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.     

Effect of medium temperature on the dispersion of WC/ZrO2/VC composite powders

This article proposed a novel method to disperse WC/ZrO₂/VC composite powders so as to attain a perfectly uniform suspension. Besides using conventional dispersing means such as adding dispersant (PEG, polyethylene glycol), mechanical stirring, ultrasonic vibration and ball milling, the temperature adjustment of dispersing-medium distilled water had also been employed. The agglomerating and dispersing mechanisms were analyzed by means of TEM observation of WC/ZrO₂/VC composite powders dispersed under five different temperatures, with the results showing that the most uniform dispersion was obtained under the temperature of 100 °C based on the criterion for conglomeration number per unit. The dispersed WC/ZrO₂/VC composite powders were dried and consequently sintered by hot-press sintering in nitrogen atmosphere at 1580 °C with pressure of 30 MPa. The testing results of mechanical properties such as relative density, hardness, bending strength and fracture toughness show that the optimal properties are obtained by using the WC/ZrO₂/VC composite powders dispersed under 100 °C. The surface crack morphologies of sintered samples are investigated and the results show that crack extended in a more tortuous path for the sample sintered from well-dispersed composite powders.     

Preparation of WC nanoparticles by twice ball milling

In order to improve the ball milling efficiency of WC powders and thus to fabricate nano-grained WC–Co cemented carbides with high mechanical properties, WC nanoparticles were prepared by twice ball milling in nylon vessels. The best technology to disperse WC powders in alcohol was investigated at first. Based on the dispersion results, 2 wt.% PEG was used with La₂O₃ as additive to improve ball milling efficiency. The particle size, crystal structure, surface morphology and surface properties were tested by a laser particle sizer, XRD, FE-SEM and FT-IR, respectively. During the first ball milling, sample d achieved the best milling performance, including average particle size (168 nm) and grain size (27.2 nm) among samples a (pure WC), b (with PEG), c (with La₂O₃) and d (with PEG and La₂O₃). La₂O₃ could greatly decrease particle size and grain size while PEG could narrow particle size distribution. During the second milling, the particle size and grain size of sample d reached 89 nm and 13.2 nm at 96 h, respectively. The results indicated that twice ball milling can greatly improve particle size and grain size compared with the first ball milling, and further narrow the size distribution. In conclusion, multiple ball milling can reduce the particle size of certain powders with suitable milling technology.     

Optimizing the synthesis of ultrafine tungsten carbide powders by effective combinations of carbon sources and atmospheres

Nanostructured WC powders can provide technologically attractive properties due to the fine microstructures obtained after sintering. Either W or WO₃ powders are used for the industrial production of WC. In both cases, the contact area between carbon and tungsten precursors has a critical influence on the reaction temperatures, which in turn affects grain growth and agglomeration of particles. Different methods have been studied to increase the reaction rates by enhancing the contact between reactants: carbon coating of tungsten powder, solid-gas reactions of tungsten powders with atmospheres containing CH₄, or mechanical activation followed by thermal activation of tungsten and carbon precursors.In this work WC-powders were obtained by mechanical activation of tungsten and carbon precursors followed by thermal activation of these mixes at temperatures up to 1100 °C. A systematic study has been carried out combining two dissimilar carbon sources (graphite and carbon black), with different atmosphere compositions (Ar, Ar-50H₂, Ar-10CO) and studying the evolution of phases at different stages of the synthesis. The results show how the efficiency of the interaction between carbon sources and atmospheres affects the completion of the synthesis. The synthesis of WC from WO₃ in H₂ containing atmospheres is enhanced when using carbon black sources, however in CO containing atmospheres the most effective interaction is with graphite.     

Mechanical properties and rapid sintering of nanostructured WC and WC–TiAl3 hard materials by the pulsed current activated heating

Tungsten carbides are primarily used as cutting tools and abrasive materials in the form of composites with a binder metal, such as Co or Ni. However, these binder phases have inferior chemical characteristics compared to the carbide phase and the high cost of Ni or Co. Therefore, low corrosion resistance of the WC–Ni and WC–Co cermets has generated interest in recent years for alternative binder phases. In this study, TiAl₃ was used as a novel binder and consolidated by the pulsed current activated sintering (PCAS) method. Highly dense WC–TiAl₃ with a relative density of up to 99% was obtained within 2 min by PCAS under a pressure of 80 MPa. The method was found to enable not only the rapid densification but also the inhibition of grain growth preserving the nano-scale microstructure. The average grain sizes of the sintered WC and WC–TiAl₃ were lower than 100 nm. The addition of TiAl₃ to WC enhanced the toughness without great decrease of hardness due to crack deflection and decrease of grain size.     

Microstructures and properties of Ni based composite coatings prepared by plasma spray welding with mixed powders

The Ni based composite coatings have been obtained by using the plasma spray welding process and mixed powders (NiCrBSi + NiCr-Cr₃C₂ + WC). Their microstructures and properties were studied. The results showed that the coatings consist mainly of γ-Ni, WC, Cr₂₃C₆, Cr₇C₃, Ni₃Si, Cr₅B₃, CrB and FeNi₃ phases, and the Ni₃Si, Cr₅B₃, CrB and FeNi₃ phases mainly segregated between the carbide grains. The carbide contents in the coatings increased with increasing the mass fractions of NiCr-Cr₃C₂ and WC powders in the mixed powders, which results in enhancing the coating hardness. The abrasive wear resistance of the coatings depends on their hardness. The higher the coating hardness, the stronger the wear resistance is. When the mixed powder (15wt%WC + 30 wt% NiCr-Cr₃C₂ + 55wt%NiCrBSi) was used, the composite coating has higher hardness and more excellent wear resistance, and the coating hardness and weight loss after wear tests are 991 HV and 8.6 mg, respectively.     

An investigation of the fabrication of tungsten carbide–alumina composite powder from WO3, Al and C reactants through microwave-assisted SHS process

Possibility of synthesis of tungsten carbide–alumina composite powder from WO₃–Al–C mixture via microwave-assisted SHS process in a domestic microwave oven has been investigated. By comparison of the results of thermodynamic calculations with experimental findings, it was found that during microwave heating of WO₃:2Al:C mixture, synthesis process initiates by vigorous exothermic reaction of WO₃ with Al which results in a great deal of heat. Major portion of tungsten carbide phase in the product is W₂C, whose formation is supposed to be related to the high thermodynamic stability of this compound at high temperatures. W₂C formation could also be related to carbon loss phenomenon in the mixture, as a consequence of some carbon burn. It has been concluded that addition of excess carbon to the initial mixture together with extension of the microwave processing time, increase the amount of WC phase in the product in expense of W₂C. Experimental results showed that only small amounts of W₂C remain in the product with around 80 mol% excess initial carbon and about 10 min of microwave heating time.     

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.     

Diffusion and solubility of Cr in WC

Although no detailed study on the Cr solubility in WC exists the compilation on the various C–Cr–W phase diagrams [1] suggests this behaviour. In order to prove this and to estimate the diffusivity of Cr in WC we prepared diffusion couples of the type Cr₃C₂–WC by joining and annealing polished fully dense counterparts of the two carbides (temperature range 1550–1750 °C). After thermal treatment the diffusion couples were cut, polished and investigated by metallography. For the measurement of the diffusion profiles the couples were subjected to WDS-EPMA (Cameca SX 100 microprobe). W, Cr, and C concentration profiles were obtained from line scans performed perpendicular to the interface. The analysis of diffusion couples of WC contacted to other carbides used for doping of hardmetals (VC, TaC, NbC, and TiC) did not yield perceptible solubility of the respective metals in WC with respect to the detection limit of EPMA.From the Cr diffusion profiles a diffusion coefficient of Cr in WC of approximately D = 1.70–2.20 × 10^?11cm²/s and an activation energy of E_A = 0.75 eV was estimated. In addition the composition of the ternary phase (W,Cr)₂C in equilibrium with WC and Cr₃C₂ could be measured. For example, in couples annealed at 1750 °C the composition reaches from (W_0.5Cr_0.5)₂C (in equilibrium with WC) to (W_0.2Cr_0.8)₂C (in equilibrium with Cr₃C₂).With the results obtained from the analysis of diffusion couples, the Cr uptake of WC powder as a function of grain size, time and temperature was calculated. Cr saturation in idealised spherical particles of 1 μm occurs only within a few minutes.     

Effects of ZrC and SiC addition on the microstructures and mechanical properties of binderless WC

ZrC-added WC ceramics and SiC-added WC–2 mol% ZrC ceramics were sintered at 1800 °C using a resistance-heated hot-pressing machine. Dense WC ceramics containing 0–1 mol% ZrC and WC–2 mol% ZrC ceramics containing 1–6 mol% SiC were obtained. The reaction products W₂C, ZrO₂ and ZrC-based solid solutions were formed in the ZrC-added WC ceramics during sintering. The relative amount of W₂C reached zero at 2 mol% ZrC, increased in the range of 2–6 mol% ZrC, and decreased again above 6 mol% ZrC. The average WC grain size decreased from 0.49 μm for the WC ceramic to 0.24 μm at 4 mol% ZrC. The SiC addition of 1–2 mol% to the WC–2 mol% ZrC ceramics caused abnormal growth of WC grains. The Vickers hardness of the ZrC-added WC ceramics decreased to 17 GPa at 2 mol% ZrC. The hardness of the SiC-added WC–2 mol% ZrC ceramics increased from 12.4 at 2 mol% SiC to 21.5 GPa at 6 mol% SiC. The fracture toughness of the ZrC-added WC ceramics decreased from 6.2 MPa m^0.5 for the WC ceramic to 5.2 MPa m^0.5 at 4 mol% added ZrC. The fracture toughness of the WC–2 mol% ZrC ceramics with 6 mol% SiC were relatively high at 6.7 MPa m^0.5. The addition of SiC to WC-based ceramics thus improved both hardness and fracture toughness.     

An investigation on the mechanochemical behavior of the Ca–C–Cu2O–WO3 quaternary system to synthesize the Cu–WC nanocomposite powder

Fundamental aspects of the reaction path in the Ca–C–Cu₂O–WO₃ quaternary system to synthesize a copper matrix nanocomposite with reinforcement particles of tungsten carbide have been studied. The mechanism of reactions was specified through the analysis of the relevant sub-reactions. In the presence of carbon as a reducing agent (without Ca), the carbothermal reaction partially occurred even after 40 h of milling. On the other hand, calcium (without C) reduced both Cu₂O and WO₃ after 15 min of milling in a self-sustaining mode. In the simultaneous presence of Ca and C, the products included Cu and W₂C as well as a significant amount of remaining unreacted W. The Cu–WC nanopowder, with no trace of W₂C, was synthesized by the addition of excess carbon to the initial mixture. SEM observations indicated that the composite powders were agglomerated and the range of the particle size was within 100 nm. Elemental mapping spectra showed a relatively uniform distribution of WC in the Cu matrix.     

In situ X-ray synchrotron diffraction study of MgB2 synthesis from elemental powders

The kinetics of MgB₂ synthesis is studied in situ by synchrotron X-ray diffraction, using pressed compacts of 200–400 μm magnesium powders mixed with three types of submicrometer, amorphous, high-purity boron powders. Reaction times for commercially available and plasma-synthesized boron powders decreases from 100 to 2 min as temperature increases from 670 to 900 °C. They can be described by diffusion-controlled models of a reacting sphere with kinetics characterized by diffusion coefficients increasing with temperature from 2 × 10⁻¹⁷ to 3 × 10⁻¹⁶ m² s⁻¹, with activation energies of 123–143 kJ mol⁻¹. Plasma-synthesized boron powders doped with 7.4 at.% carbon show no significant differences in reaction kinetics as compared to undoped powders.     

Reactive spark plasma sintering of binderless WC ceramics at 1500 °C

Binderless WC ceramics were prepared by reactive spark plasma sintering, using tungsten trioxide, tungsten and carbon black as the starting materials. Phase assemblages and microstructure of the as-sintered ceramics were investigated. It was found that graphite existed as an impurity phase due to the volatilization of WO₃, and W could compensate for the WO₃ loss to form WC with a single phase. Benefiting from the enhanced sinterability, WC ceramics with high relative density and good hardness could be obtained at temperature as low as 1500 °C.     

Electrochemical corrosion behaviors of straight WC–Co alloys: Exclusive variation in grain sizes and aggressive media

The electrochemical corrosion behaviors of straight WC–10Co cemented carbides with grain sizes of 1.2, 2.6, 6.1 and 8.2 μm, were comparatively investigated in the solutions of NaOH (pH = 13), Na₂SO₄ (pH = 7) and H₂SO₄ (pH = 1) respectively. To insure a sole variable of WC grain sizes, specific magnetic saturation values of the alloys are adjusted to be identical. The results show a good linear dependence for R_ct (charge transfer resistance) and I_corr (corrosion current density) against the grain sizes. A high sensitivity of the grain sizes to both R_ct and I_corr are identified in NaOH and H₂SO₄. In the solutions of NaOH and Na₂SO₄, the alloys with smaller WC grain sizes exhibit better corrosion resistances, while the alloys with larger WC grain sizes exhibit better corrosion resistances in H₂SO₄. Additionally, in terms of the corrosiveness, NaOH is the weakest and H₂SO₄ is the most aggressive for all the alloys. The corrosion mechanisms were discussed in light of the SEM surface observation, X-ray photoelectron spectroscope analysis and the electrical equivalent circuits for electrochemical impedance spectroscopy.     

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.     

Effect of initial particulate and sintering temperature on mechanical properties and microstructure of WC–ZrO2–VC ceramic composites

Owing to improving the mechanical properties of cemented carbides in high speed machining fields, a new composite tool material WC–ZrO₂–VC (WZV) is prepared from a mixture of yttria stabilized zirconia (YSZ) and micrometer VC particles by hot-press-sintering in nitrogenous atmosphere. Commercial WC, of which the initial particle sizes are 0.2 μm, 0.4 μm, 0.6 μm and 0.8 μm, is mixed with zirconia and VC powder in aqueous medium by following a ball mill process. The sintering behavior is investigated by isostatic pressing under different sintering temperature. The relative density and bending strength are measured by Archimedes methods and three-point bending mode, respectively. Hardness and fracture toughness are performed by Vickers indentation method. Microstructure of the composite is characterized by scanning electron microscopy (SEM). The correlations between initial particles, densification mechanism, sintering temperature, microstructure and mechanical properties are studied. Experimental results show that maximum densification 99.5% is achieved at 1650 °C and the initial particle size is 0.8 μm. When temperature is 1550 °C and particle size is 0.4 μm, the optimized bending strength (943 MPa) is obtained. The best hardness record is 19.2 GPa when sintering temperature is 1650 and particle size is 0.8 μm. The indention cracks propagate around the grain boundaries and the WC particles fracture, which is associated with particle and microcrack toughening mechanism.     

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.     

Forming and sintering behaviors of commercial α-Al2O3 powders with different particle size distribution and agglomeration

The Al₂O₃ structure ceramics have been investigated extensively in previous studies. In order to compare micron- with submicron-scale powder on forming and sintering behaviors, three commercial α-Al₂O₃ powders were studied: 0.15 μm (denoted S as small in the paper) (granulating), 0.43 μm (denoted M as middle) (granulating), and 1.8 μm (denoted L as large) (granulate-free) at d₅₀ (median size). Although the (M) powder contains hard agglomerates, it forms more easily than the (S) powder. This is principally because the (M)'s soft agglomeration strength (0.03 MPa) is weaker than (S) (7 MPa). The (L) bulk formed easily with lower pressure 10 MPa because of wider starting-particle size distribution, 0.2–15 μm. The (S) primary particles rearranged before sintering, so it postponed its sintering onset temperature to about 1200 °C. Additionally, its shrinkage rate becomes maximal and concentrated at the 2nd stage of sintering from 1300 to 1400 °C. (M) bulk revealed the longest shrinkage range from 1000 to 1500 °C because the sintering occurred with its hard agglomerates at first. Although (L) powder formed rather easily, its sintering was impeded by a much wider particle size distribution.     

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.     

High energy milling on tungsten powders

Nanocrystalline tungsten powders were produced by high energy mechanical milling, using both tungsten carbide (WC) and tungsten (W) balls as grinding media. X-ray diffraction study indicated that the lattice parameter of tungsten decreased (from 3.162 to 3.149 Å) with increasing milling time from 0 to 15 h. Considerable decrease in particle size was observed in both W and WC grinding media after 15 h of milling duration. Rietveld analysis of the X-ray data along the Williamson-Hall plots revealed that the crystallite size also decreased with increasing milling time. Chemical analyses showed that the total amount of cobalt and carbon in the milled samples were higher in WC grinding media, as compared to W grinding media. The sintered density increased from 80% to 98% from as received to milled tungsten powders, when sintered at 1790 °C. The mechanical properties of as sintered alloys were evaluated and were found to be strongly influenced by the milling time and grinding media.