University of Limoges, France
Title: Boron carbide-based ceramics for thermostructural application: Sintering by SPS and mechanical performances
Biography: Alexandre Maître
Boron carbide is a promising ceramic in the armor field and in nuclear reactors due to its low weight, its high hardness and its high capacity to absorb neutrons. These excellent properties result from unusual characteristics of B-B and B-C chemical bond. In the literature, there is a general agreement about the existence of solid solubility of carbon with the stable phase BxC and a large range extending from 8 to 20 at.% C. So, the mechanical properties of boron carbide monoliths depend on their chemical composition (i.e. carbide stoichiometry, presence of secondary phases such as free carbon) and on microstructural characteristics (i.e. porosity level, grain size). In the present work, fully-dense boron carbide monoliths exhibiting fine microstructure (i.e. submicrometric grain size) are shaped and sintered by spark plasma sintering. Two different commercial powder batches, exhibiting different stoichiometries and various amounts of secondary phases are used. Their chemical composition is well-defined by coupling different methods (TEM, XRD, IGA) and are correlated with their mechanical properties, characterized from meso- to macro-scopic scales by nano-indentation and ultrasonic pulse echography. The presence of secondary phases (graphite and boric acid) is noticed in various proportions in each powder batch. Their effect on the mechanical features of the corresponding boron carbide-based ceramics has been investigated. So, if the boric acid disappears during the sintering step, the graphite remains. However, for the considered amounts of graphite (lower than 1 wt.%), the low variation in graphite content have no significant effect on hardness and elasticity. At the opposite, the presence of oxygen in solid solution, leading to a boron oxycarbide phase, induces a decrease of both hardness and elasticity.