Development of Sustainable Superhard Materials

Hard and superhard materials are essential for a myriad of scientific, biomedical, and industrial applications. Their ability to resist indentation stems from a complex relationship among the crystal structure, chemical composition, and microstructure. One of the main difficulties in modeling the mechanical properties is that hardness is influenced by many factors, which requires extensive calculations to account for the multiple length scales ranging from local atomic interactions to long length scales encompassing microstructure. Consequently, it is not possible to employ one single method to predict the hardness of inorganic solids for various chemical systems. As a result, most known superhard materials have either been discovered through trial-and-error or by following simple design rules limiting the development of the field.

This contribution employs a combination of computational methods to identify hardness theories, machine learning techniques to screen for optimal materials, and experimental tools to verify theoretical predictions in route to developing new superhard materials. First, the influence of vacancies is explored on the mechanical behavior of ultraincompressible hard transition metal sub-nitrides through density functional theory. These studies show the synthetic conditions can be tuned to limit the occurrence of adverse vacancy softening mechanisms. Then, a methodology is developed to screen for hard materials based on high-throughput density functional theory calculations. Sustainability parameters have also been included to ensure the targeted compositions are sustainable. A machine learning method is further developed which allows the screening of more than 118,000 compounds for the highest possible hardness. The procedure highlighted two compounds, ReWC_0.8 and Mo_0.9W_1.1BC, and their subsequent synthesis and characterization confirmed they are both ultraincompressible and nearly superhard. Further examining the solid-solution behavior of Mo_2-xW_xBC showed the variation of hardness and revealed a unique balance between hardness, ductility, and sustainability which can be tuned based on transition metal ratio. Finally, the mechanism of simultaneous ductility and hardness is explored in Mo₂BC using first-principles stress-strain curves and monitoring the electronic perturbation along the deformation path. Together this dissertation provides insights on deformation mechanisms, pinpoints crystal chemical traits that generate high hardness, and provides methodologies to accelerate the advancement of hard and superhard materials.