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
Diamond is a famously strong material with outstanding properties, such as high wear resistance and hardness. For this reason, it has long been used in cutting and drilling tools, but poor thermal stability has limited its application. On page 250 of this issue1, Huang et al. report the synthesis of 'nanotwinned' diamond, in which nanometre-scale crystals share some lattice points. The authors find that the resulting material is much harder and more thermally stable than naturally occurring diamond.
The ancient Egyptians may have been the first to use diamonds in tooling, although the evidence for this is unsubstantiated. But rock drilling with diamonds has been more reliably dated to the eighteenth century2. The need for high-strength, hard-wearing drill bits for industrial drilling and oil exploration led to the development of a new class of superhard material in the 1980s consisting of diamond grains bonded with metallic cobalt. The main disadvantage of these materials is that the cobalt catalyses the breakdown of diamond to graphite at temperatures above 700 °C. A diamond composite was developed around the same time3, in which the cobalt binder was replaced by a ceramic material, silicon carbide, and was shown to be stable under harsh and severely abrasive rock-cutting conditions to temperatures in excess of 1,200 °C. However, this thermally stable diamond composite material has yet to be widely adopted as a cutting element in tools for the mining, drilling and manufacturing industries for reasons of cost.
A major drawback of diamond-based composites has been their low fracture toughness (a measure of resistance to crack propagation), which can cause them to fail catastrophically. Harder diamond composites, which have higher concentrations of diamond, have lower fracture toughness. Nevertheless, these materials have high wear resistance, and have formed the basis of long-lasting tools for industrial use, provided that the mechanical loading on them is controlled.
Hardness is not governed by composition alone; the grain size of the constituent phases of the materials is also a factor. For hard and brittle materials such as diamond composites, hardness and strength increase with decreasing grain size, as expressed by the Hall–Petch relationship4,5. Normally, such improved hardness is accompanied by a decrease in fracture toughness; this inverse relationship was a generally accepted model until nanostructured materials were thoroughly investigated for their mechanical properties. In such materials, the inverse relationship no longer holds when the grain size is less than about 100 nanometres, and fracture toughness can actually increase with decreasing grain size6. These materials, including diamond composites with constituents that have nanoscale grains, have been shown to have outstanding fracture toughness.
Grain-size-reduction techniques for improving the fracture toughness of ultra-hard materials have proved useful, but seem to have been limited by either the material involved or the technology used. Further improvements to such materials therefore seemed unlikely, unless alloyed nanograined materials with even higher intrinsic hardness could be discovered. However, Huang and colleagues1 demonstrate that a further reduction in the crucial hardness-related length scale of grains is achievable.
Researchers from the same group had previously reported7 a process for making a nanotwinned form of boron nitride — a material with a diamond-like atomic arrangement. They therefore decided to mimic that process with diamond, by subjecting carbon nanoparticles consisting of concentric graphite-like shells (known as onion carbon nanoparticles; Fig. 1) to pressures in the range of 18–25 gigapascals at temperatures of 1,850–2,000 °C. The resulting transparent material consisted of nanotwinned, nanocrystalline diamond.
The hardness of Huang and co-workers' material reached about 200 GPa; for comparison, hardness values for single-crystal diamonds range from 60 to 130 GPa, and those of nanocrystalline diamonds without nanotwins are 130–145 GPa (ref. 8). Another outstanding property is its high fracture toughness, which is greater than that of other commercially available diamond composite materials. Remarkably, the nanotwinned diamond was stable against oxidation in air at temperatures above 1,000 °C — higher than the authors expected.
Huang et al. prepared millimetre-sized pieces of their material on a laboratory scale, but it remains to be seen whether their process can be used on an industrial scale. Success will depend in part on whether starting materials of sufficiently high quality can be made. Nanocrystalline diamond has previously been sintered — fused at high temperature and/or pressure — to manufacture anvils that are used for high-pressure, high-temperature phase studies of geological materials8, and 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 long-term 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.
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Scientific Reports (2017)