Microstructure and properties of WC-8Co cemented carbides prepared by multiple spark plasma sintering

In this study, WC-8Co cemented carbides were prepared by spark plasma sintering. When the samples sintered at 1300℃ were cooled to room temperature, the samples were sintered multiple times at 1250℃. The changes in microstructure and mechanical properties of WC-8Co cemented carbides prepared by multiple spark plasma sintering were studied. The hardness of cemented carbides increased in the first two sintering, reaching 16.5 GPa. However, the hardness decreased seriously in the last two sintering. The attenuation rates of hardness were 6.2% and 2.5% due to the abnormal coarse grains. Furthermore, the crack path along the grain boundary was almost straight, causing a decrease in the indentation fracture toughness of cemented carbides. Additionally, the grains of cemented carbides were abnormally coarsened, and the morphologies of grains became unstable due to multiple sintering.     

Effects of composite binder phase on microstructure and mechanical properties of ultrafine-grained cemented carbides

In order to develop a new high-performance binder phase, four different alloys Co-Ni-Fe, Co-Ni-Cr, Co-Ni-Nb, and AlCoCrNiNb_0.5 were used as a binder in cemented carbides. The room-temperature mechanical properties and high-temperature flexural strength of cemented carbides were studied. The results show that the optimal mechanical properties for the WC-8(Co-Ni-Fe, Co-Ni-Cr, Co-Ni-Nb, and AlCoCrNiNb_0.5) can be obtained at the sintering temperatures of 1200°C, 1350°C, 1350°C, and 1300°C, respectively. Compared with cemented carbides with Co as binder phase, the hardness of the four kinds of alloys is increased, the WC grain size becomes finer, but the fracture toughness is slightly decreased. When the temperature is under 600°C, there is no visible oxidation of the four kinds of cemented carbides, and their bending strengths are basically not reduced. When the temperature increased from 600°C to 900°C, the WC-8(Co-Ni-Nb) and WC-8(Co-Ni-Fe) samples present the better high-temperature bending resistance compared with the WC-8(Co-Ni-Cr) and WC-8AlCoCrNiNb_0.5 samples, with respective decrease in bending strength of 11.7% and 7.3%.     

Influence of Cu-coated diamond addition on the microstructure and mechanical properties of WC-based cemented carbides

Cu-coated diamond enhanced tungsten carbide powder (WC)-Ni cemented carbides were successfully fabricated by spark plasma sintering method. Characterization of the phase composition and microstructure reveal that the diamond particles are well preserved and homogeneously distributed in the composites. Relative density of the samples improved from 92% to 97.6% with 2 wt% Cu-coated diamond addition. Vickers hardness and flexural strength of the samples achieved the maximum value of 2000 HV₁₀ and 950 MPa with 8 and 2 wt% addition, respectively. The fracture toughness improved from 8 to 11 MPa m^1/2 with the added content of diamond increasing from 0 to 4 wt%. The wear rate of the sample is reduced by five times with 6–8 wt% Cu-coated diamond addition. The wear mechanism mainly includes the removal of binder phase, the crushing of WC grains, and the crushing and pulling out of diamond particles.     

Effect of SiC nanowhisker addition on microstructure and mechanical properties of regenerated cemented carbides by low-pressure sintering

The hardness and toughness of regenerated cemented carbides, in general, are contradictory. Therefore, it is critical to explore regenerated cemented carbides with both high hardness and high toughness. In this study, regenerated WC-8-wt% Co cemented carbide with SiC nanowhisker were prepared by low-pressure sintering. The influence of SiCw contents on the microstructure and mechanical properties of regenerated WC-8-wt% Co cemented carbide was investigated. The results indicated that the hardness, density, flexural strength, and fracture toughness of regenerated cemented carbide first increased and then decreased with the addition of SiCw. The Vickers hardness, density, flexural strength, and fracture toughness could reach 1575 HV, 14.6 g/cm³, 2204 MPa, 16.85 MPa·m^1/2, respectively, with SiCw content 0.5 wt%, which were increased by 14.4%, 0.7%, 12.2%, and 17.3%, respectively, when compared with the regenerated cemented carbide without SiCw. The lowest friction coefficient and the best wear resistance could be also reached when 0.5-wt% SiCw was added. The fracture mechanism of the regenerated cemented carbide contained both transgranular and intergranular fracture through the microscopic observation of fracture surface via scanning electron microscope.     

Synthesis,mechanical, and thermophysical properties of high-entropy (Zr,Ti,Nb,Ta,Hf)C0.8 ceramic

Two high-entropy carbides, including stoichiometric (Zr,Ti,Nb,Ta,Hf)C and nonstoichiometric (Zr,Ti,Nb,Ta,Hf)C_0.8, were prepared from monocarbides and ZrH₂. Their sinterability, microstructures, mechanical properties, thermophysical properties, and oxidation behaviors were systematically compared. With the introduction of carbon vacancy, the sintering temperature was lowered up to 300°C, Vickers hardness was almost unaffected, whereas the strength decreased significantly generally due to the decrease of covalent bonds. The thermal conductivity shows a 50% decrease for nonstoichiometry high-entropy carbide, which is a major consequence of the lower electrical conductivity. The oxidation resistance in high temperature water vapor was not sensitive to carbon stoichiometry.     

Effects of sintering aids on microstructure,mechanical, and optical properties of AlON ceramics synthesized by SPS

Aluminum oxynitride (AlON) ceramics doped with different sintering aids were synthesized by spark plasma sintering process. The microstructures, mechanical, and optical properties of the ceramics were investigated. The results indicate that the optimal amount of sintering aids is 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO. The addition of La³⁺ and Mg²⁺ decreases the rate of grain boundary migration in ceramics, promotes pore elimination, and inhibits grain growth. The addition of Y³⁺ facilitates liquid-phase sintering of AlON ceramics. Moreover, the addition of Mg²⁺ effectively promotes twin formation in the ceramics, which hinders crack propagation and dislocation motion when the ceramics are loaded. Hence, the AlON ceramic doped with 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO exhibits a relative density of 99.95%, an average grain size of 9.42 μm, and a twin boundary content of 10.3%, which contributes to its excellent mechanical and optical properties.     

The effects of fine WC contents and temperature on the microstructure and mechanical properties of inhomogeneous WC-(fine WC-Co) cemented carbides

Inhomogeneous WC-(fine WC-Co) cemented carbides with improved hardness and toughness were successfully prepared through the addition of fine WC using planetary ball milling combined with sinter isostatic hot pressing (SHIP) technology. The inhomogeneous microstructure of the alloys consisted of coarsened WC grains and WC-Co consisting of fine WC dispersoids and Co binder phase. The increase of temperature and the addition of fine WC enhanced the sintering process. The morphologies of the coarsened WC and of the fine WC consisted of triangular and near-hexangular prisms, respectively. Due to crack path deflection and crack bridging, the prism-like coarsened WC crystals efficiently hindered cracks propagation. Intergranular fracture became predominant when adding fine WC. However, the excessively coarsened WC and some pores in alloys with 20 wt% fine WC could decrease the mechanical properties. The inhomogeneous WC-(fine WC-Co) cemented carbides with 10 wt% fine WC, sintered at 1430 °C for 40 min, could provide a combination of superior hardness and toughness.     

Effect of carbon content on microstructure and mechanical properties of WC-10Co cemented carbides with plate-like WC grain

Four sets of WC-10Co cemented carbides with different carbon content were prepared by adding the ultrafine WC powders as seeds during the in-situ sintering reaction among W, Co and C. The effect of carbon content on microstructure and mechanical properties were studied. The results show that the microstructure, phase composition and mechanical properties of WC-10Co cemented carbides with plate-like WC grains were seriously affected by the carbon content. The fast growth of WC grains with high carbon content could proceed the prismatic plane preferentially along the <1 0 $1(-)$ 0> directions, resulting in the high content of plate-like WC grains. The density increased with the increment of the carbon content and reached the maximum value, then, followed by a decline. The hardness and the transverse rupture strength of the alloy in the two-phase zone with carbon content of 5.91 wt% reached the maximum value. The existence of plate-like WC grains could impede the propagation of the cracks due to the decrease of the weakest carbide regions and the increase of the basal facets of broken WC crystals. In this case, more fracture energy was required to crack propagation and further improved the transverse rupture strength. Additionally, the plate-like WC was benefit to reduce the wear volume and bring about a better wear resistance. Thus, the alloy with the appropriate proportion of carbon content can obtain higher mechanical properties and wear resistance.     

The role of Cr addition on the processing and mechanical properties of high entropy carbides

Group VI transition metals do not form room temperature stable carbides with a rock salt structure, however, they can be incorporated into a rock salt high entropy carbide lattice. Novel 5-metal high entropy carbides (Cr, Zr, Nb, Hf, Ta)C (HEC5-Cr) were produced using spark plasma sintering and compared with 4-metal carbide (Zr_0.25Nb_0.25Hf_0.25Ta_0.25)C (HEC4) and 8-metal carbide containing Cr (HEC8-Cr). The HEC5-Cr ceramics had higher density and smaller grain size (~14 µm) compared with HEC4 (~28 µm). The solubility limit of Cr on the metal site increased from ~2.5 at% for HEC5-Cr to ~6.0 at% for HEC8-Cr, implying that the high entropy effect increased the solubility of Cr. A significant Cr enrichment was observed at the grain boundaries of HEC5-Cr, and it showed a ~14% increase in nanohardness and a similar indentation modulus compared with HEC4. The nanohardness of HEC5-Cr was up to 41.2 GPa due to increased solid solution strengthening.     

Effects of (Ti,Ta,Nb,W)(C,N) on the microstructure,mechanical properties and corrosion behaviors of WC-Co cemented carbides

Homogeneous solid-solution (Ti, Ta, Nb,W)(C,N) powders were synthesized through carbothermal reduction-nitridation method. The effects of (Ti, Ta, Nb,W)(C,N) powders on the microstructure, mechanical properties and corrosion resistance of WC-10Co cemented carbides were investigated using XRD, SEM, electrochemical test and mechanical properties tests. The results showed that cemented carbides with pre-alloyed powder addition had a similar microstructure appearance: weak core/rim structure consisting of solid-solution phase embedded in the WC-Co system. The black core and gray rim, both of which contained similar elements, were identified as (Ti, Ta, Nb,W)(C,N), but the latter contained higher amount of heavy elements.With the addition of (Ti, Ta, Nb,W)(C,N) powders, the density, transverse rupture strength and fracture toughness of samples decreased monotonously. However, the hardness rose sharply at first, reached a peak at 15 wt% solid-solution addition, then slightly decreased, and finally increased again. Results also revealed that increasing (Ti, Ta, Nb,W)(C,N) made the open circuit potential (OCP) in 1 M sulphuric acid solution more negative than that of WC-Co, and all specimens exhibited pseudo-passivation phenomenon in the test solution. In addition, increasing pre-alloyed powders led to decreasing corrosion current density, which implies that (Ti, Ta, Nb,W)(C,N) could remarkably improve the corrosion resistance of WC-Co cemented carbides.     

Effect of high-energy ball milling on the microstructure and properties of ultrafine gradient cemented carbides

Planetary low-temperature high-energy ball mill was used for preparing the mixed powders with different particle sizes by adjusting the milling time. The ultrafine grain gradient cemented carbides were prepared by sinter-HIP treatment. The effects of milling time on the gradient formation, grain growth, and mechanical properties of ultrafine grain gradient cemented carbides were investigated. The results show that the high-energy ball milling cannot effectively reduce the particle size of mixed powder with short milling time. In addition, the particle size of the mixed powder is significantly reduced, while the specific surface area is significantly increased when the ball milling time exceeds 25 hours. The gradient layer thickness and the grain size increase at the beginning and then decrease when the mixed powder particle size was decreased. Simultaneously, the density, hardness, and fracture toughness of the alloy increase gradually. On the contrary, the number of WC with abnormal grain growth is significantly increased. The thickness of the gradient layer reached 32 μm, and the mean WC grain size is 314 nm. Based on the analyzed results, an optimized gradient layer thickness, grain size, density, and hardness can be obtained when the ball milling time is 35 hours.     

Effects of SiC on phase transformation and microstructure of spark plasma sintered α/β-SiAlON composite ceramics

α/β-SiAlON/SiC composite ceramic tool materials were prepared via spark plasma sintering. The effects of content and size of SiC particles and sintering temperature on phase composition, mechanical properties, and microstructure were investigated. The results indicated that SiC restrained the transformation of β-SiAlON to α-SiAlON, but higher SiC content (≥10 wt.%) resulted in a higher Vickers hardness of the composite. The large size of SiC particles raised the densification temperature of α/β-SiAlON composites, and small SiC particles benefited to improve microstructure. There were more equiaxed α-SiAlON grains and β-SiAlON with a larger aspect ratio (${\overline{\alpha }}_{95}$ = 5.1) in the α/β-SiAlON composite containing 100 nm SiC. The sample containing 10 wt.% 100 nm SiC particles sintered at 1700°C had the optimal properties with a Vickers hardness and fracture toughness of 18.5 ± .2 GPa, 6.4 ± .2 MPa m^1/2, respectively.     

Fabrication of WC-Co cemented carbides with gradient distribution of WC grain size and Co composition by lamination pressing and microwave sintering

WC-Co composite powders with different particle sizes and Co contents were prepared by ball milling WC and Co powder mixtures for different durations. Functionally graded WC-Co cemented carbides with both Co content and WC grain size gradient were prepared by lamination pressing different WC-xCo (x?=?10, 15, 20) powder mixtures and microwave sintering the layered compacts. The WC-xCo powder mixtures were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), and the phase composition and microstructure of the functionally graded cemented carbides (FGCCs) were investigated by XRD, and FE-SEM coupled with energy dispersive spectroscopy (EDS). Mechanical behaviors of the layered WC-Co materials were measured and compared with those of WC-Co cemented carbides with single composition. The results showed that increasing the milling time from 6 to 24?h, results in the decrease of the particle size of WC-Co composite powders from 0.31 to 0.11?µm. After lamination pressing and microwave sintering, the WC-Co samples show nearly complete densification with a relative density higher than 99.7% and no ?-phase was detected in the FGCCs. The Co content and WC grain size in FGCCs decrease from the core to the surface. Homogenization of Co has hardly occurred and no cracks have formed between the layers in the sintered samples. In the inner layer, the mean WC grain size is 529?nm, while in the outer layer it is only 274?nm. Because of the difference in Co content and WC grain size, FGCCs have a Rockwell hardness of 90.75 HRA at the surface, which decreases to 86.75 HRA in the core. However the fracture toughness increases from 11.53 at the surface to 18.12?MPa?m^?1/2 in the core. The present results show that FGCCs with high outer layer hardness and high inner layer toughness were successfully prepared.     

Rapid Reactive Synthesis and Sintering of Submicron TiC/SiC Composites through Spark Plasma Sintering

Submicrometer TiC/SiC composites were fabricated by a rapid reactive sintering process through spark plasma sintering (SPS) technique using the carbon, titanium, and nanosized-SiC powders without any additive. It was found that the composite could be sintered in a relatively short time (8 min at 1480°C) to 97.9% of theoretical density. After sintering, the phase constituents and microstructures of the samples were analyzed by X-ray diffraction techniques and observed by scanning electron microscopy. The effect of nanosized and microsized SiC additives on the microstructure of TiC/SiC composites was investigated.     

Microstructure and properties of NiAl/TiC composite synthesized by spark plasma sintering of mechanically activated elemental powders

In this study, NiAl/TiC_0.95 composite was synthesized by reactive spark plasma sintering of mechanically activated elemental powders. The microstructure and properties of activated powders and sintered samples were evaluated. The elemental powders were milled after different milling times and as-mixed and 10 h milled powder mixtures were sintered by the reactive spark plasma sintering method. The phase and the microstructure changes were evaluated by x-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy, respectively. The XRD pattern of 0 h milled powder after sintering showed that Ni₃Al, Ni₂Al₃ beside NiAl and TiC_0.75 formed. While after the sintering of 10 h mechanically activated powder, the Ni₃Al and Ni₂Al₃ were eliminated and NiAl remained with TiC_0.95. The nanoindentation result of the SPSed sample showed a hardness of 12.2 ± 0.1 GPa with an elastic modulus of 25.0 ± 0.5 GPa.     

Influence of the microstructure on the thermal shock behavior of cemented carbides

The influence of single and repetitive sudden changes of temperature on the mechanical integrity of cemented carbides was investigated as a function of their microstructure. Thermal shock resistance was assessed by testing the residual flexural strength of hardmetal beams after being subjected to thermal shock by water quenching. Results indicate that hard cemented carbides tend to exhibit a superior resistance to the nucleation of thermal shock damage but a lower resistance to the propagation of this damage mechanism than tough grades, and vice versa. These trends are in agreement with those expected from the evaluation of the thermal shock Hasselman’s parameters. The evidenced strength loss after thermal shock may be related to the subcritical growth of intrinsic flaws driven by localized microcracking surrounding them. Results also point out on Ni-base hardmetals to exhibit a slightly higher resistance to abrupt changes of temperature than Co-base ones.     

Spark plasma sintering of WC-based cermets/titanium and vanadium added composites: A comparative study on the microstructure and mechanical properties

In an attempt to develop the composition and properties of W₂C-(W,Ti)C-TiC and WC-WC_1-x-VC-V super hardmetals, spark plasma sintering (SPS) method was implemented. WC powders were mixed separately with 10?wt% Ti and 10?wt% V in a high energy mixer mill and sintering processes were performed at temperatures of 2150 and 2000?°C, respectively. XRD investigations revealed the formations of TiC and (Ti,W)C as the reaction products in WC-10?wt% Ti composite. Moreover, the interfacial reaction between WC and V led to the formation of WC1-x and VC compounds. A higher bending strength (613?±?25?MPa) and fracture toughness (4.1?±?0.58?MPa?m^1/2) were obtained for WC-10?wt% V samples compared to WC-10?wt% Ti, While the WC-10?wt% Ti composite showed a higher value of hardness (3128?±?42 Vickers) in comparison to WC-10?wt% V (2632?±?39 Vickers), which can act as a super hard cermet.     

The densification behavior,microstructure, mechanical,and thermionic emission properties of LaB6-SiC composite

The LaB₆-SiC composite with the different SiC content (0, 15, 30, 36, 50, 90, and 100 wt.%,) was densified by spark plasma sintering. The effects of SiC content on the densification behavior, microstructure, mechanical, and thermionic emission properties of LaB₆-SiC composite were systemically investigated. The results show that all the rapid shrinkage occurred at the heating stage during densification, and LaB₆-36 wt.%SiC composite owned the maximum shrinkage rate of 1.5 mm/min at T = 1798°C. The highest relative density of the composite decreased from 98.18% to 95.01% as the SiC content increased from 15 wt.% to 90 wt.%, under which the morphology of LaB₆ grain evaluated from the equiaxed to elongated structure, and LaB₆ grain size varied in the range of 5.05–11.42 μm. The similar eutectic structures were observed in the LaB₆-36 wt.% SiC composite because of some LaB₆ grains melting. Both the highest fracture toughness of 5.15 ± 0.56 MPa.m^1/2 and the highest bending strength of 313 ± 4.7 MPa belonged to the LaB₆-36 wt.% SiC composite, which also exhibited thermionic emission current density of 10.74 A/cm² and work function of 2.99 eV at T = 1873 K.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

Relationship between sintering methods and physical properties of the low positive thermal expansion material Al2W3O12

Ceramic materials from the A₂M₃O₁₂ family with near-zero thermal expansion are good candidates for applications requiring high thermal shock resistance. Considering their inherently low thermal conductivity, the bulk forms of A₂M₃O₁₂ have to present Young's moduli and mechanical strength close to 100 GPa and 100 MPa, respectively, in order to compete with the state-of-the-art materials used to avoid thermal shock. The relationship between sintering, microstructure, and physical properties within the A₂M₃O₁₂ family is generally unknown while the preparation of bulks with high mechanical resistance remains a great challenge. Bulk samples of dense Al₂W₃O₁₂ (96%TD) have been obtained by pressureless three-stage sintering (TSS) and spark plasma sintering (SPS). The Young's moduli and hardness of samples prepared by SPS were 50% higher than that measured for TSS samples and more than 100% in comparison to the Al₂W₃O₁₂ bulk (91%TD). UV-Vis spectroscopy confirmed that A₂M₃O₁₂ phases are wide band-gap semiconductors (3.11 eV). When prepared by SPS, black Al₂W₃O₁₂ absorbed light within the visible spectrum due to the introduction of donor sites within the band-gap. No enhancement of the mechanism causing negative thermal expansion was observed for black Al₂W₃O₁₂. The mechanical properties achieved were significantly improved over those previously reported in literature for Al₂W₃O₁₂.