Almost all items used in everyday life are a result of the processing industry, as seen in the machining of modern metals to form appliances and car parts, precision machining parts for phones and computers, and drills for extracting oil to be used to power cars and provide reagents for plastics and chemical synthesis. Our new tools need to be harder and stronger, and as such, boride crystallography plays the dominant role in the search for novel superhard materials. Metal borides encompass a very large variety of possible structures, and are defined by the way the boron atoms are arranged: isolated boron atoms (e.g. Cr4B), single and double chains (e.g. CrB and Ta3B4), networks of boron atoms (e.g. AlB2, ReB2, WB4, ZrB12), and structures based on boron icosahedra, B12 (e.g. ZrB50 and GdB66). ReB2 was the first reported superhard metal but due to the limited supply and cost of rhenium, there has been a great incentive to investigate the possibility of using less expensive metals. This resulted in the cheaper alternative superhard borides: tungsten tetraboride (WB4) and dodecaborides (ZrB12 and YB12). Mechanical properties can be further improved through intrinsic (solid-solution hardening) and extrinsic (morphological control of the surface and grain structure) effects. The metal boride field is now expanding to the use of alloys and solid-solutions hardening with metals analogous to tungsten (e.g. Nb and Ta), thus enhancing the bulk modulus and using structures with a 3D backbone of boron atoms (“cage”-structures) increasing the material’s shear modulus. The field of superhard material compositions can be further expanded by delving into the unstable/metastable phase space stabilizing high-pressure, high-temperature phases (e.g. HfB12 and GdB12) through solid solutions formation at ambient pressure. The structural chemistry, stabilization of high-pressure phases, and properties of the family of MB12 compounds will be discussed.

Rediscovering the crystal chemistry of higher borides

2019 Fall National Meeting

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