Fundamentals and Applications of the World's Hardest Metals
- Author(s): Turner, Christopher Lawrence
- Advisor(s): Kaner, Richard B
- et al.
The application of an advanced material, in this case transition metal borides, requires a fundamental understanding of the materials’ physical and chemical properties. In this thesis, the bonding environment of boron and various borides are investigated, the solubility limits and physical properties of transition metal substitutes to form solid solutions are explored, and new insights in the causes for “superhard” behavior in the boride families are elucidated. Broadening the understanding of boron’s place in borides require a deep look into the environment boron encounters in each structure type. Pure boron exists in various allotropes, predominantly in amorphous (disordered) and crystalline (uniformly ordered), which provide similar x-ray diffraction spectra, but different solid state NMR spectra. The discrepancy arises from the unique bonding environment the boron atoms encounter.
After gaining insight into one of the simplest forms of boron, more complex species with unique structures are analyzed: aluminum diboride (not covered here in detail), rhenium diboride, and tungsten diboride (a hybrid of the two structure types). In the field of solid state NMR, the borides of greatest interest, historically, have been those of common structure type (i.e. AlB2, alternating layers of planar boron sheets and metal atoms) and in some cases unique, such as MgB2 which is superconducting. Other morphologies, like ReB2, a superhard metal (Hv ≥ 40 GPa) had until recent been overlooked; containing similar structural motifs to AlB2, such that they alternate puckered boron sheets and metal atoms, but exhibiting superhard ultra-incompressible characteristics. Traditional characterization of superhard materials involve X-ray diffraction (XRD), neutron diffraction, energy dispersive spectroscopy (EDS), and mechanical hardness testing (Vickers). The x-ray diffraction and neutron diffraction analyses provide structure information—atomic coordinates—within the unit cell. The coordination of the atoms within the structure can be deduced from the data provided by crystallographic (eg. XRD) means; this coordination may then be corroborated with NMR data. First principles calculations are growing in popularity, but simulating the more complex structure types is far from trivial, especially when randomly distributed partial occupancies and partial vacancies exist. Moving towards an application of these novel materials requires fine-tuning of the chemical and physical properties they express. These materials must be hard, tough, readily wettable, and resistant to oxidation. Therefore, the application of these compounds requires the fundamental understanding of how the properties come to be. Hybrid materials, in the form of solid solutions, allow us to evoke the desired traits. Solid solutions are materials containing a partial substitution of a parent element by another element within the structure; molybdenum partially substituted for tungsten in the tungsten tetraboride structure (WB4) would become W1-xMoxB4 where x is the percent substituted. Adding molybdenum (or any transition metal) has its limits, but as seen in a subsequent chapters, the change in hardness and oxidation resistance is in some cases significant. The emphasis on producing next generation materials for any application lies within knowing the capabilities and limitations of the material.