The effect of volume fraction of WC particles on wear behavior of in-situ WC/Fe composites by spark plasma sintering

Tungsten carbide (WC) particles have been in-situ synthesized through the reaction between tungsten particles and carbide particles by spark plasma sintering (SPS). The composites with different WC content were comparatively observed by the techniques of scanning electron microscopy (SEM), high-resolution transmission electron microscope (HRTEM), X-ray diffraction, hardness and pin-to-disc abrasive wear test. The results showed that the formed WC particles were homogenously distributed in the iron matrix with the size of smaller than 25 μm. Additionally, with the increasing of the WC content, the hardness of composites, the microhardness of matrix and the wear resistance increased, but there was no change significantly between 32 vol% WC/Fe composites and 42 vol% WC/Fe composites. The composites possessed excellent wear resistance comparing the specific wear rate determined in the present work to the martensitic wear-resistant steel under the load of 80 N after a sliding distance of ~ 950 m. The specific wear rate of the martensitic wear-resistant steel was a factor of 24 and 48 times higher than WC/Fe composites, when the content of WC was 32 vol% and 42 vol% in WC/Fe composites, respectively. The main wear mechanism was synthetic of abrasion wear and oxidation wear. The wear performance of 32 vol% WC/Fe composites didn't appear to be much different from 42 vol% WC/Fe composites, due to the WC particles in the 42 vol% composites produced stress concentration easily, which could ultimately induce the creak initiation around WC particles in the subsurface (near wear surface) and propagation to wear surface promoting the breakup of surface film.     

Wear resistance in the soil of Stellite-6/WC coatings produced using laser cladding method

The paper presents the study results of macro- and microstructure, microhardness, phase composition and practical application of Stellite-6/WC metal matrix composite (MMC) coatings produced using laser cladding technology. For the preparation of MMC coatings the Yb:YAG disc laser with a rated power equal 1 kW was used. The laser beam was controlled using a 5-axis CNC machine. The powder mixture was fed to laser beam by three-streams powder feeding system. Composite coatings were prepared to increase the durability of agricultural tools. The coatings were prepared on B27 boron steel. This steel grade is most often used into produce the tools for soil cultivation. Stellite-6/WC coatings were produced using laser beam power 550 W, feed rate 400 mm/min and powder feed rate 10 g/min. Durability tests of agricultural tools were carried out by two methods. Measurement the mass loss was the first method, while measurement the tools shape using GOM ATOS II optical scanner was the second method. It was found that Stellite-6/WC coatings contributed to increase durability of agricultural tools applied for soils cultivation.     

Three-dimensional investigation of particle-stimulated nucleation in a nickel alloy

By combining a focused ion beam (FIB) microscope with a field emission gun scanning electron microscope, it is possible to sequentially mill away ∼50 nm sections of a material by FIB and characterize, at high resolution, the crystallographic features of each new surface by electron backscatter diffraction (EBSD). The successive EBSD maps are subsequently combined to generate three-dimensional crystallographic maps of the microstructure. FIB-EBSD tomography was used to investigate the deformation and recrystallization behaviour of a nickel alloy containing coarse silica particles. The technique demonstrated unambiguously the influence of particle diameter on the misorientations generated within particle deformation zones and showed that particle-stimulated nucleation (PSN) of recrystallization occurred at particles greater than ∼1 μm. PSN also often generated groups of contiguous grains separated by both coherent and incoherent twin boundaries. It was found that much of the behaviour observed using FIB-EBSD tomography is not clearly evident in two-dimensional EBSD micrographs.     

Fe–W–C thermodynamics and in situ preparation of tungsten carbide-reinforced iron-based surface composites by solid-phase diffusion

Tungsten carbide (WC)-reinforced Fe-based surface composites were prepared by in situ solid-phase diffusion at 1423 K for 4, 6, and 8 h. The thermodynamics, phase composition, microstructure, microhardness, and wear-resistance of the Fe–W–C ternary system of the samples were examined by X-ray diffraction, scanning electron microscopy, Vickers hardness test, and wear test, respectively. Thermodynamic calculations showed that the thermodynamically favored products of the Fe–W–C system were W₂C, WC, and Fe₃C. W also exhibited a stronger carbide-forming tendency than Fe. The Gibbs free-energies of W₂C and WC, which were stable carbides, significantly decreased with increased temperature. The main phases of the composite were WC, γ-Fe, Fe₃C, graphite, and η-carbide (M₆C) with fishbone-like morphology. The longitudinal section of the composite could be easily divided into three reaction zones, namely, WC layer, “no graphite area,” and M₆C-reinforced area. WC particles in the WC layer were irregularly shaped with 0.3–12 μm particle size, with volume fraction of up to > 80%. The average microhardness value of the dense ceramic layer was 2152 HV_0.1. The maximum relative wear-resistance, which was 230.4 times higher than that of gray cast iron, was obtained at 20 N. The high wear-resistance of the composite was due to the in situ formation of dense and hard WC particulates that acted as a reinforcement phase.     

An insight into the role of the grain boundary in plastic deformation by means of a bicrystalline pillar compression test and atomistic simulation

A combination of experimental and molecular dynamics (MD) simulations is used to investigate the interaction of dislocations with a selected grain boundary (GB) in bicrystalline pillars (BCPs) with component crystals oriented for single slip and multiple slip. As a reference, single-crystalline pillars with the same orientations are also tested and compared with the BCPs. Orientations identical to the experiments are used to generate models in MD simulations. Further, electron backscatter diffraction (EBSD) measurements on the cross-section of the pillars are performed to investigate the crystal lattice rotation in correlation with the excess dislocation density. A clear change in mechanical behavior of the BCP was observed when the size of the component crystals reduced below 1 μm. The EBSD analyses of these small BCPs showed an increase in the misorientation in the vicinity of the GB. MD simulation provided atomistic insights into the dislocation nucleation process and the BCPs’ interaction with the GB. On the basis of these observations, it is concluded that in BCPs smaller than 1 μm the dislocation–GB interaction plays a more crucial role than the dislocation–dislocation interaction.     

Microstructure,grain size distribution and grain shape in WC–Co alloys sintered at different carbon activities

The properties of cemented carbides strongly depend on the WC grain size and it is thus crucial to control coarsening of WC during processing. The aim of this work was to study the effect of sintering at different carbon activities on the final microstructure, as well as the coarsening behavior of the WC grains, including the size distribution and the shape of WC grains. These aspects were investigated for five WC–Co alloys sintered at 1410 °C for 1 h at different carbon activities in the liquid, in the range from the graphite equilibrium (carbon activity of 1) to the eta (M₆C) phase equilibrium (carbon activity of 0.33). The grain size distribution was experimentally evaluated for the different alloys using EBSD (electron backscatter diffraction). In addition, the shape of the WC grains was evaluated for the different alloys. It was found that the average WC grain size increased and the grain size distribution became slightly wider with increasing carbon activity. Comparing the two three-phase (WC–Co–eta and WC–Co–graphite) alloys a shape change of the WC grains was observed with larger grains having more planar surfaces and more triangular shape for the WC–Co–graphite alloy. It was indicated that in alloys with a relatively low volume fraction of the binder phase the WC grain shape is significantly affected by impingements. Moreover, after 1 h of sintering the WC grains are at a non-equilibrium state with regards to grain morphology.     

Cladding of pre-blended Ti–6Al–4V and WC powder for wear resistant applications

In this paper, a cladding investigation to achieve uniform distribution of WC particles which is crack-free, non-porous and without delamination using a 2 kW IPG Ytterbium doped, continuous wave, fibre laser with 1070 nm wavelength was reported. The single track deposition of a pre-blended powder, 27 wt.% Ti–6Al–4V/73 wt.% WC with a particle size range of 40–120 μm was made on Ti–15V–3Cr–3Sn–3Al substrate using a co-axial nozzle and a standard powder feeding system. The laser cladding samples were subjected to various microstructure examinations, microhardness and micro-abrasion tests. The results revealed that the best clad layers were achieved at an energy density of 111.10 J.mm^?2, 15–18.3 mm.s^?1 traverse speed; (583–667) mg.s^?1 powder feed rate with substrate surface irradiated by laser beam raising its temperature to about 200 °C. This resulted in a uniform distribution of WC within the clad and the results obtained from SEM, EDS and XRD revealed that the WC particles experienced surface melting with some diffusion into the matrix, thus promoting excellent bonding with the matrix and the formation of titanium and tungsten carbides, which include TiC and W₂C. The emergence of β-Ti, TiC and W in the clad resulted in enhanced hardness values. The mean value of microhardness in clad matrix is 678 HV when measured from the top of a transverse cross section of the clad sample into the interface region with the Ti substrate which has a hardness of 396 HV. Wear tests indicated the wear resistance of the clad was seven times that of the Ti alloy substrate.     

Deformation-induced grain coalescence in an electrodeposited pure iron sheet studied by in situ neutron diffraction and electron backscatter diffraction

The plastic deformation behavior of an ultrafine-grained electrodeposited pure iron sheet with a strong {1 1 1}〈h k l〉 texture was studied by in situ neutron diffraction during tensile deformation at room temperature and by electron backscatter diffraction (EBSD). The combination of volume-averaged crystallographic orientation changes determined by neutron diffraction and the local orientation relationship determined by EBSD reveals a texture change to {1 1 1}〈1 1 0〉 and corresponding microstructural changes with tension deformation. Related to such grain rotation, grain coalescence on deformation was found using semi in situ EBSD. The results obtained are explained using a characteristic slip model, which also gives a reason for the ultrahigh Lankford value of this material.     

Effects of Ni addition and cyclic sintering on microstructure and mechanical properties of coarse grained WC–10Co cemented carbides

Coarse grained WC–10(Co, Ni) cemented carbides with different Ni contents were fabricated by sintering-HIP and cyclic sintering at 1450 °C. The effects of Ni addition and cyclic sintering on the microstructures, magnetic behavior and mechanical properties of coarse grained WC–10(Co, Ni) cemented carbides have been investigated using scanning electron microscope (SEM), magnetic performances tests and mechanical properties tests, respectively. The results showed that the mean grain size of hardmetals increases from 3.8 μm to 5.78 μm, and the shape factor Pwc decreases from 0.72 to 0.54, with the Ni content increases from 0 to 6 wt.%. Moreover, the W solubility reaches the highest value of 10.33 wt.% when the Ni content is 2 wt.%. The hardness and transverse rupture strength of WC–8Co–2Ni are 1105 HV₃₀ and 2778 MPa, respectively. The cyclic sintering is conducive to increase the WC grain size of WC–10(Co, Ni) and improves the transverse rupture strength of WC–10Co without compromising the hardness of alloys.     

A study on ultrasonic elliptical vibration cutting of tungsten carbide

Sintered tungsten carbide (WC) is a versatile metal matrix composite (MMC) material widely used in the tool manufacturing industries. Machining of this material with conventional cutting (CC) method is a real challenge compared to other difficult-to-cut materials. Ultrasonic elliptical vibration cutting (UEVC) method is a novel and non-conventional cutting technique which has been successfully applied to machine such intractable materials for the last decade. However, few studies have been conducted on cutting of WC using single point diamond tool (SPDT) applying the UEVC technique. This paper presents an experimental study on UEVC of sintered WC (～15% Co) using polycrystalline diamond (PCD) tools. Firstly, experiments have been carried out to investigate the effect of cutting parameters in the UEVC method in terms of cutting force, flank wear, surface finish while cutting sintered WC. The tests have revealed that the PCD tools in cutting of WC by the UEVC method results in better cutting performance at 4 μm depth of cut (DOC) as compared to both a lower DOC (e.g. 2 μm) and a higher DOC (e.g. 6 or 8 μm). Moreover, the cutting performance improves with the decrease in both the feed rate and cutting speed in the UEVC method like conventional turning (CT) method. A minimum surface roughness, R_a of 0.036 μm has been achieved on an area of about 1257 mm² with the UEVC performance. The CT method has also been employed to compare its cutting performance against the UEVC method. It has been observed that the UEVC method results in better cutting performances in all aspects compared to the CT method. Theoretical analysis on the UEVC method and analysis of the experimental results have been carried out to explain the reasons of better surface finish at 4 μm DOC and better cutting performance of the UEVC method.     

Reduced-temperature processing and consolidation of ultra-refractory Ta4HfC5

TaC, HfC, and WC powders were subjected to high-energy milling and hot pressing to produce Ta₄HfC₅, a composite of Ta₄HfC₅ + 30 vol.% WC, and a composite of Ta₄HfC₅ + 50 vol.% WC. Sub-micron powders were examined after four different milling intervals prior to hot pressing. XRD was used to verify proper phase formation. SEM, relative density, and hardness measurements were used to examine the resulting phases. Hot pressed compacts of Ta₄HfC₅ showed densification as high as 98.6% along with Vickers hardness values of 21.4 GPa. Similarly, Ta₄HfC₅ + 30 vol.% WC exhibited 99% densification with a Vickers hardness of 22.5 GPa. These levels of densification were achieved at 1500 °C, which is lower than any previously reported sintering temperature for Ta₄HfC₅. Microhardness values measured in this study were higher than those previously reported for Ta₄HfC₅. The WC additions to Ta₄HfC₅ were found to improve densification and increase microhardness.     

Microstructure and properties of WC–Co/3Cr13 joints brazed using Ni electroplated interlayer

The brazed joints of WC–Co cemented carbide and 3Cr13 stainless steel using Ni electroplated on Cu–Zn alloy as interlayer were investigated. The shear strength of the WC–Co/interlayer/3Cr13 joints increased firstly and then decreased with the increase of brazing temperature or brazing time. The maximum shear strength value of the brazed joints was 154 MPa at 1100 °C for 10 min. The characterizations of the WC–Co/interlayer/3Cr13 joints were studied by SEM, EDS and XRD. The results showed that the brazed joints fractured in the bulk WC–Co substrates near the interlayer. The added Ni promoted the formation of interdiffusion zone, which possessed positive effects on the bond strength of the WC–Co/interlayer/3Cr13 joints. Austenite solid solution was formed in the WC–Co/interlayer/3Cr13 joint, and the majority of austenite solid solution presented as columnar crystal. The number of austenite crystals on the WC–Co/interlayer interface was tremendously more than that on the interlayer/3Cr13 interface.     

Preparation of polycrystalline cubic boron nitride compact by high-pressure infiltration using cemented carbide

Polycrystalline cubic boron nitride (PcBN) compacts, using the infiltrating method in situ by cemented carbide (WC–Co) substrate, were sintered under high temperature and high pressure (HPHT, 5.2 GPa, 1450 °C for 6 min). The microstructure morphology, phase composition and hardness of PcBN compacts were investigated by using scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). The experimental results show that the WC and Co from WC–Co substrate spread into cubic boron nitride (cBN) layer through melting permeability under HPHT. The binder phases of WC, MoCoB and Co₃W₃C realized the interface compound of PcBN compact, and the PcBN layer formed a dense concrete microstructure. Additionally the Vickers hardness of 29.3 GPa and cutting test were performed when sintered by using cBN grain size of 10–14 μm.     

Comparison between oxidation kinetics of HVOF sprayed WC–12Co and WC–10Co–4Cr coatings

In this study, the high temperature oxidation behavior of HVOF-sprayed WC–12Co and WC–10Co–4Cr coatings were investigated. To explore the oxidation mechanism, thermo-gravimetric analysis (TGA) was applied for isothermal treatments in the range of 500–800 °C for 3 h. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the structural changes and microstructural evolutions during oxidation tests. The TGA experiments showed negligible oxidation mass gains at 500 °C for both coatings. At higher temperatures, i.e. 700 and 800 °C, the oxidation mass gains of WC–12Co were found to be much higher than those for WC–10Co–4Cr coating, respectively. The higher oxidation resistance of WC–10Co–4Cr coating probably results from the formation of compact chromium oxide layers and higher MWO₄ type tungstate (M: Co and/or Cr) to tungsten trioxide (WO₃) ratios which provide lower porosity and consequently more efficient passivation effect against oxidation. The time dependent mass gain of WC–12Co coating obeys the linear law within temperature range of 600–800 °C with apparent oxidation activation energy of ~ 104 kJ/mol. As for the oxidation of WC–10Co–4Cr coating, a negligible deviation from linear law was observed possibly due to the presence of chromium oxide and higher tungstate to tungsten trioxide ratio which hinders the diffusion process through the scales compared with WC–12Co coating. The apparent activation energy for oxidation of the WC–10Co–4Cr coating was found to be ~ 121 kJ/mol.     

Microstructure–property relationship in textured ZnO-based varistors

The relationship between microstructure texturing and electrical characteristics of a ZnO-based varistor system was investigated in comparison with a varistor system having the same chemical composition but conventional microstructure. Highly textured ZnO-based varistors were produced via the templated grain growth (TGG) technique. Stereological analysis, electron back-scattered diffractometry (EBSD) and X-ray diffractometry (XRD) were conducted to analyze texture development and orientation distribution. The degree of orientation, r, calculated from the (0 0 0 1) EBSD pole figure, was 0.34; the texture fraction, f (Lotgering factor), calculated from the XRD data, was 0.98 for the samples produced via TGG. The threshold voltages were found to be anisotropic, consistent with the observed morphological texture. The non-linear coefficients, α, did not exhibit a significant difference as a function of direction, even in the highly textured samples. However, different types of grain boundary characteristics depending on the direction were identified with 0.42, 0.69 and 1.14 eV Schottky barrier heights.     

Particle properties of submicron-sized WC–12Co processed by planetary ball milling

In this study, a conventional nano-grained tungsten carbide (WC) powder was mixed with 12 wt.% of a submicron cobalt (Co) powder in a ball mill for varying milling time periods, producing a homogeneous powder mixture which can be used to sinter near-nanocrystalline cemented carbides using short-duration sintering processes. Parameters of the wet milling process were adapted in order to maximise the mixing effect on the one hand, and to avoid particle growth during the milling process on the other. Surface analysis and microscopic examination of the milled powders showed a milling-time-dependent evolution of particle size and surface roughness. X-ray diffraction (XRD) investigation indicated a decrease of the crystallite size of WC in combination with an increase in defect density, as well as a strong increase in stacking faults in the Co. The main action of the milling mechanism is the fracturing of the WC particles. Co is distributed consistently around the WC particles. The preparation method used is a useful technique to prepare homogeneous powder mixtures of WC–Co with particle sizes below 200 nm on a laboratory scale.     

Erosion resistance of laser clad Ti-6Al-4V/WC composite for waterjet tooling

In waterjet operations, milled surfaces are left with some undesirable dimensional artefacts, thus the use of abrasion resistant mask has been proposed to improve the surface quality of machined components. In this study, the erosion performance of laser clad Ti-6Al-4V/WC composite coating subjected to plain water jet (PWJ) and abrasive water jet (AWJ) impacts to evaluate its potentials for use as waterjet impact resistant mask material and coating on components was investigated. Results showed that composite with 76 wt.% WC composition subjected to PWJ and AWJ impacts offered resistance to erosion up to 13 and 8 times that of wrought Ti-6Al-4V respectively. Scanning electron microscopy (SEM) examination of the eroded composite surfaces showed that the erosion mechanism under PWJ impacts is based on the formation of erosion pits, tunnels and deep cavities especially in the interface between the WC particles and the composite matrix owing to lateral outflow jetting and hydraulic penetration. Composite suffered ploughing of the composite matrix, lateral cracking and chipping of embedded WC particles and WC pull-out under AWJ impacts. The composite performance is attributed to the embedded WC particles and the uniformly distributed nano-sized reaction products (TiC and W) reinforcing the ductile β-Ti composite matrix, with its mean hardness enhanced to 6.1 GPa. The capability of the Ti-6Al-4V/WC composite coating was demonstrated by effective replication of a pattern on a composite mask to an aluminium plate subjected to selective milling by PWJ with an overall depth of 344 μm. Thus, composite cladding for tooling purpose would make it possible to enhance the lifetime of jigs and fixtures and promote rapid machining using the water jet technique.     

Microstructural analysis of wear micromechanisms of WC–6Co cutting tools during high speed dry machining

This original study investigates the damages of WC–6Co uncoated carbide tools during dry turning of AISI 1045 steel at mean and high speeds. The different wear micromechanisms are explained on the basis of different microstructural observations and analyses made by different techniques: (i) optical microscopy (OM) at macro-scale, (ii) scanning electron microscopy (SEM), with back-scattered electron imaging (BSE) at micro-scale, (iii) energy dispersive spectroscopy (EDS), X ray mapping with wavelength dispersive spectroscopy (WDS) for the chemical analyses and (iv) temperature evolution during machining. We noted that at conventional cutting speed Vc ≤ 250 m/min, normal cutting tool wear types (adhesion, abrasion and built up edge) are clearly observed. However, for cutting speed Vc > 250 m/min a severe wear is observed because the behavior of the WC–6Co grade completely changes due to a severe thermomechanical loading. Through all SEM micrographs, it is observed that this severe wear consists of several steps as: excessive deformation of WC–6Co bulk material and binder phase (Co), deformation and intragranular microcracking of WC, WC grain fragmentation and production of WC fragments in the tool/chip contact. Thus, the WC fragments accumulated at the tool/chip interface cause abrasion phenomena and pullout WC from tool surface. WC fragments contribute also to the microcutting and microploughing of chips, which lead to form a transferred layer at the tool rake face. Finally, based on the observations of the different wear micromechanisms, a scenario of WC–6Co damages is proposed through to a phenomenological model.     

Influence of the grain orientation spread on the pitting corrosion resistance of duplex stainless steels using electron backscatter diffraction and critical pitting temperature test at the microscale

The corrosion behavior of UNS S32202 duplex stainless steel was studied by combining electron backscatter diffraction (EBSD) measurements and critical pitting temperature tests at the microscale. The grain orientation spread (GOS) value was determined in grains of both phases from EBSD data. It was shown that austenitic sites containing extremely small ferrite grains having a GOS value greater than 1.3° were precursor sites for pitting in 4 M NaCl. The critical pitting temperature range was 45–90 °C. All the other sites of both phases remained passive up to 100 °C.     

Microstructure and corrosion resistance of the brazed WC composite coatings in aerated acidic chloride media

The paper presents the corrosion behavior of the brazed WC composite coatings by using a combination of weight loss method and electrochemical polarization measurement. The brazed WC composite coatings have lower corrosion current densities compared with the 316L stainless steel and show a superior corrosion-resistance property. A further decrease of corrosion current densities is observed for the brazed WC composite coatings with the extending of etching time. The X-ray diffusion (XRD) analysis shows that WC, γ-Ni solid solution and γ-Ni + Ni₃B eutectic solution were presented in the coating surface. The surface morphology observation and composition analysis were conducted by scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy. The insoluble oxidation products were deposited on the corroded coating surface.