title: Ultrahard nanotwinned cubic boron nitride.
abstract: Cubic boron nitride (cBN) is a well known superhard material that has a wide range of industrial applications. Nanostructuring of cBN is an effective way to improve its hardness by virtue of the Hall-Petch effect--the tendency for hardness to increase with decreasing grain size. Polycrystalline cBN materials are often synthesized by using the martensitic transformation of a graphite-like BN precursor, in which high pressures and temperatures lead to puckering of the BN layers. Such approaches have led to synthetic polycrystalline cBN having grain sizes as small as ∼14 nm (refs 1, 2, 4, 5). Here we report the formation of cBN with a nanostructure dominated by fine twin domains of average thickness ∼3.8 nm. This nanotwinned cBN was synthesized from specially prepared BN precursor nanoparticles possessing onion-like nested structures with intrinsically puckered BN layers and numerous stacking faults. The resulting nanotwinned cBN bulk samples are optically transparent with a striking combination of physical properties: an extremely high Vickers hardness (exceeding 100 GPa, the optimal hardness of synthetic diamond), a high oxidization temperature (∼1,294 °C) and a large fracture toughness (>12 MPa m(1/2), well beyond the toughness of commercial cemented tungsten carbide, ∼10 MPa m(1/2)). We show that hardening of cBN is continuous with decreasing twin thickness down to the smallest sizes investigated, contrasting with the expected reverse Hall-Petch effect below a critical grain size or the twin thickness of ∼10-15 nm found in metals and alloys.
authors: State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China.
authors: Yongjun
authors: Y
authors: Tian
authors: Bo
authors: B
authors: Xu
authors: Dongli
authors: D
authors: Yu
authors: Yanming
authors: Y
authors: Ma
authors: Yanbin
authors: Y
authors: Wang
authors: Yingbing
authors: Y
authors: Jiang
authors: Wentao
authors: W
authors: Hu
authors: Chengchun
authors: C
authors: Tang
authors: Yufei
authors: Y
authors: Gao
authors: Kun
authors: K
authors: Luo
authors: Zhisheng
authors: Z
authors: Zhao
authors: Li-Min
authors: LM
authors: Wang
authors: Bin
authors: B
authors: Wen
authors: Julong
authors: J
authors: He
authors: Zhongyuan
authors: Z
authors: Liu