Transition metal borides have attracted increasing interest due to their physical properties that combine high hardness and electrical conductivity with straightforward ambient pressure synthesis. If designed properly, superhard metallic borides could lead to improved cutting tools and wear-resistant coatings. Rhenium diboride (ReB2) is a good example of this new and growing class of superhard, metallic borides. With support from the Solid State and Materials Chemistry Program, the possibility of improving on the properties of ReB2 by making solid solutions of ReB2 with other transition metals to activate dislocation-pinning mechanisms that could create harder materials will be explored. Less expensive transition metal borides will be synthesized by replacing rhenium with the relatively inexpensive transition metal tungsten. By completely replacing Re with W and increasing the boron concentration, another interesting boride, tungsten tetraboride (WB4), can be made which appears to have comparable hardness to ReB2. After characterizing its physical properties including micro- and nano-indentation, ambient and high pressure X-ray diffraction and resonant ultrasound spectroscopy of both polycrystalline and single crystalline WB4, its structure will be reexamined using neutron diffraction. This powerful tool should enable the determination of the exact positions of the boron atoms in the unit cell, something that is currently not known. This should provide better insight into the bulk modulus, high hardness and other properties of WB4. Making solid solutions of WB4 with other transition metal elements such as Ta, Mo, Cr and Mn, will be carried out in order to potentially increase hardness and impart improved corrosion protection, thermal stability and fracture toughness. A long-term goal of this project is to explore the feasibility of using these superhard, metallic borides for cutting tools. By taking advantage of their electrical conductivity, desired shapes will be cut using electric discharge machining (EDM). Cutting tool inserts will be made by EDM, and properties such as wear resistance will be tested. Coatings will be developed and friction tests carried out. Broader impact of this project will be achieved by engaging undergraduates in the research and providing high school students and teachers with lectures and demonstrations of materials and their applications. NON-TECHNICAL SUMMARY The search for new superhard metals holds both great scientific and practical interest. Scientifically, an understanding of how and why materials known for their malleability can be turned into ultra-incompressible, superhard compounds will be gained. Practically, these metallic materials, no matter how hard, can be cut into precise shapes using a readily available process known as electric discharge machining. This will enable the exploration of their possible use for milling, sawing and drilling. Electric discharge machining will be used to turn these new materials into tools that will be tested for their ability to cut ferrous metals. Scratch resistant coatings with low friction surfaces will also be developed and tested. Broader impact of these projects will be achieved by engaging undergraduates in the research and providing high school students and teachers with lectures and demonstrations of materials and their applications.
