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.     

Rapid consolidation of nanostuctured WC-FeAl3 by pulsed current activated heating and its mechanical properties

In the case of cemented WC, Ni or Co is added as a binder for the formation of composite structures. However, the high cost of Ni or Co, the low hardness and the low corrosion resistance of the WC-Ni and WC-Co cermets have generated interest in recent years for alternative binder phases. In this study, FeAl₃ was used as a novel binder and consolidated by the pulsed current activated sintering (PCAS) method. Highly dense WC-FeAl₃ with a relative density of up to 97% was obtained within 2 min by PCAS under a pressure of 80 MPa. The average grain sizes of the sintered WC and WC-FeAl₃ were lower than 100 nm. The addition of FeAl₃ to WC enhanced the mechanical properties and increase of relative density.     

Development of non-eroding rocket nozzle throat for ultra-high temperature environment

The powder processing methods including powder metallurgy (P/M) and powder injection molding (PIM) techniques for tungsten (W)–rhenium (Re) were employed to produce a W–Re rocket nozzle. The composition of W–Re was determined by 25 wt.% of Re to avoid the formatting brittle sigma (σ) phase. The samples for analysis of the densification behavior on sintering were prepared by die pressing and cold isostatic pressing (CIP). The feedstock for the PIM process was produced by mixing the W–25 wt.% Re powder and binder system based on a wax-polymer with an optimum solid loading through the twin-extruder mixer. The injection molded specimens were debound to extract and decompose the binders via the solvent and thermal debindings. The debound samples were sintered in a hydrogen atmosphere. After sintering, hot isostatic pressing (HIP) was carried out in an argon atmosphere to enhance the density.The dilatometry experiments were performed to analyze and predict a densification behavior during sintering. The master sintering curve (MSC) model was used to characterize the densification behavior with a minimal set of preliminary experiments. The mechanical properties were evaluated through microstructure and chemical composition measured by EDX–SEM and X-ray diffraction (XRD).Finally, the eroding test was conducted using the W–25 wt.% Re rocket nozzle produced by PIM under the high temperature. After carrying out erosion tests, the erosion rate, hardness and microstructure were evaluated.     

Effect of Mo2C additions on the properties of SPS manufactured WC–TiC–Ni cemented carbides

The effect of spark plasma sintering (SPS) on the microstructure and mechanical properties of WC–Co and WC–Ni cemented carbides was studied, and compared to WC–Co produced by liquid phase sintering (LPS). There were finer WC grains with larger Co pools in the spark plasma sintered WC–Co, resulting in higher hardness and slightly lower fracture toughness than the liquid phase sintered WC–Co. The influence of the addition of 0.5–5 wt.%Mo₂C to WC-based cemented carbide containing 6.25 wt.%TiC and 9.3 wt.%Ni prepared by SPS was also studied. This addition improved the wettability between WC and Ni and lead to the improvements of microstructures, resulting in good combinations of hardness, fracture toughness and modulus of elasticity that were comparable to WC–Co based cemented carbides.     

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.     

Influence of consolidation process and sintering temperature on microstructure and mechanical properties of near nano- and nano-structured WC-Co cemented carbides

In this paper the influence of the consolidation process and sintering temperature on the properties of near nano- and nano-structured cemented carbides was researched. Samples were consolidated from a WC 9-Co mixture by two different powder metallurgy processes; conventional sintering in hydrogen and the sinter-HIP process. Two WC powders with different grain growth inhibitors were selected for the research. Both WC powders used were near nanoscaled and had a grain size of 150 nm and a specific surface area of 2.5 m²/g. Special emphasis was placed on microstructure and mechanical properties; hardness and fracture toughness of sintered samples. Consolidated samples are characterised by different microstructural and mechanical properties with respect to the sintering temperature, the consolidation process used and grain growth inhibitors in starting powders. Increasing sintering temperature leads to microstructure irregularities and inferior hardness, especially for samples sintered in hydrogen. The addition of Cr₃C₂ in the starting powder reduced a carbide grain growth during sintering, improved microstructural characteristics, increased Vickers hardness and fracture toughness. The relationship between hardness and fracture toughness is not linear. Palmqvist toughness does not change with regard to sintering temperature or the change of Vickers hardness.     

Ti (C,N)-based cermets sintered under high pressure

In this study, high pressure and high temperature sintering (HPHT) is adopted in the cermet fabrication process, and the microstructure and mechanical properties of cermets with TiC_0.5N_0.5–15WC–10Mo₂C–5TaC–10Ni–10Co (wt%) sintered under 5 GPa and different temperatures (900–1600 °C) using 6 × 14 MN cubic press are investigated. Results show that the densities of samples can reach up to 7.00 g/cm³. Vickers hardness and fracture toughness of the products are over 1727 HV30 and 7.2 MPa m^1/2 respectively. In addition, the sintering results are compared with the data that obtained from commercial samples which produced via conventional sintering technique. The conclusion is that high density and high hardness cermets can be obtained through HPHT sintering.     

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.     

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.     

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.     

Diffusion parameters of grain-growth inhibitors in WC based hardmetals with Co,Fe/Ni and Fe/Co/Ni binder alloys

The diffusion behaviour of the grain-growth inhibitors (GGI) Cr and V during early sintering stages from 950 to 1150 °C was investigated by means of diffusion couples of the type WC-GGI-binder/WC-binder. Besides Co, also alternative Fe/Ni and Fe/Co/Ni binder alloys were investigated. It was found that the diffusion in green bodies differs significantly from sintered hardmetals. Diffusivities in the binder phase were determined from diffusion couples prepared from model alloys and were found to be almost equal for Co and alternative binder alloys. The diffusion parameters determined from green bodies allowed to estimate the GGI distribution in a hardmetal during heat up. This was subsequently used to estimate an appropriate grain size of VC and Cr₃C₂ in hardmetals, which is required to ensure a sufficient GGI distribution during sintering before WC grain-growth initiates.     

Sintering behavior of W–30Cu composite powder prepared by electroless plating

Powder metallurgy technique was employed to prepare W–30 wt.% Cu composite through a chemical procedure. This includes powder pre-treatment followed by deposition of electroless Cu plating on the surface of the pre-treated W powder. The composite powder and W–30Cu composite were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Cold compaction was carried out under pressures ranging from 200 MPa to 600 MPa while sintering at 850 °C, 1000 °C and 1200 °C. The relative density, hardness, compressive strength, and electrical conductivity of the sintered samples were investigated. The results show that the relative sintered density of the titled composites increased with the sintering temperature. However, in solid sintering, the relative density increased with pressure. At 1200 °C and 400 MPa, the liquid-sintered specimen exhibited optimum performance, with the relative density reaching as high as 95.04% and superior electrical conductivity of IACS 53.24%, which doubles the national average of 26.77%. The FE-SEM microstructure evaluation of the sintered compacts showed homogenous dispersion of Cu and W and a Cu network all over the structure.     

SiC–Ti layered material prepared by binder-treated powder sintering

Ti–SiC layered material was prepared by binder-treated powder metallurgy method. Ti, SiC and C powders were ball-milled and then binder treated. Mixture of the treated powder was loaded into a stainless steel mold first and followed by loading the pure Ti powder on top of the treated powder, then compacted under a pressure of 200 MPa at 165 °C with a pressing speed of 250 mm/min. The green compacts were debinded at 500 °C for 1 h and sintered at 1500 °C for 2 h under argon atmosphere. The sintering temperature was determined by measuring the phase formation temperatures between Ti, SiC and C powders, using differential scanning calorimetric method. The reaction products after sintering were analyzed. Microstructure of the prepared Ti–SiC layered material was also studied.     

Design of multimaterial processed by powder metallurgy: Processing of a (steel/cemented carbides) bilayer material

A bimaterial, consisting of tough and hard layers, was designed to be processed by powder metallurgy. A materials selection software lead to choose steel and cemented carbide for the tough and hard components. To insure the interlayer compatibility, Fe was selected as base of the tough part and as binder of the hard part. The compositions and the temperature range enabling the simultaneous consolidation of the Fe base by transient liquid phase sintering and of the WC base by liquid phase sintering were defined from thermodynamics data. Experiments lead to define a treatment in two steps: presintering (1080 °C) for the partial densification of the Fe base then sintering (1280–1320 °C) to achieve densification and cohesion of the bimaterial. The sintering temperature increase provided a higher liquid fraction leading to interface cohesion but coarse porosity development. Bimaterial, with fair mechanical properties, were obtained by presintering at 1080 °C then sintering at 1280 °C.     

Sintering of tungsten powder with and without tungsten carbide additive by field assisted sintering technology

Tungsten powder (0.6–0.9 μm) was sintered by field assisted sintering technology (FAST) at various processing conditions. The sample sintered with in-situ hydrogen reduction pretreatment and pulsed electric current during heating showed the lowest amount of oxygen. The maximum relative density achieved was 98.5%, which is from the sample sintered at 2000 °C, 85 MPa for 30 min. However, the corresponding sintered grain size was 22.2 μm. To minimize grain growth, nano tungsten carbide powder (0.1–0.2 μm) was used as sintering additive. By mixing 5 and 10 vol.% WC with W powder, densification was enhanced and finer grain size was obtained. Relative density above 99% with grain size around 3 μm was achieved in W–10 vol.% WC sintered at 1700 °C, 85 MPa, for 5 min.     

High-frequency induction-heated consolidation of nanostructured WC and WC-Al hard materials and their mechanical properties

The rapid sintering of nanostructured WC-Al composites in a short time was investigated with a focus on the mechanical properties (hardness and fracture toughness) and consolidation using high-frequency induction heated sintering. This process allowed very quick densification to near theoretical density and prohibited grain growth in the nano-materials. The addition of Al to WC facilitated consolidation and improved fracture toughness. The hardness and fracture toughness of WC with 5 vol.% Al and WC with 10 vol.% Al composites were higher than those of monolithic WC.     

Nitridation sintering of WC–Ti(C,N)–(Ta,Nb)C–Co hardmetals

Gradient sintering of WC–Ti(C,N)–(Ta,Nb)C–Co hardmetals in vacuum and nitrogen atmosphere was investigated via interrupted sintering experiments and evolving gas analysis. Reduction of surface oxides with corresponding gas evolution was followed by spreading of the binder, shown with coercivity measurements. With low carbon balance, η phases were formed during the intermediate stages of sintering. By introducing nitrogen gas at a later stage all of the η phases could be decomposed again and a graded near-surface microstructure could be formed. If nitrogen is introduced at an early stage when porosity is still open, the TiC powder particles are nitrided, leading to a considerable uptake of nitrogen. Carbon released by this reaction must be balanced by addition of W metal. W and Co binder promote the reaction of TiC with N, whereas TaC and NbC do not. The concept of reaction sintering hardmetal by bulk nitridation was applied on two hardmetal grades and was found an effective way of introducing the nitrogen into such hardmetals.     

Effect of Cu on the microstructures and properties of WC-6Co cemented carbides fabricated by SPS

The aim of this work is to study the effect of Cu on sintering temperature, densification, microstructure and mechanical properties of WC-6Co cemented carbides fabricated by spark plasma sintering (SPS). Fine grained WC powders with an average size of 1.2 μm, were investigated. Microstructures, hardness, fracture toughness and wear resistance of WC-6(Co/Cu) cemented carbides were measured and observed using SEM, mechanical property test. The results show that the sintering temperature of WC-6Co cemented carbides can be decreased obviously with Cu added; addition of Cu reduced grain size to 0.85 μm, but led to lower density. The adding amount of Cu should be controlled within a certain range, and the samples adding the appropriate proportion of Cu can obtain higher hardness and wear resistance.     

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.     

Properties and rapid consolidation of ultra-hard tungsten carbide

Extremely dense WC with a relative density of up to 99% was obtained within 3 min under a pressure of 80 MPa using the high frequency induction heating sintering method (HFIHS) method. The average grain size of the WC was about 87 nm. The advantage of this process is not only rapid densification to obtain a near theoretical density but also the prohibition of grain growth in nanostructured materials. The hardness and fracture toughness of the dense WC produced by the HFIHS were investigated.