4,861 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
Achieving excellent tribological performance of a-C: WC film by controlling sub-nano-structure
No full textTo study the tribological behavior of a-C: WC films in the marine environment, the a-C: WC films fabricated by reactive magnetron sputter technique with different bias voltages were reciprocally sliding against Si3N4 balls in artificial seawater. Results showed that the a-C: WC film was an amorphous matrix mixed with plenty of short-range ordered structures in sub-nano scale. Excellent tribological performance of the a-C: WC film in seawater could be achieved when the sub-nano-structure possessed excellent overall mechanical properties in combination with fewer growth defects on the surface. The lowest COF and wear rate for a-C: WC film (- 30 V) in seawater were 0.12 and 6.2 x 10(-8) mm(3 )N(-l) m(-1), respectively. The results demonstrated the potential application of a-C: WC films as protective surfaces in marine environments
Achieving excellent tribological performance of a-C: WC film by controlling sub-nano-structure
No full textTo study the tribological behavior of a-C: WC films in the marine environment, the a-C: WC films fabricated by reactive magnetron sputter technique with different bias voltages were reciprocally sliding against Si3N4 balls in artificial seawater. Results showed that the a-C: WC film was an amorphous matrix mixed with plenty of short-range ordered structures in sub-nano scale. Excellent tribological performance of the a-C: WC film in seawater could be achieved when the sub-nano-structure possessed excellent overall mechanical properties in combination with fewer growth defects on the surface. The lowest COF and wear rate for a-C: WC film (- 30 V) in seawater were 0.12 and 6.2 x 10(-8) mm(3 )N(-l) m(-1), respectively. The results demonstrated the potential application of a-C: WC films as protective surfaces in marine environments
Sun dial on south side of tower, St. Clement Danes, Strand, WC
No full textMedium: pencilNot signed. Not dated."Sun dial on south side of tower, St. Clement Danes, Strand, WC" [0000.0913.000.000], Berry, John LionelArtist and Role: Berry, John Lionel,Extent: shee
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
Full text linkDissertação (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
The influence of new road infrastructure on tourism behaviour in Taiwan: the Sun Moon Lake experience
No full text[[abstract]]The construction of transport infrastructure has a considerable impact on regional development. This study examines the behaviour of tourists following the opening of a new freeway to Taiwan's Sun Moon Lake scenic area, including changes to the number of trips, duration of overnight visits, and other areas visited on the same trip. Given that these three types of behaviours are all count data, univariate Poisson lognormal models are used to analyse each of the three tourist behaviours, and then bivariate and multivariate Poisson lognormal models are implemented to examine the interactions between the three tourist behavioural responses. The model results indicate that the age of a tourist, the presence of children, place of residence, mode of transport and travel time have a significant impact on travel behaviour following the opening of the freeway.[[note]]SSC
Tribological responses of the WC/a-C film to sliding against different counterparts in air, water and oil
No full textThe WC/a-C film prepared using a reactive magnetron sputtering system was investigated. The film structure and properties were characterized, and the friction response mechanism between this film and several counterparts was investigated under different environments. The hardness and surface roughness of the counterpart as well as the match between the properties of the counterpart and the film were identified as factors that play a critical role in the friction behaviour. The environment affected the extent of wear through physical and chemical reactions, and mechanical wear with friction chemical reactions was found to be the major mechanism of influence. This study is of great practical interest, as it will help to extend the service life of WC/a-C films and minimize loss during the initial running-in period
Underregistration of neonatal deaths: an empirical study of the accuracy of infantile vital statistics in Taiwan
No full text- …
