2,004 research outputs found
Consolidation of WC-Co nanocomposites synthesised by mechanical alloying
No full textA 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
No full textHigh 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
Thin Nickel Layer with Embedded WC Nanoparticles for Efficient Oxygen Evolution
No full textDeveloping active and stable electrocatalysts for the oxygen evolution reaction (OER) is essential to enhance the efficiency of water splitting. Herein, we report a nickel/tungsten carbide (Ni/WC) composite catalyst in which WC nanoparticles are embedded underneath thin Ni layers as a highly active OER catalyst in an alkaline electrolyte. The thin Ni layer has a modulated electronic structure stemming from the interaction with the WC. The Ni/WC composite exhibits excellent OER activity and durability in a 1 M KOH solution. The turnover frequency of the Ni/WC composite (0.58 s(-1)) is increased by approximately 5.8 times relative to that of the Ni nanoparticles (0.10 s(-1)). The significant increase in catalytic activity of the Ni/WC composite can be attributed to the adsorption property change originating from the interaction between the Ni layers and the WC nanoparticles. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculation results show that the electronic structure of the Ni layers can change due to electron transfers from the WC nanoparticles to the Ni layer across their interfaces. This electronic structure change reduces the kinetic barrier of the rate-determining step of the OER pathway by lowering the O* binding energy that impedes the OER kinetics. As a result, the overall OER on the Ni/WC surface is accelerated. These findings suggest a unique strategy by which to enhance the catalytic activity of the OER in an alkaline electrolyte.
Avaliação da influência de adições de molibdênio, ferro e cromo no comportamento de sinterização e nas propriedades mecânicas do metal duro à base de níquel
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2014.Ligantes alternativos têm sido extensivamente estudados para substituir, total ou parcialmente, o tradicional ligante de cobalto em metais duros. Entre os ligantes comumente adotados, o níquel tem recebido maior atenção devido aos seus aspectos econômicos e similares propriedades mecânicas. Neste trabalho, duas rotas de processamento foram desenvolvidas, envolvendo níquel na forma de óxido e de pó metálico. Os efeitos de adições de molibdênio, ferro e cromo no comportamento de sinterização e nas propriedades mecânicas de metais duros à base de níquel foram investigados. Os compósitos foram processados por metalurgia do pó convencional. As amostras foram sinterizadas em temperaturas de 1400 a 1500 °C. Os sinterizados foram caracterizados microestruturalmente e tiveram seus valores de dureza Vickers e tenacidade à fratura determinados. A composição WC-10Ni desenvolvida apresentou valores de propriedades mecânicas compatíveis com os descritos na literatura. Análises dilatométricas revelaram pequena influência do molibdênio no comportamento de sinterização do metal duro à base de níquel. A adição de ferro e cromo mostrou um deslocamento significativo do início da retração para maiores temperaturas, reduzindo a contribuição da sinterização no estado sólido para a retração total da amostra. A adição combinada de molibdênio, ferro e cromo, também deslocou significantemente o início da retração para temperaturas maiores e indicou maior inibição do crescimento de grão. A presença de carbono em excesso nas composições apresentou forte influência no aumento da densificação. O molibdênio promoveu endurecimento por solução sólida, assim como o ferro e o cromo, mas manteve valores mais elevados de tenacidade à fratura. Dentre as amostras estudadas, as composições WC-9Ni-1Mo e WC-9Ni-0,5Mo-0,5(Fe,Cr) levaram ao melhor compromisso entre o par de propriedades dureza-tenacidade à fratura. Os valores de dureza destes materiais, em torno de 1300 HV10, expressaram-se abaixo do esperado, mas com elevada tenacidade à fratura, potencializando-os para aplicações nos setores de mineração e engenharia civil.Abstract : Alternative binders have been extensively studied to replace, totally or partially, the traditional cobalt binder in cemented carbides. Among the commonly adopted binders, nickel has received the most attention due to its economic aspects and comparable mechanical properties. In this work, two processing routes have been developed, involving nickel oxide and metallic nickel powder. The effects of Mo and Fe-Cr pre-alloyed additions on sintering behavior and mechanical properties of Ni-based cemented carbides were investigated. The composites were processed by conventional powder metallurgy. The samples were sintered at temperatures ranging from 1400 to 1500 °C. Sintered compacts were characterized microstructurally and had their Vickers hardness and fracture toughness determined. The WC-10Ni composition presented values of mechanical properties compatible with those described in literature. Dilatometric analyses revealed little influence of molybdenum on the sintering behavior of nickel-based cemented carbide. The addition of iron and chromium showed a significant shift in the onset of shrinkage for higher temperatures, decreasing the contribution of solid state sintering to the total shrinkage of the sample. The combined addition of molybdenum, iron and chromium, also significantly shifted the onset of shrinkage for higher temperatures and showed greater inhibition of grain growth. High carbon contents had a strong influence on increasing densification. Molybdenum promoted solid solution strengthening, as well as iron and chromium, but molybdenum also maintained higher values of fracture toughness. Among the samples studied, the WC-9Ni-1Mo and WC-9Ni-0,5Mo-0,5(Fe,Cr) compositions led to the better relation between hardness and toughness fracture. The hardness of these materials, around 1300 HV10, was lower than expected, but high fracture toughness was achieved, showing potential for applications in mining and civil engineering sectors
Effect of interferon-gamma on the susceptibility to Fas (CD95/APO-1)-mediated cell death in human hepatoma cells
No full textMany tumors, including hepatocellular carcinomas (HCCs), resist Fas-mediated cell death, which is one of the effector mechanisms in the host's anti-tumor response; however, this resistance can be abolished by interferon-gamma (IFN-gamma). IFN-gamma may sensitize Fas-mediated cell death in several ways, but the exact mechanism in HCCs is uncertain. In this study, we thoroughly investigated the effect of IFN-gamma on the susceptibility of one human normal liver cell line and 12 HCC cell lines to Fas-mediated cell death. We also investigated the effect of IFN-gamma on the expression of various apoptosis-related genes such. as the Fas/TNF-related genes, the bcl-2 family, and the caspase family of genes. Although most cell lines showed considerable constitutive expression of Fas, all tested cell lines resisted Fas-mediated cell death without IFN-gamma. When cells were pretreated with IFN-gamma, only three cell lines were made significantly susceptible to Fas-mediated cell death (SNU-354, SNU-387 and SNU-423); the other 10 cell lines were not affected. IFN-gamma increased the mRNA expression of Fas, TRAIL and caspase-1, and surface Fas was also increased. The strongly sensitized cell lines (SNU-354, SNU-387 and SNU-423) showed a particularly potent increment in surface Fas after IFN-gamma treatment (increase in surface Fas > 1.7-fold). This result enabled us to conclude that a potent increment of surface Fas expression is a major sensitizing mechanism of IFN-gamma. We conclude that IFN gamma cannot play a sensitizing role in most HCC cell lines and that IFN-gamma makes HCC cells susceptible to Fas-mediated cell death through a marked up-regulation of surface Fas in some HCC cells
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
Abrasive 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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