Abstract
The well known trade-off between hardness and toughness (resistance to fracture) makes simultaneous improvement of both properties challenging, especially in diamond. The hardness of diamond can be increased through nanostructuring strategies1,2, among which the formation of high-density nanoscale twins — crystalline regions related by symmetry — also toughens diamond2. In materials other than diamond, there are several other promising approaches to enhancing toughness in addition to nanotwinning3, such as bio-inspired laminated composite toughening4,5,6,7, transformation toughening8 and dual-phase toughening9, but there has been little research into such approaches in diamond. Here we report the structural characterization of a diamond composite hierarchically assembled with coherently interfaced diamond polytypes (different stacking sequences), interwoven nanotwins and interlocked nanograins. The architecture of the composite enhances toughness more than nanotwinning alone, without sacrificing hardness. Single-edge notched beam tests yield a toughness up to five times that of synthetic diamond10, even greater than that of magnesium alloys. When fracture occurs, a crack propagates through diamond nanotwins of the 3C (cubic) polytype along {111} planes, via a zigzag path. As the crack encounters regions of non-3C polytypes, its propagation is diffused into sinuous fractures, with local transformation into 3C diamond near the fracture surfaces. Both processes dissipate strain energy, thereby enhancing toughness. This work could prove useful in making superhard materials and engineering ceramics. By using structural architecture with synergetic effects of hardening and toughening, the trade-off between hardness and toughness may eventually be surmounted.
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Data availability
The data that support the findings of this study are available from the corresponding authors on reasonable request.
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Acknowledgements
This work was supported by the National Key R&D Program of China (2018YFA0703400 and 2018YFA0305900), the National Natural Science Foundation of China (51922017, 51525205, 51772011, 51672239, 51572235, 51741201, 51772260, 51972009, 51532001, 11674176 and 11874224), US National Science Foundation (EAR-1661489 and EAR-1361276) and the Fundamental Research Funds for Central Universities (YWF-19-BJ-J-94). We thank M. Veron at Grenoble Institute of Technology for the precession electron diffraction measurement.
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Y.T., Y.Y., B.X., X.-F.Z. and L.G. proposed and supervised the project; Y.G., Z.L., P.Y., X.L. and D.Y. synthesized the sample; Y.Y., W.H., B.X., B.G., Z.W., B.W. and X.-F.Z. performed the TEM characterization; Y.Y., J.W., X.Z. and Q.Z. performed the in situ bending experiments; Y.G., P.Y. and B.X. performed the Vickers indentation measurements; X.-F.Z. performed the first-principles calculations; Z.Y. performed the finite element simulations; Y.Y., Y.T., B.X., Y.W., X.-F.Z. and L.G. analysed data and wrote the manuscript. All authors participated in discussions of the research.
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Extended data figures and tables
Extended Data Fig. 1 Microstructure and X-ray diffraction patterns of nt-diamond composite synthesized at 15 GPa and 2,000 °C.
a, Low-magnification bright-field STEM image showing interlocked grains composed of nanotwins. Coloured lines indicate five different TB orientations. b, HRTEM image from the red-boxed region in a, showing interwoven nanotwins with embedded secondary phases. Inset shows the corresponding FFT pattern. In addition to the fivefold patterns from the multi-directional 3C diamond twins in the interlocked grains, extra diffraction spots from coherent non-3C diamond polytypes are clearly recognized. c, X-ray diffraction characterization. A shoulder is clearly observed at the lower-angle side of 3C diamond (111) peak near 44° (left panel of c), an indicator of polytype existence.
Extended Data Fig. 2 Hierarchical microstructure of nt-diamond composite.
a, Bright-field STEM image showing three interlocked nanograins with boundaries appearing diffuse or segmented by intersecting twin boundaries. The Moiré fringes (enclosed with yellow-dashed lines) indicate three-dimensional interpenetrating grain boundaries, which lock neighbouring grains and cause grain overlapping near the boundaries. b, c, HAADF-STEM images corresponding to the red and blue boxes in a, respectively, displaying segmented boundaries with {111} TBs (red lines) and other boundaries (cyan lines). SFs (single arrowheads) and ESFs (double arrowheads) are also marked. A 9R polytype region is also clearly recognizable in b. d, HAADF-STEM image of 3C nanotwins with coherently embedded 2H and 4H polytypes. Inset, FFT pattern corresponding to the red-boxed area, showing the symmetry of 2H and 4H polytypes. TBs and SFs are marked with red lines and arrowheads, respectively. Layer defects are marked with green ovals. Along the long axes of the ovals, the TB and stacking fault climb by one bilayer. e, HAADF-STEM image of a 9R polytype region coherently embedded in 3C diamond. The cyan dashed line indicates the grain boundary. Inset, FFT pattern corresponding to the red-boxed area, showing a tripling of (111)3C periodicity. f, Stacking sequences of 3C, 2H, 4H, 9R and 15R polytypes. g, Simulated electron diffraction pattern of 2H polytype (right) compared with experimental FFT pattern (left; inset of Fig. 2b). h, Simulated electron diffraction patterns of 9R (left) and 15R (right) polytypes compared with experimental FFT pattern (middle; inset of Fig. 2c).
Extended Data Fig. 3 Details of a localized layer defect in nt-diamond composite.
a, A high-resolution HAADF-STEM image shows a localized layer defect between 3C and 2H domains (green-dashed frame). The carbon bilayers and the TB (red line) are obviously bent because of the bulge produced by the defect. b, Electron energy-loss spectra from the defective region (green) and 3C diamond region (red). Note that the region with the layer defect exhibits a peak near 286 eV (black arrow), suggesting sp2 bonds in the layer defect and consistent with the enlarged interlayer distance. The 3C region, on the other hand, exhibits peaks typical of diamond.
Extended Data Fig. 4 Crystallographic characterization of nt-diamond composite with precession electron diffraction.
a, Virtual bright-field image. b, Crystallographic orientation map corresponding to a. Inset, the chromatic code of the cubic inverse pole figure. c, Grain and twin boundaries, among which the Σ3 boundaries are coloured in red. d, Remaining grain boundaries after removing Σ3 and Σ9 low-energy boundaries, indicating a substantial grain interlocking. The average grain size in nt-diamond composite is 22 nm as determined by precession electron diffraction.
Extended Data Fig. 5 SENB sample preparation and time-sequential SENB bending tests.
Sample preparation was with FEI (Helios Nanolab 600i). Time-sequential SENB bending tests were carried out on sample 3 after the first test shown in Fig. 3. a, SEM image of the sample after excavating craters, with a thin wall in between. b, A thin wall taken out to prepare TEM foil. c, A micro-beam shaped from the thin wall described in a. Note the precut notch on the underside of the micro-beam. d−f, SEM images from the second-round multi-cycle bending test. g−i, SEM images from the third-round test. j, k, Force−displacement curves of the first- and second-round SENB tests. Blue arrows indicate yielding points.
Extended Data Fig. 6 Comparison of fracture toughness values from SENB and indentation measurements for Y-PSZ, pure nt-diamond and nt-diamond composite.
SENB, filled bars; indentation, empty bars. The loads for indentation fracture toughness measurement were 49 N for Y-PSZ, and 19.6 N for pure nt-diamond and nt-diamond composite. Error bars indicate 1 s.d. (n = 5 for Y-PSZ and nt-diamond composite, n = 3 and 5 for indentation and SENB measurements of pure nt-diamond, respectively). ntD, nt-diamond.
Extended Data Fig. 7 An nt-diamond composite before and after in situ fracture test in TEM.
a–d, SEM observation of the test piece. a, Before the fracture test. b, After the fracture test. c, d, Enlarged SEM images corresponding to the yellow and red boxes in b, respectively. Arrows in c highlight the sinuous crack propagation path in non-3C polytypes. Double-headed arrow in d indicates a pull-out fracture in nt-diamond composite. Such pull-out fractures are common in composite materials with enhanced toughness. e, f, Plastic deformation near the crack tip from in situ TEM observation. e, HRTEM image of nt-diamond composite taken near the crack tip (corresponding to the small yellow box in the inset) where stacking faults (green arrows) can be identified. f, Inverse FFT image corresponding to red box in e. An extended dislocation with a Burger’s vector of 1/2 <110> is clearly shown by the Burger’s circuit (yellow).
Extended Data Fig. 8 Structural transformation from 9R diamond to 3C diamond.
a, Projection view of 9R and 3C diamonds with redefined unit cells. The yellow and green dashed lines represent the glide and shuffle planes, respectively. b, Stress–strain curves of 9R diamond sheared along (001)[11\(\bar{2}\)] and (001)[\(\bar{1}\bar{1}\)2] directions, corresponding to the red and blue arrows in a, respectively. The peaks of shear stress in these two directions are 103 GPa and 101 GPa, indicating no distinct easy or hard directions. c, Energy–strain curves of 9R diamond along (001)[11\(\bar{2}\)] and (001)[\(\bar{1}\bar{1}\)2] directions. d, Structural snapshots from I to II (sheared along (001)[11\(\bar{2}\)] and from III to IV (sheared along (001)[\(\bar{1}\bar{1}\)2]). Points I−IV are marked in b. The calculated energy–strain curves indicate that 9R to 3C transformation with a smaller energy barrier is preferred, rather than the collapsed layer structure. Under shear along (001)[\(11\bar{2}\)], C−C bonds across shuffle planes are tilted towards (001)[\(11\bar{2}\)], with one-ninth of bonds across glide planes broken beyond critical strain (two bonds per unit cell, marked with red cross in d). In comparison, all bonds across shuffle planes (six bonds per unit cell, marked with blue crosses in d) are broken under shear along (001)[\(\bar{1}\bar{1}\)2].
Extended Data Fig. 9 Finite element simulation results of SENB tests.
a, von Mises stress distribution in the bending test with increasing load. b, Axial strain distribution in the cantilever corresponding to a. c, The stress distribution in conventional SENB test with hinged ends. d, The stress distribution in our SENB test with fixed ends.
Supplementary information
Video 1
First-round multi-cycle bending test on Sample 3 and the corresponding force−displacement curve (shown in Figs. 3a−3d and Extended data Fig. 5j); video speed at 10 times the speed of experiment.
Video 2
Second-round multi-cycle bending test on Sample 3 and the corresponding force−displacement curve (shown in Extended data Figs. 5d−5f and 5k); video speed at 10 times the speed of experiment.
Video 3
Third-round multi-cycle bending test on Sample 3 (shown in Extended data Figs. 5g−5i); video speed at 5 times the speed of experiment. The small bulb squeezed out in the end of the second-round of bending test was torn into halves. This suggests the crack propagated through the bulb that binds the left and right sides together, a direct evidence of self-healing.
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Yue, Y., Gao, Y., Hu, W. et al. Hierarchically structured diamond composite with exceptional toughness. Nature 582, 370–374 (2020). https://doi.org/10.1038/s41586-020-2361-2
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DOI: https://doi.org/10.1038/s41586-020-2361-2
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