On the Basis of Structure and Chemical Bonding: Solubility and Superhardness of Transition Metal Borides
Materials with superior hardness are desirable in the machining and manufacturing industries, where higher hardness affords higher wear resistance and longer lifetimes for cutting tools and abrasives. Diamond is the hardest known material used for industrial applications. Its structure consists of covalent, directional carbon bonds, resulting in both a high bulk modulus and shear modulus. However, diamond’s use is limited by its cost prohibitive synthesis (requiring both high temperature and high pressure) as well as its reactivity to ferrous alloys (resulting in poor cutting performance and thermal stability above 700 �C). These shortcomings have motivated the search for alternative superhard materials (Vickers hardness, Hv ≥ 40 GPa) that are readily synthesized and capable of cutting materials at lower cost. The creation of new superhard materials has largely developed through an iterative trial-and-error process. One area of exploration is to combine light elements, such as boron, carbon, and oxygen, with highly incompressible transition metals to form covalent bonding networks capable of replicating diamond. More specifically, several transition metal boride systems exhibit exceptionally high hardness, making them an attractive alternative to traditional hard materials for industrial applications.
The primary focus of this dissertation is to examine the structure and bonding parameters required to optimize solid solution formation and grain morphology in new superhard materials. This work begins with an introduction to the factors that contribute to hardness and guide the exploration of transition metal borides. The dissertation centers on the effects of metal atom substitution on the intrinsic hardness of tungsten diboride (WB2) and rhenium diboride (ReB2) solid solutions (Chapters 2 and 4, respectively). Furthermore, secondary phases are observed to extrinsically enhance the hardness and oxidation resistance of the diboride compositions via grain boundary strengthening and precipitation hardening (Chapter 3 and 5). These solid solutions were then compiled into a collective library of various metal borides studied in our group to identify solubility trends across solid solution compositions (Chapter 6). Potential avenues in the field of superhard materials synthesis and discovery are discussed in the final chapter.