Vacuum sintering of WC-8 wt.% Co hardmetals from WC powders with different dispersity

The effect of sintering temperature and particle size of tungsten carbide WC on phase composition, density and microstructure of hardmetals WC-8 wt.% Co has been studied using X-ray diffraction, scanning electron microscopy and density measurements. The sintering temperature has been varied in the range from 800 to 1600 °C. The coarse-grained WC powder with an average particle size of 6 μm, submicrocrystalline WC powder with an average particle size of 150 nm and two nanocrystalline WC powders with average sizes of particles 60 and 20 nm produced by a plasma-chemical synthesis and high-energy ball milling, respectively, have been used for synthesis of hardmetals. It is established that ternary Co₆W₆C carbide phase is the first to form as a result of sintering of the starting powder mixture. At sintering temperature of 1100-1300 °C, this phase reacts with carbon to form Co₃W₃C phase. A cubic solid solution of tungsten carbide in cobalt, β-Co(WC), is formed along with ternary carbide phases at sintering temperature above 1000 °C. Dependences of density and microhardness of sintering hardmetals on sintering temperature are found. The use of nanocrystalline WC powders is shown to reduce the optimal sintering temperature of the WC-Co hardmetals by about 100 °C.     

Preparation,microstructure and density of nanocrystalline powders of nonstoichiometric vanadium carbide VCy

Nanocrystalline powders of VC_y with different stoichiometry and the average particle size were produced by high-temperature solid-phase synthesis and subsequent milling. The crystal structure, phase composition, microstructure and particle morphology of initial and milled VC_y powders, their specific surface area and density were studied using XRD, SEM, BET method and gas pycnometry. The anisotropy of microstrains in nanocrystalline VC_y powders was experimentally studied by XRD. It was shown that the inclusion of microstrain anisotropy and inhomogeneous broadening in the analysis make it possible to more accurately describe experimental data on diffraction reflection broadening. It was found that microstrains mainly contribute to the broadening of diffraction reflections of VC_y nanopowders produced by milling. The experimental dependence of the pycnometric density of VC_y powders on the specific surface area was established and an expression describing it was obtained. It was shown that the main reason for pycnometric density reduction of the powders with increasing specific surface area is oxygen adsorption on the surface of carbide particles.     

Fabrication of nanocrystalline WC and nanocomposite WC–MgO refractory materials at room temperature

Reactant material powders of pure WO₃, Mg and graphite have been milled at room temperature using a high-energy ball mill. After a few kiloseconds of milling (11 ks), numerous fresh surfaces of the reactant materials are created as a result of the repeated impact and shear forces generated by the balls. After 86 ks of milling, a mechanical solid state reduction is successfully achieved between the fresh Mg and WO₃ particles to form a product of nanocrystalline mixture of MgO and W. A typical mechanical solid state reaction takes place between the W particles and graphite powders to obtain fine grains of nanocrystalline WC. Towards the end-stage of ball-milling (173 ks), the nanocrystalline MgO grains (10 nm) are embedded into the fine matrix of WC to form fine nanocomposite powders (1 μm in diameter) of WC–18% MgO material with spherical-like morphology. This composite powder was then consolidated under vacuum at 1963 K, with a pressure ranging from 19.6 to 38.2 MPa for 0.3 ks, using a plasma activated sintering method. In addition, pure nanocrystalline WC powders (7 nm in diameter) obtained by removing the MgO from the milled powders, using a simple leaching technique have been also consolidated by the same consolidation technique. The consolidation step does not lead to a dramatic grain growth and the compacted samples that are fully dense still maintain their unique nanocrystalline characteristics. The elastic properties and the hardness of both consolidated samples have been investigated. A model for fabrication of refractory nanocrystalline WC and nanocomposite WC–18% MgO materials at room temperature is proposed.     

Effect of annealing on the microstructure,ordering and microhardness of ball milled cubic (L12) titanium trialuminide intermetallic powder

Ball milled powders of cubic (L1₂) titanium trialuminide modified with Mn, possessing nanocrystalline structure, were annealed at 600°C and 1000–1100°C. The best results for the calculation of the nanocrystalline grain size upon annealing from the X-ray diffraction (XRD) patterns, were obtained using the Cauchy/Gaussian approximation for both the instrumental broadening and nanocrystallite size/lattice strain separation. The nanocrystallite size increased upon annealing from 1 to 240 min at 600°C, from the initial several nanometers for the as-milled powders, to 30–140 nm for the annealed powders. This nanocrystalline grain growth is accompanied by a continuous increase of the long-range order (LRO) parameter, from zero to ∼0.8–0.9 after annealing at 600°C for 240 min. However, a phenomenal thermal stability of nanocrystalline grains is manifested in the fact that only very few powder particles exhibited the formation of micrometer-sized grains after annealing at the 1000–1100°C range. The observed differential thermal analysis (DTA) exothermic peaks around 410–430°C (peak I) and 570°C (peak II) are interpreted as the atomic re-ordering and the phase restoration peak, respectively. The observed hardening of the “outer layer” and “no core” particles upon annealing at 600°C is discussed in terms of nanograin boundaries age-hardening mechanism due to the pick-up of interstitials (carbon and/or nitrogen) and their preferential segregation at the nanograin boundaries. The reversal of the process, i.e. desegregation, might be responsible for the observed softening of the “outer layer” and “no core” particles upon annealing at the 1000–1100°C range, without any apparent microstructural changes observable under optical/scanning microscope.     

Effect of milling conditions and binder phase content on liquid phase sintering of heat treatable WC-Ni-Co-Cr-Al-Ti cemented carbides

The binder phase of WC based cemented carbides has been alloyed by adding two different aluminium compounds, AlN and TiAl₃, to mixtures comprised of WC, Ni, Co and Cr₃C₂ powders. A more efficient alloying effect is obtained by TiAl₃ additions likely due to its higher dissolution rate during liquid phase sintering. Shrinkage and melting phenomena are strongly affected by the energy of the milling process and the amount of metallic additions. The use of higher milling rotation speed induces higher oxidation of the powder mixtures and the subsequent formation of a higher volume fraction of alumina particles after sintering. Densification and WC grain growth are hindered by increasing the Al addition. Thus, full densification of alloys with higher Al additions require the use of HIP after standard vacuum sintering cycles. As-HIPed WC-Ni-Co-Cr-Al-Ti samples present a binder phase with precipitation of gamma prime similar to that found in as-cast Ni superalloys. The size, volume fraction and morphology of these precipitates has been modified by applying a standard solution treatment (1150 °C-2 h) followed by fast air cooling and subsequent aging at 600 °C and different dwelling times. Age hardening effects have been confirmed in the composition consisting of WC-12 wt% Co-12 wt% Ni-1.7 wt% Cr₃C₂-5 wt% TiAl₃ after 100 h at this temperature.     

Mechanical properties of WC–10Co cemented carbides sintered from nanocrystalline spray conversion processed powders

Mechanical properties and microstructures of nanocrystalline WC–10Co cemented carbides were investigated. The nanocrystalline WC–10Co cemented carbide powders were manufactured by reduction and carbonization of the nanocrystalline precursor powders which were prepared by spray drying process of solution containing ammonia meta-tungstate (AMT) and cobalt nitrate. The WC powders were about 100 nm in diameter mixed homogeneously with Co binder phase and were sintered at 1375 °C under a pressure of 1 mTorr. In order to compare the microstructures and mechanical properties with those of nanocrystalline WC–10Co, commercial WC powders in a diameter range of 0.57–4 μm were mixed with Co powders, and were sintered at the same conditions as those of nanocrystalline powders. TaC, Cr₃C₂ and VC of varying amount were added into nanocrystalline WC–10Co cemented carbides as grain growth inhibitors. To investigate the microstructure of Co binder phase in the WC–10Co cemented carbides, Co–W–C alloy was fabricated at the temperature of sintering process for the WC–10Co cemented carbides. The hardness of WC–10Co cemented carbides increased with decreasing WC grain size following a Hall–Petch-type relationship. The fracture toughness of WC–10Co cemented carbides increases with increasing HCP/FCC ratio of Co binder phase by HCP/FCC phase transformation.     

Structure and magnetic properties of strontium ferrite anisotropic powder with nanocrystalline structure

The phase composition, nanocrystallite size, lattice microstrain and particles morphology of a SrFe₁₂O₁₉ powder subjected to milling and subsequent annealing were studied by various methods. The investigations showed that the high-temperature annealing of the preliminarily milled powder resulted in the increase in the coercive force (μ₀Н_сi) of the SrFe₁₂O₁₉ powder up to 0.4 T owing to the formation of nanocrystalline structure (D ∼ 10³ nm) with low lattice microstrains. However, the annealed powder cannot be textured in an applied magnetic field because of random orientations of the crystallites in powder particles. A processing technique, which includes the low-temperature annealing of powder in an applied magnetic field, was suggested. It allowed us to produce the anisotropic powder of the strontium ferrite with the nanocrystalline structure that ensures the high coercive force of the powder (∼0.4 T) and possibility of the powder texturing in the magnetic field. The prepared samples textured in a magnetic field exhibit the higher both remanence (by a factor of 1.4) and energy product (by a factor of 2.1) as compared to those of isotropic SrFe₁₂O₁₉ samples.     

Low-temperature perovskite-type cadmium titanate thin films derived from a simple particulate sol–gel process

Low-temperature perovskite-type cadmium titanate (CdTiO₃) with a nanocrystalline and mesoporous structure was prepared at various Ti:Cd molar ratios by a straightforward particulate sol–gel route. The prepared sols had a narrow particle size distribution, in the range 23–26 nm. X-ray diffraction and Fourier transform infrared spectroscopy revealed that the powders contained a mixture of ilmenite-CdTiO₃, perovskite-CdTiO₃, anatase and rutile phases, depending on the annealing temperature and the Ti:Cd molar ratio. Perovskite-CdTiO₃ was the major type obtained from cadmium-prominent powders at low temperature, whereas ilmenite-CdTiO₃ was the major type obtained from titanium-prominent powders at high temperature. It was observed that the anatase-to-rutile phase transformation accelerated with decreasing Ti:Cd molar ratio. Furthermore, the ilmenite-to-perovskite phase transformation accelerated with a decrease in both the Ti:Cd molar ratio and the annealing temperature. The crystallite sizes of the ilmenite- and perovskite-CdTiO₃ phases reduced with increasing the Ti:Cd molar ratio. Field emission scanning electron microscopic analysis revealed that the average grain size of the thin films decreased with an increase in the Ti:Cd molar ratio. Moreover, atomic force microscope images showed that CdTiO₃ thin films had a columnar-like morphology. Based on Brunauer–Emmett–Taylor analysis, cadmium titanate powder containing Ti:Cd = 75:25 showed the greatest surface area and roughness and the smallest pore size among all the powders annealed at 500 °C. This is one of the smallest crystallite sizes and largest surface areas reported in the literature, and can be used in many applications in areas from optical electronics to gas sensors.     

Annealing Ni nanocrystalline on WC–Co

The effects of annealing temperature on nanocrystalline sputter-deposited Ni thin films (500 nm) deposited on WC–Co (4 wt.%) were investigated. Special attention was focused on quantitative evaluation of residual stress and Ni diffusion into the WC–Co, after heat treatment, from 873 to 1273 K. The estimated level of residual stress, as measured by X-ray diffraction, is around −1.3 ± 0.1 GPa for the as-deposited film, whereas after annealing at 1273 K it decreases significantly.Atomic force microscopy shows that high annealing temperature results into an exponential increase of the roughness. An intermixing between the nanocrystalline Ni and the Co from WC substrate occurs, as it is revealed by quantitative depth-resolved Rutherford backscattering spectrometry analysis and also supported by X-ray photoelectron spectroscopy. We ascribe a significant stress relief with the increasing annealing temperature to the diffusion process. The understanding of this process is particularly important in WC–Co parts with the surface treated with Ni in order to improve the maximum surface service temperature.     

Near-nano and coarse-grain WC powders obtained by the self-propagating high-temperature synthesis and cemented carbides on their basis. Part I: Structure,composition and properties of WC powders

Near-nano WC powders with mean grain sizes of about 200 nm were prepared by the SHS method including the reduction of WO₃ by Mg in the presence of carbon and regulating additives. The chemical leaching and refinement of the SHS reaction products allowed one to obtain stoichiometric WC containing only traces of oxygen and magnesium. The thermal reduction of WO₃ and V₂O₅ by magnesium in the presence of carbon resulted in obtaining two carbide phases of WC and complex carbide (W,V)C with the fcc crystal lattice having a grain size of less than 300 nm. It was established that the tungsten oxide reduction by magnesium in the presence of carbon cannot be used to synthesize coarse-grain WC powders. Coarse-grained WC powders were obtained using the W + C mixture heated to high temperatures by a simultaneous exothermic reaction of interaction between magnesium perchlorate Mg(ClO₄) and magnesium. The coarse-grain WC powder synthesized in such a way is nearly stoichiometric and consists of sintered round-shaped agglomerates with the average grain size of up to 16 μm and containing only traces of magnesium and oxygen. The agglomerates comprise WC single-crystals of roughly 1 μm to 8 μm in size.     

Microwave sintering of nanocrystalline WC–12Co: Challenges and perspectives

Microwave sintering of microcrystalline and nanocrystalline WC–12Co powder compacts was carried out employing different time–temperature schedules. The microcrystalline powder compacts were made from powders with particle sizes ranging from 5 to 45 μm by using methyl cellulose as the lubricant. The nanocrystalline powder compacts were made from powders having a mean WC grain size of 38 nm, without employing any lubricant. The sintered samples were characterized with respect to their densities, Vickers hardness, fracture toughness and microstructures and the challenges encountered during microwave sintering of the WC–12Co powders are discussed.     

Synthesis and processing of nanocrystalline tungsten carbide: Towards cemented carbides with optimal mechanical properties

Nanocrystalline tungsten carbide has been obtained by reduction/carburization at low temperature from precursors obtained by freeze-drying of aqueous solutions. Nanocrystalline WC powders with a adequate content of carbon were mixed with submicrometric Cobalt powder (12 wt.%), obtained by same synthesis method, and sintered in vacuum furnace. The cemented carbides fabricated from experimental powders were compared with both commercial ultrafine and nanocrystalline WC-12Co mixtures consolidated by the same route. The synthesised powders were characterized by X-ray powder diffraction, elemental analysis and scanning and high resolution transmission electron microscopy. On the other hand, density, microstructure, hardness and fracture toughness together with X-ray diffraction analysis of the sintered materials were evaluated. The cemented carbides obtained from synthesised powders exhibited a WC platelet-based homogeneous microstructure. This anisotropic growth might be due to the presence of stacking faults parallel to the basal plane in the starting WC powder, which would promote the defect-assisted preferential growth. These materials showed excellent mechanical properties, with a superior hardness/fracture toughness combination compared to materials prepared from commercial mixtures.     

The effect of heat treatment on the structure and magnetic properties of mechanically alloyed Fe-45%Ni nanostructured powders

Magnetic iron-nickel alloys generally called permalloys are of great interest due to their magnetic properties. Fe-45%Ni alloy is one of the major iron-nickel compositions, well-known for high flux density, low coercivity and their responsiveness to the magnetic annealing. In this study, nanocrystalline Fe-45%Ni alloy powders were prepared by mechanical alloying process using a planetary high-energy ball mill under an argon atmosphere. The synthesized powders were heat treated at different temperatures using a vacuum furnace. The structural properties of the as-milled and the post-heat treated powders were studied by means of X-ray diffraction (XRD) technique and transmission electron microscopy (TEM). The magnetic measurements on the powders were carried out using a vibrating sample magnetometer (VSM). The results showed that the lattice strain decreases and the crystallite size increases with annealing temperature. It was also found that the variation of coercivity is dominated by the removal of residual stress at low annealing temperatures, whereas the value of coercivity depends on the crystallite size at higher annealing temperatures.     

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.     

Synthesis and characterization of pure nanocrystalline magnesium aluminate spinel powder

Synthesis of nanocrystalline magnesium aluminate spinel (MgAl₂O₄) by mechanical activation of a powder mixture containing Al₂O₃ and MgCO₃ with subsequent annealing was investigated. Simultaneous thermal analysis (STA), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques were utilized to characterize the as-milled and annealed samples. Results showed that pure nanocrystalline spinel could be fabricated completely by 5 h of mechanical activation with subsequent annealing at 1200 °C for 1 h with a crystallite size of about 45 nm. Further milling had no significant effects on structure or phase composition of spinel phase after subsequent annealing. The nanocrystalline spinel powder obtained after 60 h of milling and subsequent annealing at 1200 °C for 1 h had a crystallite size of about 25 nm according to Williamson–Hall approach and particle sizes smaller than 200 nm. Enhanced mechanical properties were observed in samples prepared from the powder mixture and milled for a longer period.     

铋系高温超导粉体的制备

首先采用喷雾热分解法制备了铋系超导粉体，然后对得到的粉体进行焙烧处理，焙烧时采用不同的工艺参数，包括温度、保护气体的氧分压等。利用XRD，SEM等分析手段，研究了焙烧后粉体的粒度分布，形貌，相组成及其均匀性等。最后，采用粉末套管法将焙烧后的超导粉体制备成单芯超导带材，在形变热处理后，带材样品获得了超过30A的临界电流。     

Effects of fine WC particle size on the microstructure and mechanical properties of WC-8Co cemented carbides with dual-scale and dual-morphology WC grains

Dual-scale and dual-morphology WC grained WC-8Co cemented carbides comprising triangular or hexagonal fine WC grains and plate-like coarse WC grains were synthesized by vacuum sintering using Co, flaky graphite, WC, and coarse W as the starting materials. The effects of fine WC particle sizes on microstructure, relative densities, and mechanical properties of the dual-scale and dual-morphology WC grained cemented carbides were investigated. The results revealed that the growth of plate-like coarse WC grains was further promoted with the decrease in the particle size of the added fine WC; hence, their aspect ratio increased. In addition, added fine WC led to the separation of plate-like coarse WC grains so as to break their oriented arrangement and prevent their face contact; hence, plate-like coarse WC grains were completely covered by the Co binder phase. Moreover, the addition of smaller particle size of fine WC contributed to more uniform Co binder phase. When 0.4-μm WC powders was added, the aspect ratio of plate-like coarse WC grains was greater than that of plate-like WC grained cemented carbides without the addition of fine WC. The dual-scale and dual-morphology WC grained cemented carbides by adding 0.4-μm fine WC exhibited good comprehensive mechanical properties, with a transverse rupture strength of 3645 MPa, a Rockwell hardness of 91.5 HRA, and a fracture toughness of 12.3 MPa∙m^1/2.     

碳化钨颗粒尺寸对超音速火焰喷涂WC-Co涂层形成的影响

通过探讨WC颗粒对扁平粒子厚度及喷涂后WC颗粒尺寸变化的影响，研究了超音速火焰喷涂过程中WC－Co深层的沉积过程。使用具有不同WC尺寸的四种WC-Co粉末，采用JET－KOTE喷枪系统喷制了WC－Co涂层。结果发现涂层中WC颗粒的大小主要取决于原始粉末中WC的尺寸．在粉末穿越火焰的过程中，大多数WC处于固态；WC-Co涂层的沉积涉及固液两相离子的扁平化，而不是象在优化条件下金属或陶瓷材料喷涂过程中仅存在单一液相的情况。很明显WC-Co粉末中的WC的大小对涂层的形成影响很大、在超音速火焰喷涂条件下当液固粒子碰撞到已形成的涂层表面上时，其中的大颗粒WC粒子容易被反弹脱落。基于实验结果，提出了计算由液相聚积固相形成的波固两相颗粒碰撞到表面时形成扁平粒子的厚度的模型。     

Comparative kinetic and structural analyses of nanocrystalline WC powder synthesis from pre-reduced W under pure and diluted CH4 atmospheres

W nanoparticles derived from WO₃ during heating in H₂ were carburized at 900-1100 K in pure, Ar and H₂-diluted CH₄ atmospheres. It was aimed to elucidate carburization behavior of pre-reduced W powders under various atmospheres and to establish optimal conditions for the synthesis of nanocrystalline WC. Weight measurements, XRD and SEM were used to characterize the products at various stages of the reaction. At 900 K, carburization was limited to the formation of W₂C owing to slow C diffusion. At 1000 K, WC particles were obtained within ~ 75 min using the diluted gas mixtures, while under pure CH₄ atmosphere carburization reaction practically stopped due to pyrolytic carbon skin formed on particle surfaces where C supply was more than consumed. WC powders with particle size 40-65 nm and grain size 15-25 nm were synthesized at 1100 K in a short time under the gas atmospheres studied.     

Effect of the high energy milling on the microstructure of Cu-20%WC composite powders prepared with recycled WC

A mixture of Cu-20wt%WC was milled in a high energy mill for up to 50 h. The WC powder was recycled from hard metal scrap. It presents agglomerates of faceted WC particles and residual Co and Fe. In the first hours of milling, the Cu particles are deformed into plates, the agglomerates of WC are broken apart, the primary WC particles are fragmented and embedded in the Cu phase. Composite particles are formed. As milling proceeds, different plates are welded together and folded. The particle size increases. However, the hardened Cu becomes fragile and begin to break, decreasing the particle size. Both, Cu and WC are under compressive strain. Residual Co and Fe present in the recycled powder are dissolved in Cu and behave like the soft Cu phase.