3,070 research outputs found
Consolidation of WC-Co nanocomposites synthesised by mechanical alloying
Full text linkA thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the degree of Doctor of PhilosophyThe influence of mechanical alloying (MA) milling time, temperature, sintering method and microstructure on the mechanical properties of a tungsten carbide-cobalt (WC-Co) hardmetal, based on 10wt% Co, has been established. The effects of high-energy milling for 30, 60, 180 and 300 min and the interrelation between milling time and powder properties, and the resultant effects on the mechanical properties of the consolidated WC-10Co material, has been obtained for a horizontally designed ball mill. Nanostructured WC-10Co powder was synthesised after 60 min cyclic milling at room temperature with an average WC domain size of 21 nm. In direct comparison, a WC-10Co composition MA at -30°C for 60 min produced an average WC domain size of 26 nm with a higher lattice strain. WC domain size showed a slight increase with milling time, measured at 27 nm after 300 min ball milling. Extended ball milling (300 min) reduced the mean particle size from 0.148 μm for 60 min milling to 0.117 μm. Thermal analysis showed that the onset temperature of the WC-Co eutectic was related to particle size with increased milling time reducing the onset temperature from 1344°C after 60 min milling to 1312°C after 300 min milling. Onset temperature was further reduced by the addition of vanadium carbide (VC), reducing the onset temperature to 1283°C after 300 min milling. Powder contamination increased with increased milling time with Fe content measured at ~ 3wt% after 300 min ball milling. Milling at -30°C reduced Fe contamination to an almost undetectable level. Increased ball milling time resulted in decreased levels of green density with the powders milled for 30 and 300 min achieving 62.5% and 59.5% TD, respectively. Relative density increased for the powder milled at -30°C compared to the RT milled powder due to its flattened, slightly rounded morphology. A large difference in VC starting particle size compared to WC and Co led to non-uniform dispersion of the inhibitor during milling. Densification and hardness reached optimum levels for the 60 min milled powder for both pressureless sintering and sinter-HIP. Both properties decreased with increased milling time, regardless of the sintering method. Low temperature milling resulted in a higher hardness value of 1390 HV30 compared to 1326 HV30 for the 60 min, RT milled material after pressureless sintering. Densification levels of the doped materials were restricted to < 90% TD for both sintering methods due to inhomogeneity in the microstructures. Palmqvist fracture toughness (WK) of the RT milled powders increased with increased milling time and increasing WC grain size for both sintering methods. WK reached 11.6 MN.m3/2 with 300 min milling after pressureless sintering but reached 16.1 MN.m32 for the same material after sinter-HIP due to the effect of mean WC grain size and binder phase mean free path. The -30°C milled powder exhibited higher fracture toughness for both sintering methods than the 60 min, RT milled material. Spark plasma sintering (SPS) showed that the onset of densification was dependent upon particle size with the powder from 300 min milling showing an onset temperature of ~ 800°C compared to ~ 1000°C for the 60 min milled powder. The low temperature milled powder showed an onset temperature of ~ 980°C, which suggested that low temperature milling provided enhanced densification kinetics
Investigation into laser re-melting of inconel 625 HVOF coating blended with WC
Full text linkHigh velocity oxy-fuel (HVOF) spraying of Diamalloy 1005 powders mixed with WC particles onto steel (304) is considered and laser re-melting of the resulting coatings is examined. Laser re-melting process is modeled to determine the melt layer thickness while temperature increase is formulated using the Fourier heating law. The morphological and metallurgical analyses prior and post laser re-melting process are carried out using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) technique is used to determine the residual stress developed in the coating while the analytical formulation is adopted to predict the residual stress levels at the coating base material interface. The indentation tests are carried out to determine the Young’s modulus and fracture toughness of the coating prior to laser re-melting. Corrosion resistance of coating is measured using potentiodynamic polarization technique prior and post laser treatment process. The predictions of the melt layer thickness are in good agreement with experimental results. The presence of WC particles modifies temperature rise and its gradient in the coating while affecting the Young’s modulus, residual stress levels, and fracture toughness of the coating. The differences in the thermal properties of Inconel 625 powders and WC particles result in formation of small size cellular structure through polyphase solidification. WC dissolution in the central region of the large polycrystalline cells is observed due to the loss of carbon through carbonic gas formation. The results of corrosion tests prevail that significant improvement of corrosion resistance can be achieved after laser treatment process
Influencia de inhibidores de crecimiento de grano en el comportamiento tribológico de carburos cementados WC-Co a partir de polvos nanométricos
Full text linkLos carburos cementados son materiales atractivos para muchas aplicaciones industriales
debido a una combinación de propiedades mecánicas y físicas, estabilidad química y a su
excelente resistencia al desgaste. Los carburos cementados ultrafinos y nanocristalinos están
recibiendo una atención especial debido a su aplicación en el desarrollo de materiales para la
industria electrónica y automotriz. La resistencia al desgaste de estos materiales experimenta
un notable incremento cuando se reduce el tamaño de grano. La reducción en el tamaño de
grano puede obtenerse por la adición de pequeñas cantidades de inhibidores de crecimiento de
grano (especialmente Cr3C2 y/o VC), la selección del proceso y las condiciones de
sinterización.
En esta tesis se evalúa el comportamiento a fricción y desgaste por deslizamiento en seco de
carburos cementados obtenidos de mezclas ultrafinas y nanocristalinas de WC-12%pesoCo
con adición de VC y Cr3C2 como inhibidores de crecimiento de grano. Estas mezclas fueron
consolidadas mediante sinterización convencional en vacío y sinterización por chispa de
plasma. Los ensayos de desgaste por deslizamiento en seco se desarrollaron en un tribómetro
con configuración bola sobre disco utilizando bolas de WC-6Co y AISI 5210 como
contramateriales. Para los ensayos se utilizó como carga de contacto 40N y 60N, distancia de
deslizamiento de 2000m y 10000m, velocidad de deslizamiento de 0.1m/s, condiciones
medioambientales controladas.
Los resultados obtenidos han mostrado que los carburos cementados nanoestructurados
presentan una mayor resistencia al desgaste por deslizamiento en seco que los grados
ultrafinos o submicrométricos. La adición de inhibidores de crecimiento de grano a la mezcla
comercial se ha confirmado como una vía efectiva para incrementar la resistencia al desgaste,
especialmente cuando las proporciones son hasta un 1% peso y se utiliza VC como afinador.
La naturaleza elástica o plástica de las asperezas en contacto se ha manifestado en las
diferencias encontradas en el coeficiente de fricción entre los materiales obtenidos de polvos
ultrafinos y nanométricos. La sinterización por chispa de plasma, SPS, (y las condiciones de
sinterización) resultó ser el método de sinterización con el que se obtienen las mejores
propiedades tribológicas en condiciones de desgaste severo. Los parámetros de sinterización
empleados para el método tradicional, Vacío, no resultaron adecuados cuando las
proporciones de inhibidores exceden el 1%peso.
El estudio de las micrografías de las huellas de desgaste por medio de MEB, EC MEB, EDX,
reveló la coexistencia de varios mecanismos de desgaste que contribuyen al deterioro del
material. Esto se ha relacionado con las propiedades microestructurales y mecánicas de los
carburos cementados, con la naturaleza del contramaterial y con el método de procesado.
Finalmente, se ha demostrado que ejerce más influencia en la resistencia al desgaste de los carburos cementados finos un buen control microestructural que solo el incremento de la
dureza o reducción en el tamaño de grano.Espinosa Fernández, L. (2013). Influencia de inhibidores de crecimiento de grano en el comportamiento tribológico de carburos cementados WC-Co a partir de polvos nanométricos [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/29534TESI
Effect of WC addition on mechanical properties of hot-deformed Nd-Fe-B magnets
No full textAnisotropic Nd-Fe-B magnets with WC addition were prepared by hot deformation process. The effect of WC addition on the compressive and bending strength of hot-deformed Nd-Fe-B magnets has been investigated. The results suggest that the mechanical properties of hot-deformed magnets without WC addition show distinct anisotropy. Meanwhile, WC addition lowers the mechanical anisotropy in two directions parallel and perpendicular to c-axis, especially the compressive strength. The optimum mechanical properties are obtained at 1.0 wt % WC addition without sacrificing the magnetic properties. The observation for microstructure reveals that the fracture mechanism of hot-deformed magnets is a typical intergranular fracture and the crack is prior to propagate along the ribbon boundary. The improved mechanical properties might be attributed to both the grain refinement at ribbon boundaries and the reinforced effect of WC addition. (C) 2017 Elsevier B.V. All rights reserved
Research progress on preparation of cobalt-less/cobalt-free WC-based materials
Full text linkIn view of the scarcity of cobalt resources in China and the deficiency of the traditional WC–Co cemented carbides serving in the corrosive medium and high temperature environments, a series of cobalt-less/cobalt-free WC-based materials with good performance was summarized, including the WC-based cemented carbides with other metals or intermetallic compounds instead of Co, the pure WC, and the WC composites reinforced by ceramic phases. The advantages and disadvantages of the cobalt-less/cobalt-free WC-based materials were discussed, and the development trend was forecasted
Obtaining Crack-free WC-Co Alloys by Selective Laser Melting
No full textAbstractStandard hardmetals of WC-Co system are brittle and often crack at selective laser melting (SLM). The objective of this study is to estimate the range of WC/Co ratio where cracking can be avoided. Micron-sized Co powder was mixed with WC nanopowder in a ball mill to obtain uniform distribution of WC over the surface of Co particles. Continuous layers of remelted material on the surface of a hardmetal plate were obtained from this composite powder by SLM at 1.07μm wavelength. The layers have satisfactory porosity and are well bound to the substrate. The chemical composition of the layers matches the composition of the initial powder mixtures. The powder mixture with 25wt.%WC can be used for SLM to obtain materials without cracks. The powder mixture with 50wt.%WC cracks because of formation of brittle W3Co3C phase. Cracking can considerably reduce the mechanical strength, so that the use of this composition is not advised
Identification of an alkylhydroquinone from Rhus succedanea as an inhibitor of tyrosinase and melanogenesis
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A novel fluorescence-based multiplex PCR assay for rapid simultaneous detection of CEBPA mutations and NPM mutations in patients with acute myeloid leukemias.
No full textAbrasive wear behaviour of detonation sprayed WC-Co coatings on mild steel
No full textThe main objective of the present investigation is to evaluate the abrasive wear behaviour of detonation sprayed WC-Co coatings and to compare them with plasma sprayed WC-Co coatings and carburised and nitrided surfaces. Mild steel was coated with WC-12Co and WC-17Co using detonation and plasma guns. The abrasive wear rates of these coatings were determined at three different loads and compared with wear rates of as received, carburised, and nitrided mild steel. The abraded surfaces and the surfaces beneath the abraded surfaces were characterised by SEM. The results indicate that the detonation sprayed WC-12Co coating has the best abrasive wear resistance at all loads. Maximum improvement in wear resistance is obtained at an intermediate load (i.e. at 5 kg). The wear of detonation sprayed coatings is by abrasion of the soft Co matrix followed by the pullout of hard WC particles
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