De nombreuses études ont montré la faisabilité de la fabrication de pièces composites en carbure de bore et de silicium par l’infiltration de silicium fondu dans une préforme poreuse en carbure de bore (Reaction bonding). Cette méthode permet l’obtention d'un composite fortement chargé en carbure de bore (phase qui nous intéresse pour les applications balistiques), sans pour autant avoir besoin de monter à des températures de frittage de plus de 2200°C (température habituellement utilisée pour fritter le B4C). Dans notre cas la température maximale est comprise entre 1400-1600°C. Cette thèse s’intéresse plus particulièrement à l’adaptation du procédé de « reaction bonding » au chauffage sous champ micro-ondes. Les micro-ondes sont particulièrement intéressantes en ce qui concerne la rapidité du cycle thermique et le chauffage préférentiel de certaines phases (dans le cas des multi-matériaux). Pour ce faire, plusieurs verrous technologiques ont dû être levés (travail sous atmosphère et sous champs électromagnétiques, température élevée, …). Les composites obtenus sont comparés à leurs équivalents en chauffage conventionnel. Des différences microstructurales ont été observées au niveau du SiC formé lors de la réaction. Cette thèse nous a donc permis de :-trouver des conditions de fabrication de pièces en carbure de bore par chauffage micro-ondes (Argon/Hydrogéné10%, légère surpression : 1.4 bars)-montrer que les propriétés mécaniques (dureté, module d’Young,…) obtenues en four micro-ondes sont équivalentes à celles obtenus en four conventionnel (dureté : 14-20GPa) -montrer d’importante différences microstructurales du carbure de silicium formé, entre les échantillons obtenus sous vide (four conventionnel) et ceux obtenus sous atmosphère contrôlée (micro-ondes et four conventionnel).-montrer que le passage à des plus grandes tailles est possible, il est même plus simple d’infiltrer de grandes pièces que de petites à cause de l’effet de la masse sur la réponse du matériau aux champs électromagnétiques des micro-ondes.Ces résultats sont très prometteurs pour des applications balistiques : fabrication de gilets pare-balles et blindages légers. / Many studies have shown the feasibility of processing silicon-boron carbide composite by infiltration of molten silicon through a porous preform made of boron carbide (Reaction Bonding Process). Using this method, the obtained composite contains a large amount of boron carbide, which is the hardest and the most interesting phase for ballistic application. In our developed process, the maximum processing temperature is 1600°C, which is far below the usual high temperature stage/pressure conditions commonly used to sinter B4C by conventional method (respectively 2200°C and40MPa). The main goal of this thesis is to develop a novel reaction bonded process based on microwave heating. Microwaves heating has many interesting features, including fast heating process, selective heating mechanism (in case of heating multi-materials) and volumetric heating distribution. . To fulfill our goal, many technological issues need to be addressed (working in controlled atmosphere and under microwave field, high temperature ...). This thesis reports the development of this novel process, and materials made from it, exhibit similar properties compared to those made conventionally. However, some microstructural differences were observed in SiC resulting phases. This thesis has allowed to-find out the boron carbide composite piece fabrication conditions in microwave cavity (Argon/Hydrogen10%, slight overpressure: 14bars)-show that mechanical properties (hardness, Young’s modulus…) obtained are comparable to those measured on conventionally reaction bonded produced materials. -show that formed SiC has some microstructural peculiarities, between vacuum samples (for conventional) and ones obtained in hydrogenous argon (using microwave).-show that it is possible to produce larger size piece (66mm of diameter). These results are shown to be promising for ballistic applications, including the fabrication of bulletproof jacket and light armor

<p>  </p> <p>The development of a high-temperature heat exchanger made of silicon carbide (SiC) required the development of processing and joining technologies for the fabrication and integration of a prototype. Traditional ceramic forming techniques such as dry powder compaction, tape casting, or injection molding cannot effectively process complex and micron-size parts such as those required by heat exchangers to generate high surface area for improved thermal efficiency. Ceramic co-extrusion has been a successful fabrication technique to produce small structures, ceramic piezoelectric, and fibrous monolithic.</p> <p><br></p> <p>The co-extrusion process is unique in its ability to create micron-size features in two dimensions through multiple reduction steps. Using this process, the heat exchanger channels are developed to create a section with a high surface area to enhance the heat transfer between fluids.</p> <p><br></p> <p>Ceramic co-extrusion requires the development of ceramic/polymer binder systems based on SiC powder, fugitive thermoplastic binders, and low molecular weight polymeric species as processing aids. The thermoplastic binders mixed with SiC powder provided molding and extrusion capabilities to build the heat exchanger prototype. Afterward, a binder removal process and sintering were performed to densify the final component. The presence of cracks is common when working with ceramic/polymer binder systems. Ten different SiC ceramic/polymer binder systems were developed and evaluated to understand the mechanisms that generate cracks and lower the mechanical strengths of components.</p> <p><br></p> <p>A SiC heat exchanger is comprised of a main core where the fluids exchange energy and the manifolds that direct both cold and hot fluids to the respective set of channels. The integration of these components is challenging because of the high degree of covalent bonding and low self-diffusivity of SiC. Welding and other integration methods common in metals are not feasible due to the high melting point of SiC (2730 °C). Reaction bonding is a technique that has displayed the potential to integrate SiC parts by recreating the reaction of silicon (Si) and carbon (C) on an interlayer between SiC components. This work presents the development of a pressureless joining technique for SiC by reaction bonding using SiC/C loaded ceramic suspensions and the methodology to create a successful bonding region between SiC components. The approaches studied varied the thickness in the joint region to study its mechanical strength, and crystalline structure.</p>