UCLA

Superhard, ultra-incompressible transition-metal borides are exciting candidate materials for applications in cutting, forming, grinding, polishing and wear-protecting coatings. The existence of a network of directional, covalent bonds, together with a high electron density has been suggested as the key to their remarkable mechanical properties. The goal of this work is to examine how variations in bonding changes the mechanical properties of transition-metal borides. To achieve this, high-pressure diamond anvil cell (DAC) techniques are used to correlate mechanical properties with the electronic and atomic structure of these materials in an effort to understand their intrinsic hardness.

This work is divided into two parts: the first uses high-pressure Raman spectroscopy to probe the microscopic bonding structure of rhenium diboride (ReB2), one of the hardest transition-metal boride; the second investigates both elastic and plastic deformations in the inexpensive but still superhard material tungsten tetraboride (WB4) and its solid solutions using synchrotron-based in situ high-pressure X-ray diffractions.

In the first part, we aim to gain an understanding of the correlation between microscopic bonding and macroscopic properties of superhard ReB2. Pressure-dependent Raman spectroscopy and DFT calculations are used to explore lattice vibrations in ReB2. We interpret the results in terms of bond directionality and stiffness to connect hardness with bond character.

In the second part, we focus on a less expensive boride, WB4 and its solid solutions, using in situ high-pressure diffraction techniques. Two types of measurements are described. First, axial X-ray diffraction, where the X-ray beam is parallel to the compression direction and the sample is compressed hydrostatically; second, radial X-ray diffraction, where the incoming X-ray beam is perpendicular to the compression direction and the sample is confined under non-hydrostatic stress. By combining axial- and radial-diffraction measurements, we explore how the atomic network in metal borides evolves elastically and plastically under hydrostatic and non-hydrostatic pressures. With this information, we can understand how the intrinsic bonding in WB4 produces high hardness. More importantly, we can explore how changes to the electronic and physical structure arising from solid solutions formation can result in the remarkable hardness values observed for many complex WB4 based solid solutions.