Scientific Achievement
Using high-pressure radial x-ray diffraction at the Advanced Light Source (ALS), researchers found that in the superhard material, rhenium diboride, smaller grain size leads to greater yield strength (i.e., the amount of stress tolerated before permanent deformation).
The cutting edge
As we get better at making robust, durable materials that resist wear, corrosion, and extreme temperatures—think aerospace, automotive, and industrial machining applications—we also need to step up the quality of the tools we use to cut, form, and polish these hardened materials. Researchers have found that transition-metal borides are promising in this regard because of their low costs and advantageous mechanical properties. Metal borides combine highly incompressible transition metals (e.g., tungsten, rhenium, and osmium) with boron, which readily forms strong covalent bonds—a key characteristic of the prototypical superhard material, diamond. In this work, researchers used radial x-ray diffraction at high pressures to gain insight into how grain size affects the nanoscale deformation mechanics of the superhard material, rhenium diboride (ReB2).
Inspired by diamonds
Diamond is a well-known superhard material. But diamond production requires both high temperatures and high pressures, both of which naturally occur deep underground. To replicate the hardness of diamond under ambient conditions, researchers have looked for materials that incorporate two diamond-like characteristics: densely packed electrons and a highly covalent bonding network.
Electron-dense, incompressible elements can be found toward the bottom of the periodic table, but those elements are also metals, characterized by malleability and ductility. A promising strategy is to combine the metals with boron, which readily forms strong covalent bonds. The first example of a superhard metal boride following this design principle was ReB2, consisting of alternating layers of rhenium and boron. While size-induced hardening has been previously studied in softer inorganic materials, size effects in hard materials have been relatively unexplored.
High-pressure radial x-ray diffraction
In this work, the researchers synthesized ReB2 powder samples with grain sizes of 20, 50, and 60 nm. They then used high-pressure radial x-ray diffraction at ALS Beamline 12.2.2—a dedicated high-pressure beamline—to explore the effect of grain size on yield strength, which is directly related to hardness.
Read more on ALS website
Image: Schematic of radial x-ray diffraction under high pressure. A rhenium diboride (ReB2) powder sample is compressed uniaxially, creating differential compressive stresses that provide insight into the strength, deformation mechanisms, and elasticity of the material.