Diamond gets harder Composite materials that incorporate diamond are among the hardest in the world, but fail under extreme conditions. A nanostructured form of diamond, made from onion-like carbon precursors, might overcome this problem. See Letter p.250 JAMES BOLAND

materials have proved useful, but seem to have been limited by either iamond is a famously strong the material involved or the techmaterial with outstanding nology used. Further improveproperties, such as high ments to such materials therefore wear resistance and hardness. For seemed unlikely, unless alloyed this reason, it has long been used nanograined materials with even in cutting and drilling tools, but higher intrinsic hardness could be poor thermal stability has limited discovered. However, Huang and its application. On page 250 of colleagues1 demonstrate that a fur1 this issue , Huang et al. report the ther reduction in the crucial hardsynthesis of ‘nanotwinned’ dianess-related length scale of grains is mond, in which nano­metre-scale achievable. crystals share some lattice points. Researchers from the same group The authors find that the resulting had previously reported7 a process material is much harder and more for making a nanotwinned form of thermally stable than naturally boron nitride — a mater­ial with a occurring diamond. diamond-like atomic arrangement. The ancient Egyptians may have They therefore decided to mimic been the first to use diamonds in that process with diamond, by subtooling, although the evidence for jecting carbon nano­particles consistthis is unsubstantiated. But rock ing of concentric graphite-like shells drilling with diamonds has been (known as onion carbon nanopartimore reliably dated to the eight- Figure 1 | Computer model of an onion carbon nanoparticle.  Huang cles; Fig. 1) to pressures in the range 1 eenth century2. The need for high- et al. used such nanoparticles to make an ultrahard, nanostructured form of 18–25 gigapascals at temperatures strength, hard-wearing drill bits for of diamond. (Image taken from ref. 1.) of 1,850–2,000 oC. The resulting industrial drilling and oil exploratransparent material consisted of tion led to the development of a new class of the basis of long-lasting tools for industrial use, nanotwinned, nanocrystalline diamond. superhard material in the 1980s consisting of provided that the mechanical loading on them The hardness of Huang and co-workers’ diamond grains bonded with metallic cobalt. is controlled. material reached about 200 GPa; for compariThe main disadvantage of these materials Hardness is not governed by composition son, hardness values for single-crystal diais that the cobalt catalyses the breakdown of alone; the grain size of the constituent phases monds range from 60 to 130 GPa, and those of diamond to graphite at temperatures above of the materials is also a factor. For hard and nanocrystalline diamonds without nanotwins 700 oC. A diamond composite was developed brittle materials such as diamond composites, are 130–145 GPa (ref. 8). Another outstandaround the same time3, in which the cobalt hardness and strength increase with decreas- ing property is its high fracture toughness, binder was replaced by a ceramic material, sili- ing grain size, as expressed by the Hall–Petch which is greater than that of other commercon carbide, and was shown to be stable under relationship 4,5. Normally, such improved cially available diamond composite materials. harsh and severely abrasive rock-cutting con- hardness is accompanied by a decrease in Remarkably, the nanotwinned diamond was ditions to temperatures in excess of 1,200 oC. fracture toughness; this inverse relationship stable against oxidation in air at temperatures However, this thermally stable diamond com- was a generally accepted model until nano- above 1,000 oC — higher than the authors posite material has yet to be widely adopted structured materials were thoroughly investi- expected. as a cutting element in tools for the mining, gated for their mechanical properties. In such Huang et al. prepared millimetre-sized drilling and manufacturing industries for materials, the inverse relationship no longer pieces of their material on a laboratory scale, reasons of cost. holds when the grain size is less than about but it remains to be seen whether their process A major drawback of diamond-based com- 100 nanometres, and fracture toughness can can be used on an industrial scale. Success will posites has been their low fracture toughness actually increase with decreasing grain size6. depend in part on whether starting materi(a measure of resistance to crack propagation), These materials, including diamond com- als of sufficiently high quality can be made. which can cause them to fail catastrophically. posites with constituents that have nanoscale Nanocrystalline diamond has previously Harder diamond composites, which have grains, have been shown to have outstanding been sintered — fused at high temperature higher concentrations of diamond, have lower fracture toughness. and/or pressure — to manufacture anvils that fracture toughness. Nevertheless, these materiGrain-size-reduction techniques for are used for high-pressure, high-temperature als have high wear resistance, and have formed improving the fracture toughness of ultra-hard phase studies of geological materials8, and


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NEWS & VIEWS RESEARCH similar scientific applications could be predicted for nanotwinned diamond. However, the mater­ial’s creep (the tendency of a mater­ial to deform permanently in response to longterm mechanical stresses) and fatigue properties need to be measured. If the deformation mechanism changes from one that is based on crystallographic defects to one based on sliding at grain boundaries, as commonly occurs during ‘superplastic’ deformation when solid materials are heated, then methods for pinning grain boundaries would be required9. Nanodiamonds have progressed over the past decade or so from being speculative curiosities to fully functioning materials useful for a broad range of applications. Individual nanoparticles consisting of only a few hundred carbon atoms arranged into the diamond structure are being used in such diverse areas as drug delivery, bioimaging and tissue generation10. Nanodiamonds, either aggregated or disaggregated in lubrication fluids, can also form low-friction interfaces that reduce wear on moving components at both the macro- and microscale11. Equally important is the innovative and rapidly developing research on the consolidation and sintering of nanodiamonds to make solid composite materials that have a wide range of remarkable properties, such as high thermal conductivity, optical transparency, chemical inertness and high tolerance to radiation damage. These composites were initially produced on scales barely higher than that of the nanoparticles themselves, but extraordinary progress in high-pressure, high-temperature technology8 now means that the materials can be produced at sizes that have applications across several industries. The incorporation of nanotwinned, nanocrystalline diamonds into composites might lead to materials that have even more extraordinary properties. ■


Pass the ammunition Tomato plants that have been damaged by herbivorous insects emit airborne chemicals that warn neighbours of an impending attack. It emerges that the receiving plants transform these signals into defensive weapons. MARK C. MESCHER & CONSUELO M. DE MORAES


lants may seem passive, but in fact they respond in complex ways to diverse features of their environment. It is becoming increasingly clear, for example, that plants perceive and respond to environmental odours. However, almost nothing is known about the mechanisms by which plant olfaction occurs. Writing in Proceedings of the National Academy of Sciences, Sugimoto et al.1 report that when plants are exposed to odours emitted by neighbours that have been damaged by herbivorous insects, they react by transforming compounds in the odour into effective anti-herbivore defences. When insects feed on plant tissues, the assaulted plant can exhibit a range of physiological responses. For example, it may produce chemical toxins and feeding deterrents, or emit airborne volatile compounds that attract natural enemies of the feeding herbivores, such as insect predators and parasitoids (for instance, parasitic wasps, which lay their eggs in plant-feeding caterpillars)2. It has become widely accepted3–6 that plants can also use volatile emissions released by damaged neighbours as cues to prepare their own

James Boland is in the Division of Earth Science and Resource Engineering, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Pullenvale, Queensland 4069, Australia. e-mail: [email protected] 1. Huang, Q. et al. Nature 510, 250–253 (2014). 2. Tolansky, S. in Science and Technology of Industrial Diamonds Vol. 2 (ed. Burls, J.) 341–349 (Ind. Diamond Inform. Bur., 1967). 3. Wilks, E. & Wilks, J. Properties and Applications of Diamond (Butterworth-Heinemann, 1991). 4. Petch, N. J. J. Iron Steel Inst. 174, 25–28 (1953). 5. Hall, E. O. Proc. Phys. Soc. Lond. B 64, 747–753 (1951). 6. Zhao, Y. et al. Appl. Phys. Lett. 84, 1356–1358 (2004). 7. Tian, Y. et al. Nature 493, 385–388 (2013). 8. Irifune, T. & Sumiya, H. in Comprehensive Hard Materials Vol. 3 (eds Mari, D., Llanes, L. & Nebel, C. E.) 173–191 (Elsevier, 2014). 9. Suryanarayana, C. & Al-Aqeeli, N. Prog. Mater. Sci. 58, 383–502 (2013). 10. Mochalin, V. N., Shenderova, O., Ho, D. & Gogotsi, Y. Nature Nanotechnol. 7, 11–23 (2012). 11. Ivanov, M. et al. Nanosyst. Phys. Chem. Math. 5, 160–166 (2014).

defences against an impending attack — an idea that had previously been controversial3,4,7. Sugimoto et al. investigated the mechanisms by which volatile signalling between cultivated tomato plants influences their defence against larvae of the moth Spodoptera litura, an agricultural pest also known as the common cutworm. Using an experimental set-up in which airflow between individual plants was carefully controlled, the authors showed that exposure to the odours released by cutwormdamaged tomato plants significantly enhanced the ability of neighbouring plants to resist a subsequent attack. Cutworm larvae placed on plants that had been exposed for three days to the odours released by damaged plants showed both reduced growth and increased mortality compared with larvae placed on unexposed control plants. The authors’ extensive biochemical analyses of tomato-leaf tissues revealed that plants exposed to volatiles that had been released by damaged neighbours had greatly elevated levels of a single compound — (Z)-3-hexenyl­ vicianoside, or HexVic. Furthermore, they found that cutworm larvae fed an artificial diet laced with HexVic showed significantly reduced growth compared with larvae reared on an untainted diet, confirming that


Assaulted plant

(Z)-3-Hexenol → HexVic

Unassaulted neighbour

Figure 1 | Plant odours as alarm signals.  When plants are attacked by herbivorous insects, they release volatile compounds that can attract insect predators and deter further herbivory. These volatile odours also enhance the defences of neighbouring plants against attack. Sugimoto et al.1 report that tomato plants can directly transform (Z)-3-hexenol, a volatile compound released by their damaged neighbours, into (Z)-3-hexenylvicianoside (HexVic), an effective defence compound that reduces the growth and survival of the herbivores. (Figure adapted from drawings by Nick Sloff and Thomas Degen.) 1 2 J U N E 2 0 1 4 | VO L 5 1 0 | NAT U R E | 2 2 1

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Materials science: Diamond gets harder.

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