news & views elements are too expensive to be practical in large volumes. Schroers and collaborators used a common combinatorial approach based on co-sputtering 7,8 to make a library of materials consisting of thousands of compositions on a single thin film (Fig. 2a). Thousands of miniature shapeforming experiments were then prepared by etching circular holes about 0.5 mm in diameter beneath the film. A compound that generates gas at a fixed temperature was placed in each cavity before the temperature was steadily increased for the entire materials library. The gas is produced at a temperature where the glass is still relatively solid, thus pressurizing the cavity. The metallic glass begins to thermoplastically deform at a slightly higher temperature (the glass transition temperature), and the process ends at the crystallization temperature, locking in the final bubble size. By simply measuring the height of each miniature metallic glass bubble (Fig. 2b,c), Schroers and co-authors were able to characterize the shape-forming ability of each metallic-glass composition with reproducible precision. The same test also gives data, such as viscosity, that is difficult to measure by other techniques,

and may give essential new insights into the fundamentals of metallic glass stability. Therefore, with a simple measurement — the bubble height after forming — this massively parallel synthesis and shapeforming approach allowed the authors to dramatically reduce sample preparation time and to determine which BMGs can most easily be shaped into complex, useful shapes. Moreover, the same combinatorial experiments that tell which BMGs can be shaped most easily also indicate which compositions are the most stable, thus overcoming another major barrier to commercialization. In fact, there is no way at present to predict which alloys will have good glass-forming ability, and so laborious trial-and-error experiments are used to discover and develop new BMGs. The approach of Schroers and colleagues establishes the glass-forming ability of new alloys more than 100 times faster than current methods, so that studies that used to take a year can now be done in a single day. To put this in perspective, over 50 years of extensive trial-and-error were required to identify all the metallic glasses known at present; the new approach has the potential to give a similar amount of information in under two months.

Still, there is more work to be done. Commercial metallic glasses can have more than six elements, and deciding which elements to add is still more art than science. The number of possible compositions for alloys with six or more elements is vast, and tackling such a compositional space would be a challenge, even with the accelerated blow-forming approach of Schroers and co-authors. And producing controlled composition gradients of four to six elements, although possible in theory, is rarely done. The authors’ highthroughput approach should thus fast-track efforts into tapping the practical benefits of metallic glasses. ❐ Dan B. Miracle is at the AF Research Laboratory, Materials and Manufacturing Directorate, Dayton, Ohio 45433, USA. e-mail: [email protected] References Schroers, J. Adv. Mater. 22, 1566–1597 (2010). Ding, S. et al. Nature Mater. 13, 494–500 (2014). Hasegawa, R. Mater. Sci. Eng. A 375–377, 90–97 (2004). Hilzinger, H. IEEE Trans. Magnetics 21, 2020–2025 (1985). Tregilgas, J. H. Adv. Mater. Proc. 162, 40–41 (2004). Salimon, A. I., Ashby, M. F., Brechet, Y. & Greer, A. L. Mater. Sci. Eng. A 375–377, 385–388 (2004). 7. Zhao, J.‑C. Prog. Mater. Sci. 51, 557–631 (2006). 8. Green, M. L., Takeuchi, I., Hattrick-Simpers, J. R. J. Appl. Phys. 113, 231101 (2013). 1. 2. 3. 4. 5. 6.

BIOINSPIRED CERAMICS

Turning brittleness into toughness Nacre-like bulk ceramics with a unique combination of high toughness, strength and stiffness can be produced from brittle constituents by an ice-templating approach.

André R. Studart

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lthough seemingly different, the seashells of molluscs and the ceramics that are used for kitchen plates, bathroom sinks and turbine coatings have much in common. They are made primarily of brittle building blocks (95% in volume); carbonates in the case of seashells and (most often) oxides in advanced ceramics. However, whereas the latter are prone to shatter and sudden fracture, seashells are surprisingly tough. This is something that has fascinated materials scientists for decades. By means of their beautiful brick-and-mortar-like (nacreous) internal architecture (Fig. 1a) — which drives cracks through a long and tortuous path (thereby dissipating energy) before the material can be broken apart — seashells achieve a remarkable resistance to crack growth1. In fact, inorganic bridges (Fig. 1b)

present in between the submicrometre-thick bricks help distribute the externally applied forces over a larger number of bricks, thus reducing the driving force for cracking. Also, organic matter (less than 5 vol%; Fig. 1c) deforms plastically during fracture to absorb part of the energy of the propagating crack. Replicating the design principles underlying the microscopic architecture of seashells in macroscopic synthetic materials has been a long-standing challenge. Although impressive enhancements in fracture toughness combined with high strength and plasticity have been achieved by assembling strong inorganic ‘bricks’ with at least 20 vol% of organic ‘mortar’2,3, such a large fraction of organic phase limits the maximum temperature that the material can withstand. Writing in Nature Materials, Deville and colleagues now show that

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all-inorganic ceramics with a structure inspired by the nacreous layer of seashells can be made to exhibit high-temperature toughness, strength and stiffness4. Deville and co-authors used freeze casting — a well-established technique to obtain lamellar materials, where two-dimensional ice crystals are grown unidirectionally through an aqueous suspension loaded with ceramic particles5 (Fig. 1e). By controlling both the speed of the ice growth front and the concentration of particles in the suspension, freeze casting allows the particles to be excluded from the ice crystal, forcing them to assemble into long-range ordered lamellae of a few tens of micrometres in thickness. The ice is then sublimated under reduced pressures to generate a macroporous lamellar structure. Pressing such porous structures 433

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perpendicular to the aligned lamella while increasing the temperature up to about 1,500 °C results in a densified material with long-range lamellar organization (Fig. 1d). Deville and co-authors modified this approach to include 100-nm alumina particles and submicrometre silica/CaCO3 particles in a ceramic suspension of 300-nmthick alumina platelets. This was key, because the alumina platelets aligned themselves within the lamella created during the freezing process, thus generating even thinner layers. Melting of the silica/CaCO3 mixture allowed for fast sintering of the layered ceramic to form a layered microstructure that is at least one order of magnitude finer (less than 1 μm; Fig. 1d) than the microstructures of previously reported bulk lamellar ceramics, and that closely resembled those found in the nacreous layers of shells. The alumina nanoparticles partially fused during the sintering process to form bridges between adjacent platelets, thus replicating another crucial design principle of nacre. The usual approach to render ceramics tough and resistant to thermal shock is to increase the grain size of the material to at least several hundreds of micrometres. Because of the typical inverse relationship between strength and grain size, such coarsening inevitably weakens the material. Instead, by capturing the unique design of nacre, the lamellar alumina ceramics developed by Deville and colleagues reconcile strength and toughness, which are often mutually exclusive in synthetic materials6 (Fig. 2a). Indeed, the bioinspired ceramics combine the high strength and fracture toughness of submicrometre lamellar structures interconnected by stiff bridges with the unique chemical inertness,

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Figure 1 | Natural nacreous composites and bioinspired ceramics. a–c, The nacreous layer of molluscs is composed of long-range ordered submicrometre platelets for crack deflection (a), inorganic bridges for stress redistribution (b, arrows) and an organic phase for sliding and energy absorption (c, arrows). d, Nacre-like structure of the tough ceramics of Deville and colleagues4. e, Freeze-casting process used to fabricate the bioinspired ceramics4. The growth of ice crystals templates the alignment of suspended 300-nm-thick alumina platelets. Alumina and silica/CaCO3 nanoparticles added to the suspension form the inorganic bridges and the continuous phase (blue), respectively, in the sintered lamellar structure. The black triangles indicate the direction of the applied pressure. Figures reproduced with permission from: a,c, courtesy of Sylvain Deville; b, ref. 11, © 2008 Elsevier; d, ref. 4, 2014 NPG; e, ref. 5, © 2008 Wiley. b

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Figure 2 | Mechanical properties of the bioinspired nacre-like alumina ceramics of Deville and colleagues4. a–c, Comparison of strength and fracture toughness (a), resistance to crack growth (b) and crack deflection (c) of the nacreous ceramics, unstructured alumina (reference), nacre, and other advanced ceramics and composites. Panel a includes data for alumina-based composites with different ductile phases (polymer, metal or carbon) reported in the literature. In b, resistances to crack growth of nacre-like alumina at room temperature (blue and purple curves) and at 600 °C (red) are compared with that of nacre (green). 434

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news & views wear properties and high-temperature resistance expected for ceramic materials. Toughening is evidenced by the increasing resistance of the material against crack growth (known as a rising R-curve behaviour; Fig. 2b), and results in the very tortuous crack path observed during fracture of the nacre-like ceramic as compared with unstructured alumina (Fig. 2c). This is quite remarkable considering the fact that the material is made entirely of brittle ceramic building blocks. In fact, the levels of toughness achieved exceed by an order of magnitude than those of advanced ceramics of the same composition and grain size, and are comparable to those of the toughest polymer-infiltrated bulk nacre-like composites reported thus far 2 (Fig. 2a). This indicates that toughening is possible even in the absence of an energy-absorbing polymer phase. The lack of a polymer phase allows the nacre-like alumina ceramics of Deville

and co-authors to maintain their strength and toughness even at 600 °C (Fig. 2b), a temperature that would be sufficient to degrade any organic constituent. Still, the authors’ freeze-casting approach could potentially produce all-inorganic lamellar ceramics with unprecedented strength and toughness at higher temperatures by employing inorganic building blocks that are more heat resistant. These materials could extend the lifetime or operating temperature of refractory linings for metallurgical ovens and combustion chambers of aircrafts. By replicating design concepts of a biological material using building blocks with functional properties that are superior to those of natural constituents, Deville and collaborators’ work nicely illustrates how bioinspired approaches can lead to materials that easily outperform stateof-the-art technologies7. This and other recent examples8–10 of bioinspired materials design suggest that an increasing number

of processing routes that effectively capture the design principles of natural materials are soon expected to emerge. ❐ André R. Studart is at the Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland. e-mail: [email protected] References 1. Meyers, M. A., Chen, P. Y., Lopez, M. I., Seki, Y. & Lin, A. Y. M. J. Mech. Behav. Biomed. Mater. 4, 626–657 (2011). 2. Munch, E. et al. Science 322, 1516–1520 (2008). 3. Bonderer, L. J., Studart, A. R. & Gauckler, L. J. Science 319, 1069–1073 (2008). 4. Bouville, F. et al. Nature Mater. 13, 508–514 (2014). 5. Deville, S. Adv. Eng. Mater. 10, 155–169 (2008). 6. Ritchie, R. O. Nature Mater. 10, 817–822 (2011). 7. Studart, A. R. Adv. Mater. 24, 5024–5044 (2012). 8. Erb, R. M., Libanori, R., Rothfuchs, N. & Studart, A. R. Science 335, 199–204 (2012). 9. Erb, R. M., Sander, J. S., Grisch, R. & Studart, A. R. Nature Commun. 4, 1712 (2013). 10. Mirkhalaf, M., Dastjerdi, A. K. & Barthelat, F. Nature Commun. 5, 3166 (2014). 11. Meyers, M. A., Lin, A. Y.‑M., Chen, P.‑Y. & Muyco, J. J. Mech. Behav. Biomed. Mater. 1, 76–85 (2008).

NATURAL MATERIALS

Armoured oyster shells

The remarkable properties of a bivalve shell that enable it to protect the animal against its predators could inspire the design of new lightweight armour materials.

Robert O. Ritchie

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ature designs materials as hierarchical architectures with complex composite structures spanning the nano to near-macro length scales to create unique combinations of properties that are often difficult to achieve with synthetic materials. Indeed, the quest to understand such amazing natural structures and their intriguing mechanisms has provided a fresh stimulus for the development of new man-made materials with unprecedented properties and performance. This is particularly evident in the search for superior lightweight armour for use in personal protection and military applications. Nature has been adept at creating lightweight armours for over 500 million years, both as highly flexible dermal armours, for example fish scales, or more rigid shells, such as those of molluscs1. Despite wide variations in composition, architecture and structure, there is distinct commonality: bio-armours consist of sets of rigid plates — varying in thickness from several 100 μm for small fish to over 100 mm in the case of dinosaurs — connected by collagen fibres or muscles. Flexibility is generally achieved

through the overlapping or juxtaposition of plates. In comparison, Kevlar (aramid fibre)-based synthetic materials2, which are used in many personal armour applications at present, are not especially lightweight (~20 kg is added to a soldier’s load). Hence, there remains considerable insight that could be drawn from studying natural systems for the fabrication of improved synthetic armour materials. Writing in Nature Materials, Li and Ortiz3 report on the penetration resistance and spatial localization of damage as a consequence of effective energy-dissipation mechanisms in the shell of the bivalve marine mollusc Placuna placenta. The translucent optical properties of the outer shell have resulted in its use in window panes as a cheap alternative to glass in parts of Asia, and hence, P. placenta is also called the ‘window pane’ oyster 4. The key to the shell’s defense properties lies in the presence of graded nano/microstructures. Li and Ortiz explain how in the P. placenta shell, these graded structures comprise an outer layer to resist and localize penetration damage, and a fracture-resistant base to absorb

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the excess energy without plate failure. The latter has the effect of maintaining a ‘multi-hit’ capacity. The first role of armour is to arrest the penetrating object, which is generally achieved by spatially localizing any comminuted zone directly beneath the penetrator. In man-made armour, this is often achieved through the use of a hard surface layer to minimize local plasticity, in fish scales, for example, it is achieved by a highly mineralized outer layer 1,5, and in abalone shells by a prismatic layer of mineral laths aligned perpendicular to the surface to optimize wear and penetration resistance6. The second role of armour is to accommodate the deformation; graded material properties throughout the thickness of the armour are used to ensure that the larger deformations are in the inner regions, which are tough enough to support greater amounts of plastic deformation than the hard, but brittle, outer shell. Fish scales achieve this with a collagen-based inner layer 1,5,7. Amazingly, in certain fish, lamellae of collagen fibrils are arranged in the form of a spiral staircase (Bouligand) structure; 435

Bioinspired ceramics: Turning brittleness into toughness.

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