Synthesis and characterization of ultra-incompressible superhard borides.

摘要

The search for ultra-incompressible, superhard materials holds both scientific and practical interest due to their use as abrasives, cutting tools and coatings. The quest for such materials, however, rarely strays from high pressure synthetic methods, which require gigapascals of applied pressure. This dissertation illustrates how light elements can be incorporated into transition metals to replace weak metallic bonds with strong covalent bonds to improve a material's mechanical properties.;By combining rhenium, a high valence electron density metal, with boron, a covalently bonding p-block element, we can synthesize rhenium diboride at ambient pressure. Hardness testing and scratches left on a diamond surface assert the superhard nature of ReB2. High pressure diffraction experiments indicate that ReB2 is ultra-incompressible and able to support a remarkably high differential stress. In an effort to understand the plastic response of ReB2, the indentation size effect observed in hardness measurements is analyzed within the context of conventional plasticity theories. The low load behavior of ReB2 is accurately described by a strain gradient plasticity model. The results suggest that the ability of ReB2 to scratch diamond is explained by its superhard nature at low loads.;The anisotropic properties of ReB2 are studied using crystals prepared from a metal flux. Hardness testing indicates that the (002) plane possesses the highest hardness. The elastic anisotropy is determined using indentation moduli, confirming the hardness anisotropy. Electrical resistivity measurements demonstrate that ReB2 is the hardest metallic material.;In an effort to increase the hardness via particle size effects nanocrystalline ReB2 was consolidated using spark plasma sintering. A linear correlation was found to exist between hardness and pellet density with a maximum predicted hardness of 56 GPa.;The mechanical properties of ultra-incompressible hard borides, Ru 1-xOsxB2, are also studied. Bulk modulus and hardness vary linearly with composition, while the differing behavior among end-member can be explained by relativistic effects, core electron density, and differences in the cohesive energy of the parent metals.;Finally, several projects are summarized that are predicted to form new superhard compounds: Tri-arc crystal growth of borides, ReB2-structured materials, metal tetraborides, and boron-rich crystals.