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Nanotwinned diamond with unprecedented hardness and stability


Although diamond is the hardest material for cutting tools, poor thermal stability has limited its applications, especially at high temperatures. Simultaneous improvement of the hardness and thermal stability of diamond has long been desirable. According to the Hall−Petch effect1,2, the hardness of diamond can be enhanced by nanostructuring (by means of nanograined and nanotwinned microstructures), as shown in previous studies3,4,5,6,7. However, for well-sintered nanograined diamonds, the grain sizes are technically limited to 10−30 nm (ref. 3), with degraded thermal stability4 compared with that of natural diamond. Recent success in synthesizing nanotwinned cubic boron nitride (nt-cBN) with a twin thickness down to 3.8 nm makes it feasible to simultaneously achieve smaller nanosize, ultrahardness and superior thermal stability5. At present, nanotwinned diamond (nt-diamond) has not been fabricated successfully through direct conversions of various carbon precursors3,6,7 (such as graphite, amorphous carbon, glassy carbon and C60). Here we report the direct synthesis of 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 and thermal stability, with Vickers hardness up to 200 GPa and an in-air oxidization temperature more than 200 °C higher than that of natural diamond. The creation of nanotwinned microstructures offers a general pathway for manufacturing new advanced carbon-based materials with exceptional thermal stability and mechanical properties.

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Figure 1: Onion carbon nanoparticles and a bulk sample synthesized at 10 GPa and 1,850 °C.
Figure 2: A nt-diamond bulk sample synthesized at 20 GPa and 2,000 °C.
Figure 3: Typical mechanical properties of nt-diamond and its comparison with other tool materials.
Figure 4: Typical thermal stability of a nt-diamond sample.


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This work was supported by the National Natural Science Foundation of China (51121061), the Ministry of Science and Technology of China (2011CB808205 and 2010CB731605), the National Natural Science Foundation of China (51332005, 51172197, 11025418 and 91022029) and the US National Science Foundation (EAR-0968456).

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Authors and Affiliations



Y.J.T. conceived the project. Y.J.T., D.L.Y., B.X. and Y.B.W. designed the experiments. Q.H. synthesized onion carbon precursors. Q.H., D.L.Y., B.X., Y.J.T., Y.B.W. and Z.S.Z. performed the HPHT experiments, W.T.H. performed TEM observations, and B.W. performed molecular dynamics simulations. Y.J.T., B.X., D.L.Y., Y.M.M., Y.B.W., J.L.H. and Z.Y.L. analysed the data. Y.J.T., B.X., Y.M.M. and Y.B.W. co-wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to Yongjun Tian.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Schematic icosahedral model of a ten-shell onion carbon.

The icosahedral-quasicrystal-like model of an onion carbon particle was relaxed from a nested buckyonion of C60, C240, C540, C960, C1,500, C2,160, C2,940, C3,840, C4,860 and C6,000. This model was constructed with the same classical molecular dynamics technique as that used in our previous work5. The spacings between adjacent shells in the model vary from 0.300 nm to 0.340 nm.

Extended Data Figure 2 Phase transformation of onion carbon compacts at HPHT.

XRD patterns of onion carbon precursor (Raw) and seven samples recovered from different conditions indicated by P (in GPa)–T (in °C) pairs. The inter-shell spacing of the starting onion carbon nanoparticles is 0.3485 nm. For the two samples recovered from 8 GPa/2,000 °C and 15 GPa/1,200 °C, the onion carbon structure does not show significant alteration except that the inter-shell spacing decreases to 0.3305 and 0.3361 nm, respectively. Cubic diamond appears when the applied pressure is more than 10 GPa and temperature is more than 1,400 °C, with an accompanying new carbon phase recognized in the black opaque samples synthesized at 1,850 °C or below. A small amount of residual onion carbon can be detected in the sample recovered from 15 GPa/1,400 °C. At pressures of 18–25 GPa and temperatures of 1,850–2,000 °C, the recovered samples changed from translucent to transparent, and only the diffraction peaks of cubic diamond can be seen in XRD patterns. Weak shoulders of the (111) peaks of diamond (red arrows) appear in three samples synthesized at pressures of 18−20 GPa and temperatures of 1,850−1,950 °C. Asymmetry in the (111) and (220) peaks of diamond was often observed in the samples synthesized at pressures below 20 GPa and temperatures below 1,950 °C.

Extended Data Figure 3 XRD patterns of a sample recovered from 10 GPa and 1,850 °C.

All the recorded d spacings of visible diffraction peaks are listed in Extended Data Table 1. Insets: two peaks overlapping the cubic diamond reflections. Most of these extra reflections can be indexed with a monoclinic structure (M-diamond) as shown in Extended Data Table 1.

Extended Data Figure 4 TEM images, electron energy loss spectrum (EELS) and SAED measurements on a sample recovered from 10 GPa and 1,850 °C.

a, TEM image showing interlaced twins. b, HRTEM image corresponding to the area in the red box in a. A monoclinic M-diamond (M) domain is observed between two cubic diamond (C) domains. c, EELS spectra of M and C phases. All the C–C bonds are sp3 hybridized in both M and C phases. d−f, SAED patterns along the [010], [150] and [130] zone axes of M, respectively, recorded by rotating an M crystal. (111) and (200) spots of the twinned C phase, overlapping with some spots of the M phase as a result of coherent growth, are marked by red circles and boxes, respectively. The determined orientation relations between M and C phases are M(001)//C(111) and M[010]//C[011].

Extended Data Figure 5 HRTEM observations of three nt-diamond bulk samples synthesized at different HPHT conditions.

ac, HRTEM and corresponding TEM (inset) images of three representative samples, O-366 (a), P-368 (b) and M-363 (c) as listed in Extended Data Table 2. TBs are marked with red arrows. The measured average twin thicknesses are 5.2 nm for sample P-368, 5.4 nm for sample O-366 and 7.9 nm for sample M-363; the smaller the average twin thickness, the higher the hardness. The full width at half-maximum (FWHM) of the (111) peak is mainly related to the nanograin size: samples O-366 and P-368 have a larger FWHM as a result of their smaller nanograin size. Both pressure and temperature can promote the phase transformation of onion carbon to diamond. The probability of stacking faults and the volume fraction of M-diamond decrease with elevated synthesis temperature and pressure, as confirmed by our HRTEM observation. The abundant stacking faults in the nanotwins result in the appearance of a shoulder near the (111) peak (Extended Data Fig. 2), for example in the XRD pattern of sample O-366. The asymmetries of the (111) and (220) peaks of diamond shown in Extended Data Fig. 2 can be attributed to planar faults and the secondary phase in microstructure. On the one hand, a twin fault can itself produce peak asymmetry; on the other, M-diamond also contributes to peak asymmetry because of peak overlap, as demonstrated in Extended Data Fig. 3.

Extended Data Figure 6 Comparison of Vickers indenter tip before and after hardness and fracture toughness tests of nt-diamond.

a, b, Scanning electron microscopy images of the square pyramid diamond tip before (a) and after (b) the tests of nt-diamond. A load of 9.8 N was used during the hardness and toughness tests. As shown in b, the indenter, with a dark imprint of 6.9 μm × 6.9 μm on the tip matching the permanent indentation on the tested nt-diamond, shows no visible plastic deformation. c, d, Photographs of indentations on the standard calibration block equipped by microhardness tester KB 5 BVZ. The indentations were formed at a load of 1.96 N before (c) and after (d) the tests, with the same tip as shown in a and b. The indenter tip produced an almost identical indentation (or standard hardness value) on the calibration block after the nt-diamond tests. These calibration results ensured the accuracy, repeatability and reliability of the unprecedented hardness and exceptional toughness values of nt-diamond reported in the present study.

Extended Data Figure 7 Comparison of in-air oxidation resistance of bulk nt-diamond with other diamonds measured at a heating rate of 10 °C min−1.

a, Comparison of the onset oxidation temperatures determined from measured thermogravimetry curves. The onset temperature was 1,056 °C for a bulk nt-diamond, 805 °C for a natural diamond crystal, 725 °C for synthetic diamond powders and 680 °C for a nanograined diamond4. b, Comparison of the onset oxidation temperatures determined from the exothermic trough in the measured heat flow curves of DSC. The onset temperature was 1,035 °C for the nt-diamond, 750 °C for the natural diamond and 705 °C for the synthetic diamond. The exothermic peaks located at 1,280 °C and 1,320 °C for the nt-diamond were probably due to the presence of finer nanotwins. The above-measured oxidation temperatures are consistent with those determined from the corresponding thermogravimetry curves.

Extended Data Figure 8 Atomic arrangements of a {111} Σ = 3 twin boundary in cubic diamond.

The twin boundary is projected along the 〈011〉 direction. Because of the stacking sequence of ABC for diamond structure, the minimum twin thickness is 3d111, where d111 is the planar distance along the direction of 〈111〉 in the unit cell of cubic diamond.

Extended Data Table 1 Comparison of d spacings (dobs) observed from XRD and SAED with those of proposed M-diamond structure and cubic diamond
Extended Data Table 2 Vickers hardness HV (GPa), Knoop hardness HK (GPa) and fracture toughness KIc (MPa m0.5) for six transparent pure (XRD standard) nt-diamond bulk samples

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Huang, Q., Yu, D., Xu, B. et al. Nanotwinned diamond with unprecedented hardness and stability. Nature 510, 250–253 (2014).

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