First-Principles Studies On Conductive Superhard Materials

In this dissertation, the first-principles calculation method based on density function theory is used to study the physical properties of conductive super-hard materials, such as their ideal strengths. The conductive materials have wide applications in national defense, industry and daily life, where the hardness or strengths of conductive materials may become vital in their applications. It is of prime importance both theoretically and practically to study and design conductive materials with super- or ultra-hardness.In the first two chapters of this dissertation, we give a briefly introduction to the research background and basic theories of our studies.In chapter three, we present the studies on a new kind of light elements super-hard material, the diamond-like BC₃ structure (d-BC₃). We calculate its tensile and shear strengths under different stain loads. The calculated electronic density of states reveals that d-BC₃ is metallic not only at equilibrium, but also under large tensile and shear deformation. With the analysis of tensile and shear strengths under strains along various high symmetrical directions, we found that d-BC3 structure is one of the hardest conductor studied to date. We also proposed and studied two types of meta-stable layered BC3 structures. One of these layered BC3 structures can become the precursor that will lead to the synthesis of d-BC3 at much reduced pressure and temperature.In chapter four, we present the results of extensive studies on Fe₄N for its potential applications as high density magnetic recording heads and recording materials because of its high hardness and large saturation magnetization. Due to the nearest distance between the strong N-Fe bonding chains in <001> direction in Fe₄N structure, previous theoretical and experimental studies of Fe₄N believe that the (001) plane is the crystal plane with the highest hardness or strength in Fe₄N structure. We carried out first principles calculations of tensile and shear strengths for Fe₄N on its (001), (011), (111) and (112) low index crystal planes. Our results show that the (011) plane of Fe₄N has superior mechanical properties with a shear stress 35% higher than those of other planes, possessing the highest scratching hardness suitable for designing high density magnetic recording heads. Our studies call for experimental efforts to grow high quality single phase Fe₄N films in (011) crystalline direction for designing better high density magnetic recording heads.In chapter five, we study a new type of ultra-hard materials, OsB₂. Synthesizing super-hard materials with only light covalent elements (such as B, C and N) in mass production is usually expensive, since their processes require extremely high pressures and temperatures. Recently, a new design principle is proposed to synthesize ultra-hard materials by combining light covalent elements (such as B, C and N) with electron-rich transition metals to obtain materials with good electric conductivity and superior mechanical strength. Generally speaking, light covalent elements can form strong, directional covalent bonds with high resistance against structural shear deformations, while the high density of valence electrons from transition metals results in high bulk modulus that prevents the material structures from being squeezed together, both of which enhance the resistance of the structures against large inelastic deformations, leading to increased hardness. A primary example and among the first synthesized following this principle is OsB₂, which shows a experimental Vickers hardness of 30 GPa, over that of steel, on its (001) plane, applicable in designing abrasives and cutting tools for ferrous metals as well as scratch-resistance coatings. However, our first- principles calculation results show highly anisotropic shear strength distributions in certain crystalline planes of OsB₂. For instance, the peak shear stress (about 10 GPa) in the [010] direction on the (001) plane of OsB₂ is only one third of that (about 30 GPa) in the [100] direction on the same plane. This prevents OsB₂ from being used as cutting tools for steels. Our calculations demonstrate that simple hardness experimental tests may not accurately detect the strength properties of OsB₂ and highlight the importance of exploring atomistic deformation and instability modes in designing new ultra-hard materials. In chapter six, we study the effects of normal pressures beneath indenters on the hardness of materials in indentation hardness tests. Rhenium diboride (ReB₂), another typical crystal structure recently synthesized by combining covalent elements with transition metals, has attracted considerable interest for its high scratching hardness capable of scratching the diamond surface. However, the measured Vickers hardness (Hv) of ReB₂ is unexpectedly low ranging from 20GPa to 30GPa, which is well below that of diamond (about 100GPa). To explain this seemingly contradictory hardness behavior of ReB₂, we report first-principles calculations of ideal shear strengths of ReB₂ by neglecting the normal pressures beneath indenters (similar to scratch hardness experiment) and including the normal pressures beneath indenters (similar to indentation hardness experiment) in the calculations. Our results show that the normal pressure beneath the indenter indeed reduces the shear strength of ReB₂. The calculated ideal indentation strength of ReB₂ under a Vickers indenter is about 26 GPa, which agrees well ReB₂ with that of experiments. Detailed atomistic physical mechanisms to explain the reduction of indentation strength of 2 by the normal pressures beneath indenters are discussed in the thesis.