The 4th International Conference on Matter and Radiation at Extremes (ICMRE 2019)

491 / 2019-03-18 20:21:56

Small Onion-like BN Leads to Ultrafine-twinned Cubic BN

onion-like BN, nanotwinned cBN, size effect, hardness, high pressure and high temperature (HPHT)

Plenary track > High Pressure Physics and Materials Science

Nanotwinned cubic boron nitride (nt-cBN) possessing remarkable hardness, toughness, and stability has attracted widespread attention due to its distinct scientific and industrial importance. The key for nt-cBN synthesis is to adopt an onion-like BN (oBN) nano-precursor and induce phase transition under high pressure. Unfortunately, the technologically important nt-cBN material has not been thoroughly investigated, such as the structure effect of precursor.
In this work, oBN materials with different nanoparticle sizes were prepared, and then subjected to pressure at appropriate synthesis condition to obtain nanopolycrystalline cBN. We found that the size change of oBN used greatly affected the mechanical performance of products. Accompanied by the size decrease of precursor from ~320 nm to 90 nm, the Vickers hardness values of nanostructured products continued to improve from 61 GPa to 108 GPa. This finding could be attributed to the fact that large oBN nanoparticles possessed more flattened, orderly and graphite-like shell layers, in sharp contrast to the highly wrinkled and imperfect layers in small-diameter nanoparticles, thus resulting in the apparent reduction of ultrafine-twin substructure in synthetic products. This study revealed that only small oBN precursor could produce complete ultrafine nt-cBN with outstanding performance. A practical route was proposed to further improve the performance of this technologically important material.