UC San Diego

Today’s emerging technologies in high temperature applications are placing difficult demands on structural materials. These applications, including hypersonic flight and nuclear energy generation, are often limited by the maximum operating temperature of the structural and coating materials. The current list of materials that can withstand these extreme environments is short and a new material space must be explored to meet current and future demands. High entropy carbides, which comprise complex solid solution mixtures of five or more metals and carbon, represent a new class of ultra-high temperature ceramics that allow for unique properties compared to the traditional binary carbides. In this work, five-component, rocksalt structured, transition metal carbides are synthesized in bulk via mechanical alloying and current and pressure assisted densification and homogenization. The carbides are investigated for phase composition in the pursuit of randomly mixed, single-phase, solid solutions. The effect of entropy on mixing is predicted via a newly formulated entropy descriptor, the Entropy Forming Ability (EFA), that is developed using experimental data and density functional theory. The EFA is then employed in choosing candidate compositions for experimental synthesis. In total, a list of 12 compositions are investigated with 9 forming single-phase and, therefore, high entropy carbides. Phase composition and the extent of mixing is investigated via electron microscopy with energy dispersive x-ray spectroscopy, transmission electron microscopy, x-ray diffraction, and x-ray absorption fine structure. These high entropy carbides allow for electronic structures that are not available to the binary or ternary carbides. Particularly, the use of entropy to stabilize single phase structures allows for the inclusion of elements that are not stable in the rocksalt structure at room temperature. In this work, the inclusion of molybdenum and tungsten into rocksalt structured carbides allows for more electron rich compositions that exhibit increased metallic character, which is described by the increased energy distance between the Fermi level and the pseudo-gap. This parameter is used to predict and tune mechanical properties and leads to the synthesis of compositions with enhanced plasticity and high fracture toughness. Elastic properties, hardness, and fracture toughness are investigated via acoustic wave speed, nanoindentation, and microindentation techniques.