Designing New Superhard Metal Borides

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Non-Technical Abstract The creation of, and mastery over, ever-harder materials is an endeavor at least as old as chemistry itself. The importance of hard materials to tools, technology, and the societies they sustain is so self-evident that it is common archaeological practice to name pre-historic human eras by the tooling resources employed, i.e. Stone Age, Bronze Age, etc. In the modern age, superhard materials are needed for high-performance cutting tools, especially for preventing tool damage when cutting advanced ceramics and superalloys. Considering the contributions of the machining industries to the global economy, it is clear that a new generation of cutting tools could have a significant impact worldwide; the world market for superhard materials is projected to reach over $20 billion within the next 5 years. Unfortunately, very few superhard materials exist and those that do (such as diamond) need to be synthesized under extreme pressure and heat, thus making them too expensive for many applications. With support from the Solid State and Materials Chemistry program in the Division of Materials Research, the research team is developing the next generation of superhard materials than can be produced at ambient pressure - superhard metal borides. Training the next generation of materials chemists and bringing the excitement of this project to the general public will provide broader impact. The Principal Investigator serves as the faculty advisor to the Student Members of the American Chemical Society at UCLA. This position has been successfully used to reach many middle and high school students through visits to their schools, fostering a love for chemistry for both undergraduates and K-12 students. The co-Principal Investigator is actively developing new ways to teach science with high school teachers through her position as the Director of Outreach for the California NanoSystems Institute. Each graduate student involved in this proposal helps with outreach projects and mentors undergraduates. The undergraduates will assist the graduate students in research, thereby increasing the future graduate student pool in materials chemistry. Technical Abstract Ten years ago, the Principal Investigators suggested that new superhard materials that could be synthesized at atmospheric pressure (unlike diamond or cubic BN) could be compositionally designed by incorporating covalent bonding into high valence electron density metals. The covalent bonds prevent shear and the electron density adds incompressibility. Transition metal borides exemplify these parameters by providing high valence electron density, multiple boron-boron covalent bonds, and unique crystallographic structures. This idea was first demonstrated with osmium diboride (incompressible), and then with rhenium diboride and tungsten tetraboride (both of which are superhard and incompressible). With support from the Solid State and Materials Chemistry program in the Division of Materials Research, the research team refines and expands on these materials by focusing on structure and bonding motifs. The superhard design toolkit can be enlarged by studying the mechanical properties of new tungsten tetraboride solid-solutions to form dodecaboride-type structures, increasing electron density through doping with iron, and by examining the lower borides of tungsten, which complement the 3-D boron network of tungsten tetraboride by containing 0-D, 1-D, and 2-D boron structures. This family of compounds serves as a model system for understanding how the nature of B-B and B-metal bonds affect hardness. Each composition can be studied using high-pressure radial diffraction to obtain lattice plane specific information about failure and slip. The eventual goal is the development of a third design parameter to explicitly elucidate the structures and bonding motifs that should most improve macroscopic mechanical properties based on microscopic bonding. In this way, the Principal Investigators hope to uncover additional fundamental design rules that can be used to rationally synthesize new members of an increasingly complex family of ultra-hard yet easy to synthesize materials.