<p>Cemented carbides based on tungsten carbide and cobalt arecommonly produced by a powder metallurgy route including liquidphase sintering. The pressed compact densifies to almost halfits volume during sintering due to pore elimination. Thesintering behaviour changes with material composition, such ascarbide grain size, binder fraction, carbon content andaddition of cubic carbides.</p><p>This thesis is devoted to the study of constitutivebehaviour, in particular densification, and the microstructuralevolution during cemented carbide sintering. Dimensionalchanges are monitored using dilatometry with and withoutapplied external load. The microstructural evolution isinvestigated with light optical microscopy and scanningelectron microscopy. Thermodynamic calculations are used asreference.</p><p>Constitutive relations are derived for uniaxial viscosity,viscous equivalent of Poisson’s ratio and sintering stressbased on relative density and temperature. The relations areextended to a model describing sintering shrinkage withexplicit dependencies on carbide grain size and binder content.The model is divided in three stages of which two pertain tothe solid state and the third to liquid phase sintering. Solidstate shrinkage is suppressed in a material with coarsecarbides and in the stage of liquid phase sintering grain sizestrongly influences the uniaxial viscosity. The binder contentaffects primarily the later densification.</p><p>The effects of carbon content and grain size distribution onshrinkage have been studied. High carbon content enhancesshrinkage rate, but the effect of grain size distribution israther small. The mean carbide grain size is insufficient todescribe densification for very broad distributions only.</p><p>Shrinkage occurs through rearrangement andsolution-reprecipitation. Rearrangement is studied through theevolution of the pore size distribution and simulatedgenerically using a discrete element method.</p><p><b>Keywords:</b>Cemented carbides, Sintering, Constitutiverelations, Microstructure, Densification, Modelling</p>

Recent studies indicate B4C nanopowder may provide additional advantages without loss of established properties. In this study, preliminary forms of graphite-coated B4C nanopowders on the order of 20-40 nm with various additives were sintered and analyzed. Methanol washing was performed on the powders to remove most of the B2O3 impurity usually present. XRD analysis of the powders verified the nanograined nature and, to some extent, the amount of amorphous material within the powders. A dilatometer furnace was used to track the dimensional changes during sintering, and densities of sintered samples were compared to green compact densities. The onset of sintering occurred at various temperatures depending on the dopant and its amount, most often occurring at higher temperatures than expected. This was likely due first to volatilization of residual B2O3 and then to the graphite coatings of the powders preventing direct B4C-B4C contact. Double-stage sintering, where sintering is either slowed, arrested or reversed and then re-accelerated, occurred in all but one sample. Samples with sintered densities greater than 93% theoretical density were hot isostatically pressed (HIP) with the expectation that the post-HIP density would be 100% theoretical density. Ultimately, post-HIP densities increased less than 2% compared to sintered densities.

Thesis (M.S.)--University of Delaware, 2007. / Principal faculty advisors: Jingguang G. Chen, Dept. of Chemical Engineering; and Thomas P. Beebe, Jr., Dept. of Chemistry & Biochemistry. Includes bibliographical references.

Thesis (Ph.D.)--Texas Christian University, 2009. / Title from dissertation title page (viewed Oct. 30, 2009). Includes abstract. Includes bibliographical references.

Thesis (M.Sc.(Applied Science: Metallurgy))--University of Pretoria, 2009. / Abstract in English. Includes bibliographical references.

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Les carbures de métaux de transition (MC) sont des matériaux réfractaires et présentant une sélectivité optique intrinsèque qui se caractérise par une grande absorbance dans l’uv-visible tout en ayant une faible émittance dans l’infrarouge. Cette particularité leur vaut de faire l’objet de nombreuses études où ils sont envisagés comme matériau absorbeur dans les centrales solaires à concentration (CSP). Cependant, compte tenu des températures de fonctionnement de ces dispositifs (au-delà de 1000°C), les carbures de métaux de transition présentent une limitation majeure liée à leur relative tenue à l’oxydation. L’idée de la présente étude est donc d’associer ce type de matériau au carbure de silicium (SiC) qui à l’heure actuelle est utilisé comme absorbeur de ces technologies notamment du faite de sa grande réfractarité et de sa tenue à l’oxydation (jusqu’à 1400°C). La première partie de la démarche expérimentale a donc consisté en l’identification parmi une série de carbures (HfC, ZrC et TiC) celui présentant les meilleures caractéristiques en terme de sélectivité optique. Puis dans une deuxième partie, différentes voies de synthèse (colloïdale, moléculaire) mettant en œuvre des précurseurs métalliques (alcooxydes, colloïdes) et des sucres, ont été étudiées pour synthétiser des composites de type SiC-MC. L’influence des paramètres expérimentaux ainsi que de la composition ont été étudiées sur d’une part l’aptitude de chaque méthode à conduire aux phases recherchées et d’autre part sur la microstructure ainsi que leurs propriétés optiques. L’analyse de ces dernières a été effectuée d’abord sur des poudres afin de discriminer les échantillons sur la base de leur composition puis sur des pastilles qui ont été obtenues par deux procédés de frittage (HP et SPS) dont l’analyse des résultats constitue la dernière partie de l’étude.Ce travail de recherche a permis de conclure que l’association d’un carbure de métal de transition au carbure de silicium permettait d’aboutir à un composite SiC-MC présentant une certaine sélectivité spectrale et donc susceptible de jouer le rôle d’absorbeur dans les CSP. / Transition metal carbides (MC) belong to the category refractory materials. They have an intrinsic optical selectivity, which is characterized by a high absorbance in the UV-visible region and a low emittance in the infrared range. This feature is at the origin of many studies on these materials where they were expected to play the role of absorber in an Concentrating Solar Power plant (CSP). However, given the operating temperatures of such devices (beyond 1000 ° C), the transition metal carbides have a major limitation related to their relatively low resistance to oxidation. The idea of this study is to associate such material to the silicon carbide (SiC), which currently is used as absorber in CSP systems due to its good thermomechanical properties and resistance to oxidation (up to 1400 ° C).Therefore, the first part of the experimental approach consisted in the identification among a series of carbides (HfC, ZrC and TiC) the one presenting the best characteristics in terms of optical selectivity. Then, in the second part of the study, many synthesis routes (molecular, semi-molecular and colloidal) implementing different metal precursors (alcooxydes and colloidal solution) and a carbon source (sugar) were studied according to their ability to conduct to SiC-MC type composites. The influence of the experimental parameters as well as the one of the chemical composition has been investigated. The aim was first to evaluate the ability of each synthesis routes to conduct to the expected phases and also their impact on the microstructure and the optical properties. These latter have been first studied on powders in order to discriminate the samples on the base of their compositions and then the analysis have been made on densified materials by HP or SPS.This research has permitted to conclude that the combination of a transition metal carbide with silicon carbide conduct to a composite MC-SiC presenting a certain spectral selectivity and that such a material could play the role of absorber in CSP system.

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