Search

Recent biomedical engineering development has made temporary implants not only possible but a certain widespread reality in the short term. These implants can be used in the healing process of bones or as vascular stents. Inside the patient, they are progressively degraded and absorbed, completely disappearing after fulfilling their purpose. Notable amongst their many advantages is the avoidance of an extraction surgery. Magnesium, an essential biological element with low density but good mechanical properties, similar to those of human bones, is a perfect candidate for implant applications. However, Mg has to be alloyed to tailor its corrosion and mechanical properties to match specific requirements and prevent premature mechanical failure. Unfortunately, a usual alloying element of Mg, aluminium, is suspected to be related to cancer and Alzheimer’s occurrence. Therefore Al-free Mg-based alloys are the scope of this work. In this sense, biocompatible alloying elements were added to Mg in small quantities intending to improve corrosion and mechanical behaviour thanks to the solid solution phenomenon, aiming to avoid detrimental excessive formation of secondary phases. Ca, Mn and Sr were the alloying elements used in this work, all three essential nutrients. More specifically, two ternary (Mg-Ca-Mn, Mg-Mn-Sr) and a quaternary (Mg-Ca-Mn-Sr) alloy systems were studied. Favourable alloy compositions were predicted and determined through thermodynamic calculations. Microstructural and mechanical behaviour analyses were performed on as-cast produced samples. After characterization, a correlation between mechanical properties, composition phase content and grain size was obtained. Based on this relation, recommended values for alloying elements were obtained. These are 0.5wt.% Ca, 0.5wt.% Mn and 0.75wt.% Sr. At these percentages solid solution strengthening occurs. A higher percentage of alloying elements leads to appearance of secondary phases that can be deleterious for

The SPS technique (Spark Plasma Sintering), also known under the acronym FAST (Field Assisted Sintering Technique) is a rapid sintering method in which a uniaxial load and a high intensity pulsed current are simultaneously applied. The current can pass through the die and the punches made in a conductive material and through the powder to densify, if it is electrically conductive, and heats the system by Joule effect at very high heating rates up to one thousand degrees per minute. These particular sintering conditions allow to sinter a broad range of materials very quickly and at lower temperatures than in conventional sintering. This technique is original and quite recent. However, there exists a misunderstanding of the involved mechanisms. For example, the existance of sparks or plasma in the powder to densify has not yet been prooved. That is why the acronym FAST is probably more accurate than the acronym SPS. In this work, we try to evaluate what are the real advantages and limitations of the SPS technique comparing with more conventional sintering techniques (Hot Pressing). The first part of this document concerns a parametrical study of the technique based on tests realised on alumina. This well-known material is often used as a reference in the study of solid-state sintering. We also evaluate the influence of the electrical and thermal properties of the material to sinter by the study of the densification of composites. The systems chosen for this part of the thesis are Al2O3-TiC and AlN-TiC, taking into account the high electrical conductivity of TiC and the high thermal conductivity of AlN. Moreover, in the applied sintering conditions, no reaction occurs between the different phases of the composites. We try to give hypotheses concerning the sintering mechanisms and the potential effects of the electric current and/or field by using an isothermal analysis method but also the master sintering curves theory (non-isotherm) wich leads to the determination of, La technique SPS (Spark Plasma Sintering), également connue sous l’acronyme FAST (Field Assisted Sintering Technique) est une méthode de frittage rapide dont le principe consiste en l’application simultanée d’une pression uniaxiale et d’un courant pulsé de grande intensité. Ce courant traverse le moule et les pistons en matériau conducteur et la poudre à densifier, pour autant qu’elle soit conductrice, et chauffe le système par effet Joule jusqu’à des vitesses pouvant atteindre mille degrés par minute. Ces conditions de frittage particulières permettent de densifier rapidement bon nombre de matériaux en un temps relativement court et à des températures généralement plus faibles qu’en frittage conventionnel. Cette technique de frittage est originale et relativement nouvelle. Cependant, les mécanismes sous-jacents ne sont pas encore bien compris/connus à l’heure actuelle. Ainsi, l’existence, dans la poudre à densifier, d’étincelles ou d’un plasma au cours du frittage, est toujours sujet à débat, raison pour laquelle l’acronyme FAST devrait, en principe, être préféré à l’acronyme SPS. Dans ce travail, nous tentons d’évaluer quels sont les réels avantages et limitations de la technique SPS en la comparant à des méthodes de frittage plus conventionnelles (Hot Pressing). La première partie du document concerne donc une étude paramétrique de la technique sur base d’essais réalisés sur l’alumine. Ce matériau bien connu est souvent utilisé comme référence dans l’étude du frittage à l’état solide. Nous évaluons aussi l’influence des propriétés électriques et thermiques des matériaux à fritter en étudiant la densification de composites. Les systèmes choisis sont Al2O3-TiC et AlN-TiC, tenant compte de la conductivité électrique élevée du TiC et de la grande conductivité thermique de l’AlN. En outre, dans les conditions expérimentales choisies, aucune réaction ne se produit entre les différentes phases des composites. Nous tentons de dégager des pistes concernant les mécanismes, (FSA 3) -- UCL, 2011

Bulk alumina foams were generated by slip casting of suspensions combined with particle-stabilized emulsion droplets as pore templates. Because of the volatile nature of the template, the green foams could be consolidated by sintering without a debinding step. The porous structure of the consolidated foams was analyzed nondestructively using image analysis of microfocus computer tomography (micro-CT) data. The fraction of open and closed pores, as well as the pore sizes and pore size distributions, were calculated from the generated 3D micro-CT datasets and correlated to the applied processing parameters. The anisotropy in the produced structures was assessed and elucidated by a detailed analysis of segmented tomography data. The results indicate that the width of the pore size distribution and the average macropore size, ranging from 225 to 136 mu m, depend on the slip rheology. Any anisotropy caused in the structure was linked to the template preparation but is only present in a narrow zone near the bottom of the foams. Although the macroporosity (>= 20 mu m) was only 24%-31%, tomographic analysis showed that over 90% of these macropores were interconnected through >= 20-mu m-diameter windows.