The processing of boron carbide powder including sintering optimization, green body optimization and sintering behavior of nano-sized boron carbide was investigated for the development of complex shaped body armor. Pressureless sintered B4C relative densities as high as 96.7% were obtained by optimizing the soak temperature, and holding at that temperature for the minimum time required to reach terminal density. Although the relative densities of pressureless sintered specimens were lower than that of commercially produced hot-pressed B4C, their (Vickers) hardness values were comparable. For 4.45cm diameter and 1.35cm height disk shaped specimens, pressureless sintered to at least 93.0% relative density, post-hot isostatic pressing resulted in vast increases in relative densities (e.g. 100.0%) and hardness values significantly greater than that of commercially produced hot-pressed B4C. The densification behavior of 20-40nm graphite-coated B4C nano-particles was studied using dilatometry, x-ray diffraction and electron microscopy. The higher than expected sintering onset from a nano-scale powder (15008C) was caused by remnant B2O3 not removed by methanol washing, keeping particles separated until volatilization and the carbon coatings, which imposed particle to particle contact of a substance more refractory than B4C. Solid state sintering (1500-18508C) was followed by an arrest in contraction attributed to formation of eutectic liquid droplets of size more than 10X the original nano-particles. These droplets, induced to form well below known B4C-graphite eutectic temperatures by the high surface energy of nano-particles, are interpreted to have quickly solidified to form a vast number of voids in particle packing, which in turn, impeded continued solid state sintering. Starting at 22008C, a permanent liquid phase formed which facilitated a rapid measured contraction by liquid phase sintering and/or compact slumping.

Abstract Densification and mechanical properties (hardness, fracture toughness and flexural strength) of the SiC-TiB2 composite were studied. Pressureless sintering experiments were carried out on samples containing 0 to 50 vol % of TiB2 created by an in-situ reaction between TiO2 and C: 2TiO2 + B4C + 3C 2TiB2 + 4CO Al2O3 and Y2O3 were used as sintering additives to create a liquid phase and promote densification at sintering temperatures ranging from 1820 to 1940oC. The sintered samples were subsequently heat-treated at temperatures ranging from 1850 to 1970oC It was found that the presence of TiB2 formed by the above reaction serves as an effective obstacle to crack propagation thus increasing both the strength and fracture toughness of SiC while maintaining high a hardness of the sintered samples. Densities higher than 98 % TD were achieved depending on both the sintering temperature and heat treatment conditions. From a density viewpoint, the optimum volume fraction of TiB2 was from 12 to 24 vol %. Typical microstructures for samples with this volume fraction of TiB2 consist of TiB2 particles (< 5m) uniformly dispersed in a matrix of elongated SiC plates. The presence of TiB2 particles in the matrix of SiC inhibited exaggerated grain growth of the SiC grains and activated additional toughening mechanisms. The subsequent heat treatment of the sintered samples improved mechanical properties. The optimum sintering and heat treatment temperatures were 1940 and 1970oC, respectively. The maximum flexural strength of 593 MPa was obtained in sample with 12 vol % TiB2. A maximum fracture toughness of 6.6 MPa•m1/2 was measured in samples containing 24 vol % TiB2. While both fracture toughness and strength increased with the presence of TiB2 particles, hardness on the other hand decreased from ~18 GPa in samples without TiB2 to 16.4 and 15.9 GPa in samples with 12 and 24 vol % TiB2, respectively. A theoretical analysis was conducted to model the effect of microstructure on the fracture toughness of SiC-TiB2 composites and was experimentally verified. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2009-09-30 23:04:32.565

No description available.

The class of hyperbolic conservation laws model the phenomena of non-linear wave propagation, including the presence and propagation of discontinuities and expansion waves. Such nonlinear systems can generate discontinuities in the so-lution even for smooth initial conditions. Presence of discontinuities results in break down of a solution in the classical sense and to show existence, weak for-mulation of a problem is required. Moreover, closed form solutions are di cult to obtain and in some cases such solutions are even unavailable. Thus, numerical algorithms play an important role in solving such systems. There are several dis-cretization techniques to solve hyperbolic systems numerically and Finite Volume Method (FVM) is one of such important frameworks. Numerical algorithms based on FVM are broadly classi ed into two categories, central discretization methods and upwind discretization methods. Various upwind and central discretization methods developed so far di er widely in terms of robustness, accuracy and ef-ciency and an ideal scheme with all these characteristics is yet to emerge. In this thesis, novel upwind and central schemes are formulated for various hyper-bolic systems, with the aim of maintaining right balance between accuracy and robustness. This thesis is divided into two parts. First part consists of the formulation of upwind methods to simulate genuine weakly hyperbolic (GWH) systems. Such systems do not possess full set of linearly independent (LI) eigenvectors and some of the examples include pressureless gas dynamics system, modi ed Burgers' sys-tem and further modi ed Burgers' system. The main challenge while formulating an upwind solver for GWH systems, using the concept of Flux Di erence Splitting (FDS), is to recover full set of LI eigenvectors, which is done through addition of generalized eigenvectors using the theory of Jordan Canonical Forms. Once the defective set of LI eigenvectors are completed, a novel (FDS-J) solver is for-mulated in such a manner that it is independent of generalized eigenvectors, as they are not unique. FDS-J solver is capable of capturing various shocks such as -shocks, 0-shocks and 00-shocks accurately. In this thesis, the FDS-J schemes are proposed for those GWH systems each of which have one particular repeated eigenvalue with arithmetic multiplicity (AM) greater than one. Moreover, each ux Jacobian matrix corresponding to such systems is similar to a unique Jordan matrix. After the successful treatment of genuine weakly hyperbolic systems, this strategy is further applied to those weakly hyperbolic subsystems which result on employ-ing various convection-pressure splittings to the Euler ux function. For example, Toro-Vazquez (TV) splitting and Zha-Bilgen (ZB) type splitting approaches to split the Euler ux function yield genuine weakly hyperbolic convective parts and strict hyperbolic pressure parts. Moreover, the ux Jacobian of each convective part is similar to a Jordan matrix with at least two lower order Jordan blocks. Based on the lines of FDS-J scheme, we develop two numerical schemes for Eu-ler equations using TV splitting and ZB type splitting. Both the new ZBS-FDS and TVS-FDS schemes are tested on various 1-D shock tube problems and out of two, contact capturing ZBS-FDS scheme is extended to 2-dimensional Euler system where it is tested successfully on various test cases including many shock instability problems. Second part of the thesis is associated with the development of simple, robust and accurate central solvers for systems of hyperbolic conservation laws. The idea of splitting schemes together with the notion of FDS is not easily extendable to systems such as shallow water equations. Thus, a novel central solver Convection Isolated Discontinuity Recognizing Algorithm (CIDRA) is formulated for shallow water equations. As the name suggests, the convective ux is isolated from the total ux in such a way that other ux, in present case other ux represents celerity part, must possess non-zero eigenvalue contribution. FVM framework is applied to each part separately and ux equivalence principle is used to x the coe cient of numerical di usion. CIDRA for SWE is computed on various 1-D and 2-D benchmark problems and extended to Euler systems e ortlessly. As a further improvement, a scalar di usion based algorithm CIDRA-1 is designed for v Euler systems. The scalar di usion coe cient depends on that particular part of the Rankine-Hugoniot (R-H) condition which involves total energy of the system as a direct contribution. This algorithm is applied to a variety of shock tube test cases including a class of low density ow problems and also to various 2-D test problems successfully. vi

Zirconium carbide (ZrC) powders were produced by carbothermal reduction reactions using fine-scale carbon/metal oxide mixtures as the starting materials. The reactant mixtures were prepared by pyrolytic decomposition of solution-derived precursors. The latter precursors were synthesized via hydrolysis/condensation of metal-organic compounds. The first step in the solution process involved refluxing zirconium alkoxide with 2,4 pentanedione ("acacH") in order to partially or fully convert the zirconium alkoxy groups to a chelated zirconium diketonate structure ("zirconium acac"). This was followed by the addition of water (under acidic conditions) in order to promote hydrolysis/condensation reactions. Precursors with variable carbon/metal ratios were produced by varying the concentrations of the solution reactants (i.e., the zirconium alkoxide, "acacH," water, and acid concentrations.) It was necessary to add a secondary soluble carbon source (i.e., phenolic resin or glycerol) during solution processing in order to obtain a C/Zr molar ratio close to 3 (as required for stoichiometry) in the pyrolyzed powders. The phase development during carbothermal reduction was investigated for oxide-rich carbon-deficient and slightly carbon-rich compositions. The reaction was substantially completed after heat treatments in the range of ~1400-1500oC. The crystallite sizes were in the range of ~100-130 nm. However, some oxygen dissolved in the lattice and some free carbon was present. Heat treatment at temperatures >1600oC was required to complete the reaction. The dry-pressed powder compacts, with varying C/Zr molar ratios, were pressureless sintered to relative densities in the range of ~98-100% at 1950oC.

Densification behaviour and mechanical properties (hardness, fracture toughness and flexural strength) of the SiC-TiC composite system were studied. Pressureless sintering experiments were conducted on samples containing 0 to 30 vol % TiC created through an in-situ reaction between TiO2 and C: TiO2 + 3C -> TiC + 2CO. Sintering of the compacts was carried out in the presence of Al2O3 and Y2O3 sintering additives which promoted densification at sintering temperatures ranging from 1825 to 1925°C. It was determined that the presence of synthesized TiC particles served to effectively toughen the composite through crack deflection, impedance and bridging. An increase in fracture strength and hardness was also observed. Densities in excess of 98 % theoretical density were achieved depending on the sintering conditions and volume fraction of TiC phase. The SiC grain size and morphology was analyzed as a function of TiC volume fraction. The presence of TiC particles in the SiC matrix inhibited the exaggerated grain growth of the SiC grains and activated additional toughening mechanisms. The SiC grains were found to be roughly equiaxed with very fine TiC particles preventing significant elongation. The optimal sintering conditions for room temperature mechanical properties required slow heating through the reaction zone (1300 to 1520°C) followed by a 1 h dwell at 1885°C. At this temperature, the maximum flexural strength of 566 MPa was measured in samples containing 5 vol % TiC. Conversely, a maximum fracture toughness of 5.7 MPa·m0.5 was measured in samples containing 10 vol % TiC sintered at 1900°C. The hardness was shown to increase very little, from ~19.8 GPa in the monolithic SiC samples to 20.1 GPa in samples containing 5 vol % TiC. A theoretical analysis was conducted to model the effect of porosity and grain morphology on the mechanical properties of the SiC matrix and was experimentally verified. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2010-09-21 15:20:02.797

Dans cette thèse, on étudie des systèmes de particules en interaction dont le comportement est lié à certaines équations aux dérivées partielles lorsque le nombre de particules tend vers l’infini. L’équation de Kuramoto-Sivashinsky modélise par exemple la propagation de certains fronts de flamme, la topographie de la surface d’une couche mince en cours de croissance, et fait apparaître des structures macroscopiques. Un modèle de particules en interaction par un couplage harmonique des vitesses, attractif aux premières vitesses voisines, répulsive aux secondes voisines, associée à des collisions élastiques, produit des profils de vitesses analogues aux fronts de flamme. On observe également la création et l’annihilation d’agrégats de particules. Un autre modèle, où les particules fusionnent lors des collisions en préservant masse et quantité de mouvement, et avec uniquement attraction au plus proche voisin, permet de retrouver un modèle de type gaz sans pression avec viscosité. Ces modèles sont étudiés théoriquement, en particulier les facteurs de mise à l’échelle des forces d’interaction sont précisés pour obtenir les équations correctes dans la limite du grand nombre de particules. Des simulations numériques confirment la validité et la pertinence des modèles. / This work is concerned by systems of interacting particles, which are linked to partial derivative equations when the particle number becomes large enough. The Kuramoto-Sivashinsky equation is actually modeling as well the front flame propagation as the morphology of growing interfaces, in deposition, for example. Moreover, surface periodical macroscopic structuring is occurring. An interacting particle model through an harmonic velocity coupling, attractive with the first velocity-neighbor and repulsive for the second neighbors, associated with elestic collisions. This model thus provides us with velocity profiles close to those of front flame propagation. Creation and annihilation of particle clusters is also observed. Another model, where particle are merging during collisions, while retaining mass and momentum conservation and with only nearest neighbor attraction, allows to recover a viscous pressureless gas model. These models are studied using mathematical tools. Especially interaction scaling factors are determined for obtaining the suitable equations in the large particle number limit. The numerical simulations confirm the relevance of the models.

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No description available.

No presente trabalho, peças com gradiente funcional de Al2O3 com reforço de NbC foram planejadas com o intuito de obter um MGF (material com gradiente funcional) com uma alta dureza e boa tenacidade à fratura a partir de diferentes técnicas de sinterização. Os MGFs apresentam-se como uma excelente alternativa quando é necessária a união de materiais com propriedades térmicas ou mecânicas muito diferentes, já que possuem uma transição suave de propriedades ao longo do corpo, como consequência de uma mudança gradual do teor das fases. No planejamento dos MGFs foram utilizadas análises dilatométricas para compatibilizar o comportamento durante a sinterização de cada camada e assim minimizar as tensões que ocorrem durante a retração, responsáveis por trincas e delaminações. Nos ensaios de dilatometria observou-se que a diferença máxima de retração entre os compósitos de teor de NbC variando de 5 a 30 %p é de 4,85%, assim, foram projetados MGFs com um passo de 5% de NbC, reduzindo esta diferença para 2,73%. Compósitos monolíticos de Al2O3 com diferentes teores de NbC foram sinterizadas a 1500ºC num forno convencional sob atmosfera de grafite. As densidades das peças sinterizadas foram inferiores a 90% da densidade teórica (DT), o que comprometeu a dureza dos compósitos (10 a 14 GPa), inferiores que as durezas dos materiais originais. Uma das maiores dificuldades no processamento destes compósitos foi sua densificação, prejudicada devido à presença de partículas de alta refratariedade na matriz de alumina, pelo que foi realizado um estudo do efeito da nióbia (Nb2O5) como auxiliar de sinterização nos compósitos Al2O3-NbC. Utilizando 0,5 %mol deste aditivo foi possível melhorar as densidades dos compósitos que, segundo os resultados da microscopia eletrônica de varredura (MEV), devido a uma densificação em presença de fase líquida. No entanto, a melhora na densidade é efetiva para temperaturas inferiores a 1450°C devido provavelmente à sobrequeima. Devido às dificuldades para obter peças densas a partir desses compósitos, foi utilizado o processo de Spark Plasma Sintering (SPS), o qual foi bem sucedido para obter peças com gradiente funcional com boa densidade (> 98 %TD) e livres de trincas. O gradiente projetado foi mantido com sucesso após a sinterização e, graças à alta densidade das peças, foi possível atingir altas durezas (até 24,3 GPa) e tenacidade à fratura ~5 MPa.m1/2, fazendo com que estes materiais apresentem potencial de aplicação como ferramentas de corte. / In the present work, Al2O3/ NbC graded composites were designed in order to obtain a FGM with a high hardness and good fracture toughness from different sintering techniques. Functionally graded materials (FGM) present enormous potential on matching materials that have different mechanical and thermal properties via a gradual transition throughout the body, as a consequence of a gradual transition of the phase content. For designing of FGMs, a dilatometric analysis was used for matching the sintering behavior of each layer in order to minimize the thermal strains occurring during shrinkage and that are responsible for cracking and delamination. It was observed that the maximum difference in shrinkage between these composites is 4.85%, which could be reduced to 2.73% if a FGM with step of 5% NbC is produced. Monolithic composites of Al2O3 with different amounts of NbC were sintered at 1500°C in a conventional oven under an atmosphere of graphite, the density of the sintered pieces were less than 90 %TD, compromising the hardness of the composites (10 to 14 GPa) that were lower than the hardness of the original materials. Due to the fact that one of the greatest difficulties in processing these composites is a densification, which is impaired due to the presence of high refractory particles in the alumina matrix, a study of the effect of the niobia (Nb2O5) as a sintering aid in the composite Al2O3-NbC was conducted, with addition of 0.5 mol% was possible to improve the density of the composites, due to the liquid state sintering according to the SEM results. However, this effect is only effective until a temperature of 1450°C probably due to the overfiring. In order to obtain dense bodies from these composites, Spark plasma sintering (SPS) was used. This process was successful for producing functional graded bodies with good density (> 98% TD) and crack free. The designed gradient was successfully maintained after sintering and due to the high density of FGMs, it was possible to attain high hardness (up to 24.3 GPa) and a fracture toughness of ~ 5 MPa.m1/2, showing that these materials have a good potential application as cutting tools.