Shanlin Hu from the University of California, Los Angeles showed that transition metal borides can behave as incompressible superhard materials. Using rhenium diboride (ReB2) they were able to determine that the hardness increased as its grain size reaches critical values.
Using a grained mixture of solid sodium chloride, rhenium and boron, the team packed the precursors and then heated them in an Argon flow where the salts were molted and allowed to precipitate. Once the sample is washed, they obtained nanocrystals of ReB2 that ranged between 50nm to 100nm depending on the temperature and washing time.
ReB2 was placed in a high pressure diamond anvil while x-ray beams were passed through the sample to obtain information about the changes in structure due to a pressure of up-to 59 GPa.They plotted the differential strain to pressure relation of ReB2 at bulk, 45 nm and 20 nm sizes to find that, as the nanocrystal structure approached 20 nm, the structures had a greater differential strain than the bulk material. This could possibly be because the grain boundary is competing with the grain interior dislocations.
If you want to harden your knowledge of ultra-incompressible superhard materials, please click here to watch Dr. Hu’s 2021 MRS Spring Meeting and Exhibit presentation on ReB2 nanocrystals.