed powder. The WC-FeAl composites are obtained by sintering the mixed powders with the pulse current sintering technique. Transverse rupture strength is improved by reducing the oxygen content in the mixed powder whereas the difference of microstructure and composition is not observed clearly. Some results suggest that oxidation of FeAl during the preparation of the WC-FeAl mixed powder affects the mechanical properties of WC-FeAl products. It is concluded that oxygen content is very important for controlling the mechanical properties of WC-FeAl products.
Composites containing ultrafine tungsten carbide/cobalt (WC-Co) cemented carbides and 30 vol% cubic boron nitride (cBN) were fabricated by spark plasma sintering technique. The effect of cBN particles on the densification behavior, microstructure, and mechanical properties of the composites were investigated. According to SEM observation of microstructure, cBN particles are uniformly distributed and have excellent bonding with the cemented carbide matrix. X-ray diffraction analysis shows that there is no indication of phase transformation from cBN to hBN. The addition of very hard dispersed cBN to the WC-Co promotes an increase of hardness, but a decrease of flexural strength. The density and the hardness of cBN-WC-Co composites increase with the increase of the sintering temperature. However, it has the highest hardness of 2,170 HV at 1,370 °C and then the hardness decreases with the further increase of the sintering temperature.
This work addressed on size control of WC grains in WC-FeAl composite fabricated from vacuum sintering techniques. Carbide additives and oxygen were candidates for the size controller of WC grains. The carbide additives such as VC and Cr3C2 suppressed the WC grain growth, which was similar to WC-Co system. As a result, they improved Vickers hardness, but degraded transverse rupture strength. On the other hand, some amount of oxygen included in the composite also suppressed the WC grain growth but improved both the hardness and the strength. It is concluded that oxygen is one of the most effective size controller of WC grains in the WC-FeAl system.
This paper addressed on the changes in constituents and FeAl compositions by oxidation in WC–FeAl composite – one of the alternative materials of WC–Co hard material. Two kinds of WC–FeAl samples having different oxygen content were obtained by controlling the powder preparation processes. Fe3W3C and various oxides including α–Al2O3 were present as semi-products in the sample of higher oxygen content whereas AlFe3C0.5 and slight amount of α–Al2O3 were present in the sample of lower oxygen content. The composition of FeAl phases depended on the oxygen content; the ratio of Fe in the FeAl phases was larger in the composite of higher oxygen content. It was found that the oxygen content strongly influenced the constituents and FeAl composition, which linked with WC grain growth in WC–FeAl composites.
WC and WC–Al2O3 materials without metallic binder addition were densified by spark plasma sintering in the range of 1800–1900 °C. The densification behavior, phase constitution, microstructure and mechanical properties of pure WC and WC–Al2O3 composite were investigated. The addition of Al2O3 facilitates sintering and increases the fracture toughness of the composites to a certain extent. An interesting phenomenon is found that a proper content of Al2O3 additive helps to limit the formation of W2C phase in sintered WC materials. The pure WC specimen possesses a hardness (HV10) of 25.71 GPa, fracture toughness of 4.54 MPa·m1/2, and transverse fracture strength of 862 MPa, while those of WC-6.8 vol.% Al2O3 composites are 24.48 GPa, 6.01 MPa·m1/2, and 1245 MPa respectively. The higher fracture toughness and transverse fracture strength of WC-6.8 vol.% Al2O3 are thought to result from the reduction of W2C phase, the crack-bridging by Al2O3 particles and the local change in fracture mode from intergranular to transgranular.
The main aim of this two-part article is to present and arrange the collected important information on widely applicable Ni-based self-fluxing alloy coatings and to point to the variety of investigations considering the period from 2000 to 2013. A survey of the investigations of these coatings obtained by the following deposition technologies will be given: flame spraying, high-velocity oxy-air fuel spraying, detonation gun spraying, electric arc spraying, plasma spraying, plasma-transferred arc welding, and laser cladding.The treated subject is very extensive and is divided into two parts (in the first part wear and corrosion investigations are presented; in the second part the focus is on investigating the impact of external mechanical loading, residual stresses, and the microstructure of coatings).In the first part, following the general facts (terms, deposition technologies, types of investigations, and application), a description and systematization is given of numerous investigations of wear and corrosion as well as cases of the coexistence of wear and corrosion processes.
Cubic boron nitride/Tungsten carbide-cobalt (cBN/WC–Co) composites were fabricated at low temperature by pulse electric current sintering (PECS) using Ni–P as sintering additives to
promote low temperature densification. Effect of c–BN addition on the densification behavior, microstructure and mechanical properties of WC–Co composites was investigated. It was found that near-fully dense WC/Co with 0 ~ 20 vol% cBN composites could be obtained at temperatures between 1100°C and 1200°C under 50 MPa, while no phase transformation of
cBN into the low-hardness hexagonal hBN. The 20 vol% cBN/WC–Co sample sintered at 1100°C under a pressure of 50 MPa for 10 min has a combination of hardness 16.5 GPa
and fracture toughness KIC 13.7 MP·m1/2. The homogeneously dispersed cBN particles in the WC/Co composites promoted a slight increase of hardness, but a significant improvement of fracture toughness. The cBN particle bridging or deflection and pullout contribute to the high toughness.
The synthesis of titanium carbonitride (TiCN) powders by thermal plasma using extended arc thermal plasma reactor and the effect of TiCN reinforcement for the development of advanced WC–Co cermets has been studied with respect to hardness and fracture toughness. These classes of materials are being investigated for future application in wear-resistant seals, cutting tools, etc. Metallurgical reactions and microstructural developments during sintering of cermets and functionally graded cemented carbonitrides are being investigated by analytical methods such as differential thermal analysis/thermo-gravimetric analysis, X-ray diffraction and analytical Scanning electron microscopy with energy dispersive X-ray spectroscopy. By an in-depth understanding of the complex phase reactions and the mechanisms that govern the sintering process and metallurgical reactions, new cermets and different types of functionally graded cemented carbonitrides with desired microstructures and properties have been attempted to develop. The significant improvement of micro-hardness was observed with optimal concentration of TiCN reinforcement addition in WC–Co system without sacrificing much fracture toughness value of the composite cermets.
For the production of hard, high temperature and abrasion resistant parts, like water-jet nozzles or pressing tools for forming glass lenses, binderless cemented carbide is used. In this work, the consolidation of tungsten carbide with additions of VC and Cr3C2 grain growth inhibitors is studied. Tungsten carbide powder dry or wet milled was consolidated by dry pressing, debindering and gas pressurized sintering and, alternatively, by spark plasma sintering. The effect of adding VC and Cr3C2 to binderless tungsten carbide on the grain growth was studied with contents being 0; 0.1; 0.3; 0.5; 0.7 and 1.0 wt.%. Samples with an ultrafine microstructure free of abnormal grain growth, a hardness of 25.5 GPa and a fracture toughness of 7.2 MPa·m1/2 were archived by conventional sintering. Both carbides reduce grain growth, but with Cr3C2 a finer microstructure can be achieved at lower amounts. Compared to the same amount of Cr3C2, the addition of VC results in smaller grains but lower hardness and fracture toughness.
An experimental study of the variation of transverse-rupture strength with composition and grain size has shown that the strength reaches a maximum for values of the mean free path between carbide particles of 03 to 0.6 microns. The fracture originates in and proceeds through the carbide grains mainly. Impact strength and hardness also were recorded.
Slip in single crystals of tungsten monocarbide has been observed around Vickers pyramid hardness indentations. The slip plane is {l100} at room temperature and the slip directions are postulated to be the and . The Vickers hardness value on a (0001) face is approximately twice the value determined for a (1100) face. This is attributed to the ease of motion of dislocations with Burgers vectors as opposed to the dislocations having Burgers vectors since the former can dissociate into partial dislocations. The occurrence of a {1100} slip plane is explained on the basis of the fact that a minimum number of only two covalent tungsten-carbon bonds need be broken for a unit of slip on this plane.
Dense diamond/cemented carbides were fabricated at around 1300°C, 41MPa for 3min under meta-stable conditions for diamonds by pulsed-electric current sintering. Diamond particles were coated with SiC to prevent the graphitization of diamond with molten binder during sintering. No graphitization of diamond was confirmed by microstructure observation and Raman scattering analysis. The combination of SiC coating of diamond and rapid sintering was effective to fabricate diamond/cemented carbides. These new materials, composed of diamond and cemented carbide, showed a 50% larger fracture toughness and 10 times better wear resistance compared with conventional cemented carbides, and five times better machinability compared with diamond compacts. The diamond/cemented carbides are applicable to wear-resistant sliding tools such as centerless blades, work rests, and stoppers.
Characterization of the flame sprayed and furnace fused NiCrBSiC alloy coatings with two different carbon contents and 15–45 wt.% WCCo addition is described in terms of microstructure, microhardness and differential thermal analysis. Microstructural development of these coatings before and after fusing treatment is discussed to identify the precipitates in the coatings. Optimum fusing conditions (time and temperature) for wear-testing sample were investigated in terms of microhardness and porosity of the coatings. Wear performance of these coatings was also investigated by two- and three-body abrasive and dry sliding wear experiments. The coating of 35% WC with NiCrBSiC is showing the best quality (the highest hardness and the lowest porosity). However, 25% WC addition is showing the best wear resistance for Sugaru abrasive wear test, while 40% WC addition is showing the best wear resistance for DSRW (dry sand rubber wheel) abrasive wear test. It is also shown that the dry sliding wear resistance of the 20% (and/or 30%) WCNiCrBSiC composite coating is almost 10 times better than that of the quenched and tempered JIS SUJ2 bearing steel.
Reactant material powders of pure WO3, Mg and graphite have been milled at room temperature using a high-energy ball mill. After a few kiloseconds of milling (11 ks), numerous fresh surfaces of the reactant materials are created as a result of the repeated impact and shear forces generated by the balls. After 86 ks of milling, a mechanical solid state reduction is successfully achieved between the fresh Mg and WO3 particles to form a product of nanocrystalline mixture of MgO and W. A typical mechanical solid state reaction takes place between the W particles and graphite powders to obtain fine grains of nanocrystalline WC. Towards the end-stage of ball-milling (173 ks), the nanocrystalline MgO grains (10 nm) are embedded into the fine matrix of WC to form fine nanocomposite powders (1 μm in diameter) of WC–18% MgO material with spherical-like morphology. This composite powder was then consolidated under vacuum at 1963 K, with a pressure ranging from 19.6 to 38.2 MPa for 0.3 ks, using a plasma activated sintering method. In addition, pure nanocrystalline WC powders (7 nm in diameter) obtained by removing the MgO from the milled powders, using a simple leaching technique have been also consolidated by the same consolidation technique. The consolidation step does not lead to a dramatic grain growth and the compacted samples that are fully dense still maintain their unique nanocrystalline characteristics. The elastic properties and the hardness of both consolidated samples have been investigated. A model for fabrication of refractory nanocrystalline WC and nanocomposite WC–18% MgO materials at room temperature is proposed.
To control the grain growth of nanocrystalline WC–Co cemented carbides by phosphorus (P) doping was studied in spark plasma sintering (SPS). Experimental results show that the use of small amount of phosphorus dopant to chemically-activated sintering of the nanocrystalline WC–Co composites could enhance the sinterability, promote the low temperature densification, and decrease the sintering temperature of the WC–Co nanocomposites. With only 0.3wt.% P additions, full density WC–7Co cermets were obtained at temperature as low as 1000°C, which is 100°C lower than that of the undoped counterparts in spark plasma sintering of the nanocomposites. By phosphorus doping in WC–Co nanocomposites, the grain growth of WC–Co cemented carbides could be inhibited to some extent, as a result WC–7wt.% Co–0.3wt.% P cemented carbides with average WC grain size of 200nm were achieved, which can be attributed to the lower sintering temperature resulting in a decrease of grain boundary mobility. Mechanical properties measurements show that P doping could lead to a great increment of hardness, but at the same time a little sacrifice of fracture toughness of the nano-WC–Co cemented carbides.
The activation of self-propagating combustion reactions in the WC–Ni system was investigated by using an electric field. Self-sustaining combustion in elemental reactants with composition corresponding to WC–x wt.%Ni (0⩽x⩽10) composites can be activated only when the imposed field is above a threshold value. The nature of combustion products depended on the magnitude of the field. The products contained the high temperature phase (WC), low temperature phase (W2C) and mixed compounds of type Ni2W4C. WC diffraction peaks in X-ray diffraction diagram shifted right when moving from the reactants region to the final product one or increasing electric field. The effect of the relative density of the reactants compact on the synthesis of WC–Ni was also investigated. Moreover, the reaction mechanism of WC–Ni is proposed. X-ray and microscopic analysis of the quenched combustion front suggests that the synthesis of WC is a process involving the solid diffusion of carbon into a carbide layer. Molten Ni reacts with W and W2C to form intermediate phase, which decomposes into WC and Ni. Molten Ni redounds the transformation from W2C to WC phase.
Evacuation temperatures of 1000/sup 0/C activated titanium monocarbide, of 600/sup 0/-1000/sup 0/C activated tungsten monocarbide, and of > 1000/sup 0/C activated tantalum monocarbide for the hydrogenation of ethylene at O/sup 0/C. The tantalum carbide activity was about one order of magnitude lower than that of the other two carbides. Changes in surface area during activation corresponded to the activation temperatures but were small. The activation temperatures corresponded to the temperatures at which metal oxide oxygen is removed, but hafnium, zirconium, and niobium monocarbides were not activated for ethylene hydrogenation at temperatures to 1100/sup 0/C, at which surface oxygen was completely removed.
Diamond growth occurs at high temperatures and pressures in the presence of certain molten metals which serve as solvent catalysts. The zones of pressure and temperature in which diamond growth occurs have been determined for a number of metals. These zones are bounded on the low‐temperature side by the melting point of the metal‐carbon eutectic at pressure. They are bounded on the high‐temperature side by the diamond‐graphite equilibrium line. This experimentally determined equilibrium line agrees very closely with the theoretical extrapolation of the thermodynamically calculated line proposed by Berman and Simon, viz.,
WC and dense WC–xvol.%Co materials, with grain size of <1 μm, were synthesized by high-frequency induction heated combustion synthesis (HFIHCS) method in one step from elemental powders of W, C and Co within several minutes. In the absence of cobalt additive, WC can be formed, but its relative density was low (about 70%) under simultaneous application of a 60 MPa pressure and the induced current. However, in the presence of just 5 vol.% cobalt, the density increased to 98.5% under the same experimental conditions. The effects of Co content on the hardness, fracture toughness, and the relative density of the dense WC–xvol.%Co composites were investigated. Also, the effect of the carbon ratio to the tungsten on the mechanical properties was researched. The hardness of the WC–Co composites decreased with increasing Co content, while the fracture toughness increased. The maximum fracture toughness and hardness were 15.1 MPa m1/2 at WC–20vol.%Co and 1928 kg/mm2 at WC–5vol.%Co.
Ultrafine WC–Ni–VC–TaC cemented carbides with different amounts of cubic boron nitride (cBN) were fabricated by spark plasma sintering, and the microstructure and mechanical properties of the as-prepared cermets were investigated. Scanning electron microscopy observations showed that the size of WC grains in the cermet samples was 0.2–0.4 μm. After the addition of cBN, the samples were still quite dense with the highest relative density of almost 98% when the addition fraction of cBN was 50 vol.%, although some micropores might exist in the samples. X-ray diffraction results indicated that no phase transformation of cBN was detected. The relative density and hardness of the cemented carbides increased with the addition fraction of cBN, but their strength decreased. When the fraction of cBN increased from 0 up to 50 vol.%, the hardness of the samples increased from 2100 to 3200 HV, but the flexural strength decreased from 1950 to 1250 MPa.Graphical abstractResearch highlights► Ultrafine WC–Ni–VC–TaC–cBN cemented carbides were fabricated. ► Almost full dense cermet of high hardness and high strength. ► Combining enhancement method from sparking plasma sintering, WC grain growth inhibitor and addition of superhard materials.
Nanometer WC–Co powder was hot pressed in order to develop new processes and improve the properties of WC–Co cemented carbides. Density and hardness were measured. The microstructures of sintered alloys were observed by SEM. And the grain size of WC in alloys was also measured. The results show that hot pressing can lower the sintering temperature by about 100 °C, increases the density (more than 99%) and circumscribes the growth of grain size (smaller than 1 μm) of alloys. Hardness can also be improved up to HRA 92 by this process. Hot pressing is an effective method to get WC–Co cemented carbides with fine grain size and good properties.
Mechanical properties and microstructures of nanocrystalline WC–10Co cemented carbides were investigated. The nanocrystalline WC–10Co cemented carbide powders were manufactured by reduction and carbonization of the nanocrystalline precursor powders which were prepared by spray drying process of solution containing ammonia meta-tungstate (AMT) and cobalt nitrate. The WC powders were about 100 nm in diameter mixed homogeneously with Co binder phase and were sintered at 1375 °C under a pressure of 1 mTorr. In order to compare the microstructures and mechanical properties with those of nanocrystalline WC–10Co, commercial WC powders in a diameter range of 0.57–4 μm were mixed with Co powders, and were sintered at the same conditions as those of nanocrystalline powders. TaC, Cr3C2 and VC of varying amount were added into nanocrystalline WC–10Co cemented carbides as grain growth inhibitors. To investigate the microstructure of Co binder phase in the WC–10Co cemented carbides, Co–W–C alloy was fabricated at the temperature of sintering process for the WC–10Co cemented carbides. The hardness of WC–10Co cemented carbides increased with decreasing WC grain size following a Hall–Petch-type relationship. The fracture toughness of WC–10Co cemented carbides increases with increasing HCP/FCC ratio of Co binder phase by HCP/FCC phase transformation.
Conventional binderless cemented carbide is known as WC-3% TiC-2% TaC cemented carbide with a mean WC grain diameter of about 2 μm, which does not include a binder phase.This alloy, however, has no binder phase and therefore low strength. The authors investigated the effects of mean diameter of raw WC powder on mechanical characteristics, and found that the smaller the mean diameter of raw WC powder, the lower the temperature at which sintering is possible and the higher the hardness and strength becomes. An investigation was also made on the effects of grain growth suppression additives on the alloy using 0.6 μm diameter WC powder, which offers the highest mechanical characteristics, with the objective of enhancing characteristics through finer grains. Hardness increased with additional amounts of Cr3C2 and VC. Strength peaked at a certain additive amount, with superior values of HRA = 95.5 and transverse-rupture strength = 1.8 GPa.The microstructure was found to be composed of only very fine and uniform carbides; a mirror surface of 7 nm Rtm was obtained by mirror polishing. These characteristics make it possible for this alloy to be used in various optical applications.
Pure WC powder of about 200 nm in diameter was sintered by the spark plasma sintering process without the addition of any binder phase. The initial ultrafine particle size was basically maintained after sintering. The binderless WC sintered at 1773 K for 240 s showed almost full densification and exhibited excellent mechanical properties.
A number of advanced PVD coating designs based on Ti–Al–N–C–B were evaluated in metalcutting. Monolayer PVD TiN, TiAlN, TiB2 and different variants of TiAlN multilayer coatings were deposited on WC-6 wt.% Co hardmetal inserts. The coatings were applied either by cathodic arc processes or a high-ionization magnetron sputtering process. The coated tools were evaluated in milling of ductile and gray cast irons with and without coolant, and in turning of Inconel 718 and a hypereutectic AlSi alloy. The TiAlN-multilayer coated tools showed the best performance in dry milling applications; the TiAlN-monolayer coated tools performed better under wet milling. The observed results are consistent with a model that takes into consideration the inherent residual stresses within the coating, the stresses during machining, and the bonding strength of the coating layers to the substrate. In Inconel 718 turning, the TiAlN-multilayer coating showed some performance advantage over the TiAlN-monolayer and the TiN/TiCN/TiAlN-multilayer coating particularly at higher speed. In the turning of the aluminum alloy, PVD TiB2 had performance advantage over PVD TiAlN and PVD TiN, which could be correlated with their relative hardness values.
The microstructure and mechanical properties of nanograin-sized WC-Co composites were investigated and compared with those of conventional cermets. The dislocation density in the nanometer-sized WC crystals is lower than in the conventional ones, and no inclusions are observed in them. Nanostructured composites have higher tungsten content in the binder phase and a higher ratio of the cobalt. An amorphous phase is observed in the binder phase of the nanostructured samples. Hardness and surface toughness were investigated by performing Palmqvist indentations at loads from 0.025 to 40 Kg. The hardness increases with decreasing binder mean free path of dislocation in the binder phase. The high hardness of nanostructured cemented carbides results not only from the ultrafine microstructure, but also from alloy strengthening of the binder phase itself. The variations of hardness with load suggest that the finer grade conventional carbides have higher microfracture strength, and the nanostructured WC-Co composites are superior to the conventional ones in this respect. Bulkfracture toughness is related to cracks developing through the phases of the material. Palmqvist indentation toughness measurements show that the toughness decreases with increasing hardness in conventional composites, whereas the increase of hardness in nano-structured composites does not further reduce their bulk fracture toughness. This implies that different dominant toughening mechanisms exist in the conventional and nanostructured composites.
The majority of carbide cutting tools in use today employ hard coatings because coatings offer proven benefits in terms of tool life and machining performance. Continuing development of the chemical vapor deposition (CVD) coating process, the most widely used technique, has produced complex multilayer coatings tailored for specific applications and workpiece materials. These coatings include alumina layers of different crystal structures, and TiCN layers applied by high- or moderate-temperature (MT-CVD) processes. Over the last decade, coatings applied by physical vapor deposition (PVD) have gained acceptance in applications requiring sharp edges or those featuring interrupted cuts. Originally limited to TiN coatings, the PVD offering now includes TiCN and TiA1N coatings which provide better high-speed performance and increased abrasive wear resistance. In the area of superhard coatings, improvements in deposition processes and coating adhesion have resulted in diamond-coated carbide tools that have begun to play an important role in machining non-ferrous and non-metallic materials. This paper presents the state of the art in hard coatings for carbide cutting tools including discussion of coating characteristics and applications.
Dense WC with grain size of 0.43 μm was synthesized by high-frequency induction combustion synthesis from milled elemental powders of W and C. The milled W powders had a grain size in the range 45–73 nm. Dense product (98.5%) could be obtained within 2 min under a pressure of 60 MPa. Due to loss of carbon (by interaction with surface oxides), products made from stoichiometric powders (W:C = 1:1) contained the sub-carbide W2C. With excess carbon, products containing the WC phase only were obtained. The effect of initial grain size (of W) and the W:C stoichiometry on the grain size of the product WC was investigated. The grain size of WC increased with an increase in the amount of excess carbon. The maximum values for fracture toughness and hardness obtained for the dense WC were 4.8 MPa m1/2 and 2708 kg/mm2, respectively.
The rapid sintering of nano-structured WC hard materials in a short time is introduced with a focus on the manufacturing potential of this spark plasma sintering process. The advantage of this process allows very quick densification to near theoretical density and prohibition of grain growth in nano-structured materials. A dense pure WC hard material with a relative density of up to 97.6% was produced with simultaneous application of 60 MPa pressure and electric current of 2800 A within 2 min. A larger current caused a higher rate of temperature increase and therefore a higher densification rate of the WC powder. The finer the initial WC powder size the higher is the density and the better are the mechanical properties. The fracture toughness and hardness values obtained were 6.6 MPa m1/2 and 2480 kg/mm2, respectively under 60 MPa pressure and 2800 A using 0.4 μm WC powder.
A sintering method for the rapid sintering of a nanostructured hard metals in short time is introduced with a focus on the manufacturing potential of this novel high-frequency induction heated sintering (HFIHS) process. The advantage of this process allows very quick densification to near theoretical density and prohibition of grain growth in nanostructured materials. In this work, we developed a new process of sintering for nanostructured WC–15vol.%Co hard metals. A highly dense WC–15vol.%Co with a relative density of up to 99.4% was produced with simultaneous application of 60 MPa pressure and induced current within 1 min. The average grain size was about 258 nm and mean free path was about 11.6 nm. The fracture toughness and hardness values obtained were 11.9 MPa m1/2 and 1992 HV30, respectively. Also, we produced dense WC–Co hard metals with a relative density of 97% without applying pressure.
Fine grained submicron cemented carbides with a market share of 10–15% of total carbide consumption are characterized by an almost optimal combination of hardness and toughness. They are mainly used in machining wood and wood based materials, plastics, composites, ferrous and non-ferrous metals as well as in chipless operations such as in can tooling, high pressure punches and for wear parts. Tailored submicron cemented carbide has widened the application range of carbide and has led to a multifold improvement in abrasive wear life.
WC/Ni coating was formed by laser cladding of a W/C/Ni powder blend. The formed WC crystals have rectangular or quadrangle cross-section shapes with size of 2–30 μm. Step, twist and cross growth morphologies of WC formation were observed. The coating contains WC, CW3, WNi, FeW3C, Fe6W6C, W3O, W, C, and (Fe,Ni) phases.
WC and WC–VC materials without metallic binder addition were densified by pulsed electric current sintering (PECS) in the 1600–1900 °C range. The densification mechanism, microstructure and mechanical properties of the pure WC and VC-doped WC carbides were investigated. The densification of WC–VC materials was accompanied with the formation of a cubic (V, W)C solid solution. Although no abnormal WC grain growth was observed, the (V, W)C grains were observed to grow rapidly with increasing VC content. A maximum Vickers hardness (HV10) of 27.39 GPa in combination with an indentation toughness of 4.38 MPa m1/2 was obtained for the PECS pure and 1 wt% VC doped WC grades. The hardness was found to linearly decrease down to 21.41 GPa with further addition of VC up to 16 wt%, with a slightly decreased fracture toughness.
Rapid grain growth during the early stage of sintering has been found in many nano material systems including cemented tungsten carbide WC–Co. To date, however, there have been few reported studies in the literature that deal directly with the kinetics or the mechanisms of this part of grain growth. In this work, the grain growth of nanosized WC during the early stages of sintering was studied as a function of temperature and time. The effects of other influencing factors, such as the initial grain size, cobalt content, and the grain growth inhibitor VC, were investigated. The kinetics of the grain growth process was analyzed and the evolution of the morphology of WC grains during heating-up was studied using high resolution scanning electron microscopy. The results showed that the grain growth process consists of an initial stage rapid growth process which typically takes place during heat-up and the normal grain growth during isothermal holding. The initial rapid grain growth is at least partially attributed to the process of coalescence of grains via elimination common grain boundary. The preferred orientation between WC grains within the aggregates is considered a favorable condition for coalescence of grains, hence rapid grain growth. The solution–reprecipitation process is considered a mechanism of coalescence.