Ultrahard nanotwinned cubic boron nitride and diamond

Ultrahard nanotwinned cubic boron nitride and diamond
Dongli Yu1, Bo Xu1, Wentao Hu1, Yanming Ma2, YanbinWang3, Julong He1, Zhisheng Zhao1,
Bin Wen1, Quan Huang1, Yufei Gao1, Kun Luo1, Zhongyuan Liu1 & Yongjun Tian1
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,
Qinhuangdao 066004, China.
State Key Laboratory for Superhard Materials, Jilin University, Changchun, 130012, China.
Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60439, USA.
Diamond and cubic boron nitride (cBN) are the well-known superhard materials that
have a wide range of industrial applications. Although diamond possesses the highest
hardness in all materials, the lower thermal and chemical stabilities have limited its
applications. cBN has a better stability, but its hardness is much smaller than diamond.
Nanostructuring is an effective way to improve hardness of cBN by virtue of the Hall–Petch
effect. Here we report the formation of cBN with a nanostructure dominated by fine twin
domains of average thickness 3.8 nm. This nanotwinned cBN (nt-cBN) was synthesized from
specially prepared oBN precursor with the onion-like nested structures. The nt-cBN bulk
samples are optically transparent with a striking combination of physical properties such as an
extremely high Vickers hardness exceeding 100GPa, a high oxidization temperature of
1294oC and a large fracture toughness over 12MPam1/2 (well beyond the toughness of
commercial cemented tungsten carbide, 10MPam1/2). Basing on same principle, we have
achieved great success in the direct synthesis of nanotwinned diamond (nt-diamond) with an
average twin thickness of ~5 nm, using a precursor of onion carbon nanoparticles at high
pressure and high temperature, and the observation of a new monoclinic crystalline form of
diamond coexisting with nt-diamond. The pure synthetic bulk nt-diamond material shows
unprecedented hardness, toughness and thermal stability, with Vickers hardness up to ~200
GPa, fracture toughness values of 9.7 to 14.8 MPam1/2 and an in-air oxidization temperature
more than 200 oC higher than that of natural diamond. Our experimental results on
nt-diamond and nt-cBN confirm that there is continuous hardening at nanotwinning sizes
down to 5nm for nt-diamond or 3.8nm for nt-cBN, which is in stark contrast with the sharp
softening of metals at these nanometre scales. We therefore predict that pursuing
microstructure with thinner nanotwin sizes may lead to findings of covalent materials with
even superior properties. The successful syntheses of nt-diamond and nt-cBN show that
nanotwinning microstructure is an effective route for simultaneously enhancing the hardness,
fracture toughness and thermal stability of superhard materials. The new materials possess the
combined advantages of ultrahigh hardness, high thermal stability, and extremely high
fracture toughness, providing a wide range of new industrial applications.
Keywords: nt-diamond, nt-cBN, hardness, fracture toughness, thermal stability