Structural design and microstructure of composite hollow lattice. (a) Illustration of multiscale design of the composite hollow lattice; (b) SEM image from a tilted view of a composite hollow lattice; (c) zoomed SEM image of a composite hollow lattice, with an inserted heatmap of Raman signals from G peak of carbonized polydopamine (C-PDA); (d) TEM image of the tube wall of a hollow strut from the composite hollow lattice, showing that the thickness of alumina nanolayer is 15?nm and that the thickness of C-PDA nanolayer is 8?nm.
Structural design and microstructure of composite hollow lattice. (a) Illustration of multiscale design of the composite hollow lattice; (b) SEM image from a tilted view of a composite hollow lattice; (c) zoomed SEM image of a composite hollow lattice, with an inserted heatmap of Raman signals from G peak of carbonized polydopamine (C-PDA); (d) TEM image of the tube wall of a hollow strut from the composite hollow lattice, showing that the thickness of alumina nanolayer is 15?nm and that the thickness of C-PDA nanolayer is 8?nm.
Deformation and failure mechanisms of bending-dominated hollow lattices. (a) The deformation induced by uniaxial compression test at the nodes of a lattice can be divided into: beam stretching, beam bending, and node bending. (b) Finite element modeling (FEM) of uniaxial compression of a hollow composite lattice shows that the stress is concentrated at the nodes of the lattice. (c) The bulging hollow nodes from different lattices are shown. The FEM visualizations are taken from compression at a strain of 2.61%. Below the FEM results are the corresponding experimental observations of the nodes from different lattices. The SEM images are taken at the compressive strain of 33%. (d) Failure at the vertical struts in different lattices is compared. The strut of a ceramic lattice shows bucking and kinking in the middle; while the strut of a composite lattice merely inclines with a crack finally initiated at the spot where the strut is joined with horizontal ones.
Deformation and failure mechanisms of bending-dominated hollow lattices. (a) The deformation induced by uniaxial compression test at the nodes of a lattice can be divided into: beam stretching, beam bending, and node bending. (b) Finite element modeling (FEM) of uniaxial compression of a hollow composite lattice shows that the stress is concentrated at the nodes of the lattice. (c) The bulging hollow nodes from different lattices are shown. The FEM visualizations are taken from compression at a strain of 2.61%. Below the FEM results are the corresponding experimental observations of the nodes from different lattices. The SEM images are taken at the compressive strain of 33%. (d) Failure at the vertical struts in different lattices is compared. The strut of a ceramic lattice shows bucking and kinking in the middle; while the strut of a composite lattice merely inclines with a crack finally initiated at the spot where the strut is joined with horizontal ones.

The ideal material should be mechanically strong and reliable but ultralight. Nature achieves this with ease, combining soft, light materials in periodic arrangements like bricks and mortar to create composite materials with improved strength, stiffness, and toughness. It has proved trickier, however, to mimic nature’s cleverness with materials in the lab.

Now, a team of US and Chinese researchers led by Gary J. Cheng at Purdue University believes has come up with a strategy to overcome the ‘rule of mixtures’ and create ultralight, mechanically reliable composite materials [Deng et al., Materials Today (2018), https://doi.org/10.1016/j.mattod.2018.03.027].

Ceramic materials are light and strong, but inherently brittle. The advent of 3D printing, however, is enabling the manufacture of composite materials with complex internal micro- and nanostructures that can compensate for this shortcoming. Hollow 3D architectures offer the prospect of ‘mechanical metamaterials’ with greatly reduced weight but improved stiffness and strength. To date, however, such mechanical metamaterials have fallen into one of two categories, truss structures dominated by stretchable lattices (S-Lattices) and those dominated by bending lattices (B-Lattices). The struts in S-Lattices cannot rotate or bend, so these structures have high stiffness but buckle under compression. By contrast, B-Lattices are more flexible and resistant to impact damage from large strains because their struts can rotate and bend.

Instead, Cheng and his team created a bending-dominated hollow nanolattice material (B-H-lattice), with excellent strength, recoverability, and cyclability, which is coated with a carbonized polymer nanolayer to reduce buckling during deformation.

“We found that the mechanical properties of a 3D ceramic nanolattice can be significantly improved by a nanolayered ceramic/carbon nanolattice with much better stiffness, cyclability, and stiff/strength-weight ratio,” he explains.

The B-H-lattice is made up of bow-tie-like units supported by interconnected vertical struts. The complex architecture is formed using a UV-curable resin template, which is fabricated with lithography. Alumina is deposited on the template along with an exterior nanolayer of carbonized polydopamine, a mussel-inspired biopolymer. Finally, the template is removed leaving a metamaterial that shows high stiffness, low density, recovers without buckling under strains (of up to 55%), and is stable under cyclic loading (up to 15% strain).

“The novelty of our material lies in its non-buckling behavior,” says Cheng. “This is the first time such a recovery mechanism has been realized.”

The new type of metamaterial can accommodate large strains by bending, while suppressing buckling during deformation. The metamaterial’s properties are outstanding, even when compared with natural materials, he says.

Now the team wants to demonstrate large-scale production.

“In the next 10 years, our research could benefit applications like ceramic-based multifunctional structures, mechanically stable energy storage devices, lightweight high-strength materials, and energy dissipation structures,” says Cheng.