Fabrication of full density near-nanostructured cemented carbides by combination of VC/Cr3C2 addition and consolidation by SPS and HIP technologies

The aim of present work is to study the effect of VC and/or Cr₃C₂ in densification, microstructural development and mechanical behavior of nanocrystalline WC-12wt.%Co powders when they are sintered by spark plasma sintering (SPS) and hot isostatic pressing (HIP). The results were compared to those corresponding to conventional sintering in vacuum. The density, microstructure, X-ray diffraction, hardness and fracture toughness of the sintered materials were evaluated. Materials prepared by SPS exhibits full densification at lower temperature (1100 °C) and a shorter stay time (5 min), allowing the grain growth control. However, the effect of the inhibitors during SPS process is considerably lower than in conventional sintering. Materials prepared by HIP at 1100 °C and 30 min present full densification and a better control of microstructure in the presence of VC. The added amount of VC allows obtaining homogeneous microstructures with an average grain size of 120 nm. The hardness and fracture toughness values obtained were about 2100 HV₃₀ and close to 10 MPa m^1/2, respectively.     

VC, Cr3C2 and NbC doped WC–Co cemented carbides prepared by pulsed electric current sintering

WC–12 wt.% Co grade cemented carbides doped with 0.9 wt.% VC, NbC or Cr₃C₂ grain growth inhibitor were consolidated by pulsed electric current sintering (PECS), also known as spark plasma sintering (SPS), in the solid state at 1240 °C for 2 min. The microstructure and properties of the PECS material grades are compared with those of pressureless sintered grades, liquid phase sintered at 1420 °C for 1 h. Microstructural and hardness characterization revealed that both the chemical composition and sintering technique play an important role on the WC grain growth and final mechanical properties. To obtain a nanometer sized WC–Co microstructure, it is essential to carefully select the grain growth inhibitor in addition to the application of a fast thermal densification cycle by means of spark plasma sintering.     

Change in relative density of WC-Co cemented carbides in spark plasma sintering process

The relative density of WC-Co cemented carbides during spark plasma sintering (SPS) was analyzed.Based on the change in displacement of the ram in the SPS system,the relative densities in the sintering process can be achieved at different temperatures.The results indicated that densification of the samples started at near 900℃,the density rapidly reached its maximum at the increasing temperature stage,in which the temperature was lower than the sintering temperature of 1200℃,and most of the densification took place in the stage.Besides,the theoretcal values were consistent with the experimental results.     

VC- and Cr3C2-doped WC–NbC–Co hardmetals

This study compares the microstructure and mechanical properties of plain and 0.9 or 3.6 wt% VC- or Cr₃C₂-doped WC–12 wt% Co hardmetals with 40 wt% NbC, prepared by pulsed electric current sintering (PECS) in the solid state for 4 min at 1240 °C and conventional pressureless liquid phase sintering (CS) for 1 h at 1420 °C. The addition of VC or Cr₃C₂ was found to inhibit grain growth of the residual WC grains, whereas the size of the solid solution (Nb,W,V/Cr)C grains was hardly influenced. The type of grain growth inhibitor and densification temperature however, strongly influenced the composition of the NbC solid solution formed, which was thermodynamically and experimentally assessed.     

Tailored sintering of VC-doped WC–Co cemented carbides by pulsed electric current sintering

WC–12 wt.% Co powder mixtures with 0, 0.45 or 0.9 wt.% VC additions were consolidated by solid state pulsed electric current sintering (PECS) for 2 min at 1080–1240 °C. The influence of the sintering condition and VC concentration on the densification, WC grain growth and mechanical properties of the cemented carbides were investigated. Finite element simulation revealed that the radial temperature gradient inside the sintering powder compacts could be homogenised using a carbon felt insulation surrounding the graphite die set-up.     

Microstructure and mechanical properties of ultrafine WC–Ni–VC–TaC–cBN cemented carbides fabricated by spark plasma sintering

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.     

Properties and microstructure of VC/Cr3C2-doped WC/Co cemented carbides

This paper deals with the effects of codoped VC/Cr3C2 and sintering temperature on the magnetic and mechanical properties of ultra-fine grained WC-12%Co alloys. Results show that the synergistic action of doped VC/Cr3C2 in optimal proportion enhances both the hardness and transverse rupture strength (TRS) of the alloys, with more homogeneous microstructure. When the alloy is sintered at 1430℃ and with 0.5% Cr3C2/0.2% VC, the TRS reaches 3786 MPa, the hardness is 91.7 HRA and the grain size srnaller than 0.6 μm. The numerical analyses on grain growth during the sintering process show that both VC precipitating on the WC grain boundary and Cr3C2 dissolving in the Co phase decrease the solid/liquid interfacial energy γ, the process of dissolution and reprecipitation is greatly retarded and the coarsening of WC grains is inhibited.     

Mechanical alloying and spark plasma sintering of the intermetallic compound Ti50Al50

This study investigates the microstructures and mechanical properties of Ti₅₀Al₅₀ alloys prepared via mechanical alloying (MA) starting from elemental powders. The process of the spark plasma sintering (SPS) has also been studied. It is found that the nanocrystallization process of the Ti–Al alloy proceeds and the sintering temperature can control the microstructure of alloy. The sintering of the compacts is carried out at the temperatures of 1100–1200 °C with a compaction pressure of 30 MPa and a heating rate of 30 °C min⁻¹. Specimens with high densities and approaching the equilibrium state can be obtained in short time by spark sintering than conventional sintering. Such shorter high temperature is important to prevent grain growth.     

WC-Co based cemented carbides with large Cr3C2 additions

Five model alloys based on WC-16.6 vol% Co with different amounts of Cr₃C₂ (0–12 vol%) substituting WC were studied. Their microstructures were characterised with X-ray diffraction, scanning electron microscopy and transmission electron microscopy in combination with energy dispersive X-ray analysis. Thermodynamic calculations on the C-Co-Cr-W system were carried out using the Thermo-Calc software. The microstructures were related to previously published results on properties (magnetic coercivity, hardness, transverse rupture strength and indentation fracture toughness) of these materials. The aim of this work was to investigate whether the positive effect of small Cr₃C₂ additions on grain refinement and various properties remains also for large Cr₃C₂ additions. For Cr₃C₂ additions larger than 2 vol%, the most obvious effect on the microstructure was the formation of a chromium- and cobalt-rich M₇C₃ carbide. The Cr/Co ratio of this phase depends on the amount of Cr₃C₂ substituting WC. Some large WC grains were present in the WC-Co material, but not in the materials with Cr₃C₂ additions. Apart from this, no major changes of the WC grain size with Cr₃C₂ content were observed. The partial replacement of tough binder by presumably brittle M₇C₃ carbide might explain the lower toughness of these materials.     

Comparison of ZrB2-SiC ceramics with Yb2O3 additive prepared by hot pressing and spark plasma sintering

This work compared phase composition, microstructures and mechanical properties of ZrB₂—20 vol.% SiC—5 vol.%Yb₂O₃ prepared by hot pressing (HP) and spark plasma sintering (SPS) in vacuum. Despite the same raw material composition, the two densification techniques led to the appearance of different secondary phases. The HP ceramics had coarsened microstructure, predominantly intergranular fracture characteristic and high fracture toughness. The SPS ceramics exhibited refined microstructure, transgranular fracture model and high bending strength.     

Fabrication and mechanical properties of ultrafine grained WC-10Co-0.45Cr3C2-0.25VC alloys

The ultrafine grained WC-10Co-0.45Cr₃C₂-0.25VC alloys were fabricated through planetary ball milling and low pressure sintering. The effects of the cobalt particle size, milling speed and sintering temperature on the microstructure, hardness and fracture toughness of the ultrafine grained alloys were investigated using optical microscopy, scanning electron microscopy and mechanical testing. The results showed that the mechanical properties of the low pressure-sintered alloys substantially depend on the milling speed and sintering temperature. At the same time, the hardness and fracture toughness of the samples can be increased from 1703 MPa and 8.90 MN m^−3/2 to 1789 MPa and 11.21 MN m^−3/2, respectively, when the cobalt particle size is reduced from 17 μm to 1.4 μm.     

Densification behaviour of pure molybdenum powder by spark plasma sintering

Pure molybdenum was sintered with SPS under various temperatures, external pressures and heating rates. The microstructure of the specimens representing the different sintering conditions was investigated by classical metallographic methods. The relative density, the microhardness and the chord length distribution were measured. Linear shrinkage, depending on time or temperature, was calculated from piston travel, which was recorded during sintering process. These results show that the main part of consolidation takes place during fast heating up. The densification behaviour is controlled mainly by sintering temperatures and applied pressure. The molybdenum powder was successfully consolidated by SPS in very short times. A relative density of 95% was reached by sintering temperatures of 1600 °C and external pressure of 67 MPa.     

Effect of VC addition on sinterability and microstructure of ultrafine Ti(C, N)-based cermets in spark plasma sintering

The effect of vanadium carbide (VC) addition on the sinterability and the microstructure of ultrafine Ti(C, N)-based cermets consolidated through spark plasma sintering (SPS) was systematically investigated using optical microscope, scanning electron microscope (SEM) with X-ray energy dispersive spectrometer (EDS), X-ray diffractometer (XRD) and transmission electron microscope (TEM). Our results reveal that the addition of VC increases the porosity of sintering body and depresses the sinterability of Ti(C, N)-based cermets. It is also found that the VC addition has a significant influence on the microstructure of ultrafine Ti(C, N)-based cermets, which inhibits the dissolution of titanium-containing compounds and the formation of inner rim phase and outer rim phase, thus preventing from grain growth. Owing to the depressed dissolution and precipitation, nitrogen liberation is mitigated, therefore resulting in less amount of graphite phase in the samples. In substance, VC changes the solubility of metallic elements in the binder, which makes more elements of Mo and W to be reserved in the binder and thus greatly decreases the content of titanium dissolved into the binder. The re-building solubility rule determines the development of phases and microstructure.     

Densification and mechanical properties of fine-grained Al2O3-ZrO2 composites consolidated by spark plasma sintering

Alumina matrix composites containing 5 and 10 wt% of ZrO₂ were sintered under 100 MPa pressure by spark plasma sintering process. Alumina powder with an average particle size of 600 nm and yttria-stabilized zirconia with 16 at% of Y₂O₃ and with a particle size of 40 nm were used as starting materials. The influence of ZrO₂ content and sintering temperature on microstructures and mechanical properties of the composites were investigated. All samples could be fully densified at a temperature lower than 1400 °C. The microstructure analysis indicated that the alumina grains had no significant growth (alumina size controlled in submicron level 0.66-0.79 μm), indicating that the zirconia particles provided a hindering effect on the grain growth of alumina. Vickers hardness and fracture toughness of composites increased with increasing ZrO₂ content, and the samples containing 10 wt% of ZrO₂ had the highest Vickers hardness of 18 GPa (5 kg load) and fracture toughness of 5.1 MPa m^1/2.     

Hydrogen storage performance of Mg-based composites prepared by spark plasma sintering

The hydrogen storage capacities, hydrogen absorption mechanism and hydride stability of Mg-based composites prepared by spark plasma sintering (SPS) were investigated in this paper. The results showed that the composites had a double-phase microstructure of Mg phase and V-based solid solution, with nanocrystalline magnesium existing at their sintering interface. With the addition of the V-based solid solution in 20% volume fraction, the composite exhibited a maximum reversible hydrogen storage capacity of 4.2 wt.% at 573 K, compared with that of pure Mg of almost zero. DSC results indicated that the hydride decomposition temperature of MgH₂ decreased sharply from 708 K in pure Mg to 636 K and to 591 K as the volume of V-based solid solution increased from 20 vol.% to 50 vol.%. With the addition of V-based solid solution, the hydrogen absorption kinetics of pure Mg was greatly improved at 573 K, and its hydrogen absorption mechanism changed from surface reaction control to diffusion control in the composite. Based on these experimental results, a model was put forward to describe the hydrogen absorption/desorption mechanism in these composites.     

等离子体烧结制备TiNi合金的表征

将高能球磨制备的原子比为1:1的Ti Ni合金粉进行等离子体真空烧结。利用XRD、EDS和SEM对合金粉和烧结样进行了成分与微观形貌的表征,同时对烧结样进行了硬度测试。结果表明:球磨22 h后Ti Ni粉呈非晶态粉末,球磨30 h后的TiNi合金粉发生了明显的固相反应,生成了TiNi、Ni_3Ti、Ti_3Ni_4等物相。等离子体烧结样的物相是Ti Ni,Ni_4Ti_3、Ni_3Ti和Ti_2Ni。平均晶粒尺寸约2μm,平均硬度(HV)达到9000 MPa,自然时效1年后的平均硬度达到6800MPa,是常规电弧熔炼法制备的Ti Ni合金的2~5倍。     

Rapid fabrication of the Ti3SiC2 bonded diamond composite by spark plasma sintering

A mixture of Ti/Si/TiC/diamond powders was employed to fabricate the Ti₃SiC₂ bonded diamond composite using the spark plasma sintering-reactive synthesis method. The addition of diamond does not inhibit the synthesis of Ti₃SiC₂ in the sintered product. In the matrix Ti₃SiC₂ grains developed lamellar morphology with an average length size of 5-10 μm. Ti₃SiC₂ matrix displays good pullout strength with diamond, and the Ti₃SiC₂ bonded diamond material exhibits good wear resistance.