LETTER doi:10.1038/nature13381 Nanotwinned diamond with unprecedented hardness and stability Quan Huang1*, Dongli Yu1*,BoXu1*, Wentao Hu1*, Yanming Ma2, Yanbin Wang3, Zhisheng Zhao1, Bin Wen1, Julong He1, Zhongyuan Liu1 & Yongjun Tian1 Although diamond is the hardest material for cutting tools, poor synthesized from C60, amorphous carbon and glassy carbon, but Knoop thermal stability has limited its applications, especially at high tem- hardness decreases significantly to 70286 GPa (ref. 6). The observed peratures. Simultaneous improvement of the hardness and thermal hardness deficiency seems to originate from intergranular fracturing stability of diamond has long been desirable. According to the Hall2 along poorly sintered grain boundaries, rather than the reverse Hall2 Petch effect1,2, the hardness of diamond can be enhanced by nanostruc- Petch effect resulting from grain-boundary sliding6. Technically, the turing(bymeansof nanograinedand nanotwinned microstructures), synthesis of well-sintered nanograined diamond while maintaining a as shown in previous studies3–7. However, for well-sintered nano- smaller grain size remains a challenge. grained diamonds, the grain sizes are technically limited to 10230 nm Nanotwinning is an effectivemechanism for acquiring a smaller char- (ref. 3), with degraded thermal stability4 compared with that of natural acteristic size of microstructure, because twin boundaries possess lower diamond. Recent success in synthesizing nanotwinned cubic boron excess energy than grain boundaries. It has been verified experimentally nitride (nt-cBN) with a twin thickness down to ,3.8 nm makes it that, at nanoscale, twin boundaries show a hardening effect identical to feasible to simultaneously achieve smaller nanosize, ultrahardness those of grain boundaries for metals11,12. Ubiquitously nanotwinned struc- and superior thermal stability5. At present, nanotwinned diamond tures have been introduced into superhard materials through the success- (nt-diamond) has not been fabricated successfully through direct con- ful synthesis of nt-cBN with an average twin thickness of ,3.8 nm at versions of various carbon precursors3,6,7 (such as graphite, amorphous HPHT5. These nt-cBN bulk samples have a superior combination of 5 carbon, glassy carbon and C60). Here we report the direct synthesis high hardness, high toughness and high thermal stability . The syn- of nt-diamond with an average twin thickness of ,5 nm, using a thesis of nt-diamond has not yet been reported but is highly desirable precursor of onion carbon nanoparticles at high pressure and high in view of the excellent performance of nt-cBN. temperature, and the observation of a new monoclinic crystalline Experience in the synthesis of nt-cBN through an onion-like BN pre- form of diamond coexisting with nt-diamond. The pure synthetic cursor suggested the use of onion carbon as precursor in the fabrication bulk nt-diamond material shows unprecedented hardness and ther- of nt-diamond. Onion carbon, a high-energy metastable carbon consist- mal stability, with Vickers hardness up to ,200 GPa and an in-air ing of concentric graphite-like shells (Extended Data Fig. 1), is structur- oxidization temperature more than 200 6C higher than that of nat- ally similar to onion-like BN and can be produced in large amounts13.A ural diamond. The creation of nanotwinned microstructures offers a high concentration of puckered layers and stacking faults in onion car- general pathway for manufacturing new advanced carbon-based ma- bon may provide the key for the nucleation of nt-diamond at HPHT, as terials with exceptional thermal stability and mechanical properties. for nt-cBN5. In fact, isolated onion carbon particles have been observed Diamond is the hardest, stiffest and least compressible crystalline toconvert into diamond nanocrystalsunder intense electron irradiation material with exceptionally high thermal conductivity. Tools made of even at ambient pressure14. diamond are widely used for cutting and shaping hard substances such The onion carbon nanoparticles (,20250 nm in diameter) used in as stones, glasses and ceramics. However, diamond is energetically unsta- our study were characterized by transmission electron microscopy (TEM) ble relative to graphite under ambient conditions, with an inherent draw- to contain numerous puckering and stacking faults (Fig. 1a). X-ray dif- back of poor thermal stability. In air, the onset oxidation temperature fraction (XRD) characterization of the onion precursors and recovered is ,800 uC for natural diamond8,9, resulting in the severe wear of dia- samples after HPHT treatments is presented in Extended Data Figs 2 mond tools at high temperatures. and 3. The inter-shell spacings of untreated onion carbon were centred The synthesisof materialsharder than natural diamond haslong been on 0.3485 nm. When treated below 10 GPa and 2,000 uC, onion carbon sought10. The Hall2Petch relation1,2 offers a general pathway to enhan- retained the original nested crystal structure. Samples recovered from cing hardness by decreasing characteristic size of microstructures (for 10215 GPa and 1,400–1,850 uC were black and opaque (Fig. 1b inset), example grain size or twin thickness). Nanograined diamond has been and contained cubic diamond and an unidentified carbon phase. This successfully synthesized through direct conversions of certain carbon latter phase has not been observed before and seems to be inherently precursorsat high pressureand hightemperature (HPHT)3,6,7. The pres- related to the specific structural transformation of onion carbon precur- sure and temperature conditions6 needed to synthesize nanograined dia- sors at HPHT. Transparent samples were recovered from 18225 GPa monds are much higher than those for growing single-crystal diamonds and 1,85022,000 uC, with pure cubic diamond as indicated by the XRD in the industry. High pressure is necessary to control grain size effec- patterns. The synthetic temperature of cubic diamond from onion car- tively by suppressing atomic diffusion, which promotes growth. Nano- bon was ,450 uC lower than that from graphite3,6, allowing easier indus- grained diamonds synthesized from pure graphite at 2,30022,500 uC trial fabrication. and 12225 GPa reach a grain size of 10–30 nm, with a high Knoop Typical TEM and high-resolution TEM (HRTEM) images of a black hardness of 110–140 GPa (ref. 3) but a reduced onset oxidation tem- opaque sample (synthesized at 10 GPa and 1,850 uC) are shown in perature of ,680 uC in air4. At lower temperatures (,1,800 uC), nano- Fig. 1b, c and Extended Data Fig. 4a, b. Cubic diamond was the dom- grained diamonds with a smaller grain size (5210 nm) have been inant phase, with lamellar {111} nanotwins. The new secondary carbon 1State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China. 2State Key Laboratory for Superhard Materials, Jilin University, Changchun 130012, China. 3Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60439, USA. *These authors contributed equally to this work. 250 | NATURE | VOL 510 | 12 JUNE 2014 ©2014 Macmillan Publishers Limited. All rights reserved LETTER RESEARCH a b c M 0.5 mm C M M C 10 nm 100 nm 5 nm Figure 1 | Onion carbon nanoparticles and a bulk sample synthesized at (C) domains form a {111} twin boundary (TB). Several M-diamond (M) 10 GPa and 1,850 6C. a, HRTEM image of onion carbon nanoparticles. domains are associated with cubic diamond twins containing stacking faults b, TEM image of the sample showing nanotwinned microstructure. Inset: (SFs). Fast Fourier transforms of M-diamond and cubic diamond, shown in the photograph of the black opaque sample (,2 mm in diameter). c, HRTEM upper and lower insets, respectively, indicate that lattices of M-diamond and image of the area marked with the red box in b. Two adjacent cubic diamond cubic diamond are coherent. phase was clearly seen with HRTEM. The d spacings deduced from interrupted by interlocked areas where adjacent nanocrystals inter- selected-area electron diffraction (SAED) patterns (Extended Data Fig. 4d–f) sected and merged, making it difficult to determine individual nano- and XRD data (Extended Data Table 1) of this new phase do not match grains unambiguously. The nanotwins were predominantly thinner any reported carbon phase. The new phase (denoted M-diamond) had than 10 nm. Figure 2c shows a twin thickness distribution derived from a monoclinic structure with lattice parameters of a 5 0.436 nm, b 5 444 nanotwins on the basis of HRTEM measurements. The average 0.251 nm, c 5 1.248 nm and b 5 90.9u. All the C2C bonds were sp3 thickness, ,5 nm, is the smallest microstructural size so far achieved in hybridized, as indicated by the electron energy loss spectrum measure- diamonds. In our transparent nt-diamond samples, stacking faults were ments (Extended Data Fig. 4c), similar to those in cubic diamond. In also observed in nanotwins (Fig. 2b and Extended Data Fig. 5). These the TEM images, thin, elongated (and occasionally polygonal) M- stacking faults, due to extensive twinning, altered the stacking sequence diamond domains intersected adjacent nanotwinned cubic diamond of (111) planes in diamond15 and produced weak shoulders of the strong (C-diamond) domains, forming coherent boundaries parallel to the dia- (111) reflection in the XRD patterns (Extended Data Fig. 2). These ob- mond (111) planes. The orientation relations between M-diamond (M) served planar faults together with the secondary phase of M-diamond and C-diamond (C) as determined from SAED were M(001)//C(111) also caused the asymmetries in both the (111) and the (220) peaks of and M[010]//C[011] (Extended Data Fig. 4d–f). diamond (Extended Data Figs 2 and 3). The HRTEM images of a transparent pure nt-diamond sample (syn- A hardness value should be determined by the asymptotic region of thesized at 20 GPa and 2,000 uC; Fig. 2a inset) revealed that C-diamond the hardness–load curve10,16. We found that our samples reached asymp- contained a high density of lamellar {111} nanotwins (Fig.