Small onion-like BN leads to ultrafine-twinned cubic BN

Nanotwinned cubic boron nitride (nt-cBN) with 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. Here, we found that the size change of oBN used greatly affected the mechanical performance of products. With the precursor size decreasing from ~320 to 90 nm, the Vickers hardness of nanostructured products improved from 61 to 108 GPa, due 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 the synthetic products. This study reveals 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 important material. 拥有极高硬度、 韧性和稳定性的纳米孪晶立方氮化硼(nt-cBN)在材料领域备受关注. 前期研究表明, 在高温高压条件下以洋葱式氮化硼(oBN)为前驱物是合成nt-cBN的关键. 本文研究发现, 前驱物oBN的粒径变化会显著影响最终产物的组织结构和机械性能. 随着前驱物oBN的粒径从~320 nm减小到~90 nm, 合成的纳米结构块材的硬度从61 GPa逐渐提高到108 GPa. 表明大粒径的oBN拥有和普通六方氮化硼类似的平整、有序的外层结构, 这与小粒径的oBN纳米粒子中存在大量弯曲、褶皱的氮化硼原子层及高密度层错的结构特点形成鲜明对比; 大粒径oBN前驱物中的这些有序结构显著减少了最终产物中超细孪晶亚结构的含量, 从而导致相应产物硬度的下降. 本研究表明, 只有粒径足够小的oBN前驱物才能合成出拥有高密度超细纳米孪晶结构且性能优异的nt-cBN. 为进一步提高此类纳米孪晶材料的性能提供了一种切实可行的方案.