The word “metamaterial” conjures up visions of matter interacting with electromagnetic waves to bend the waves around objects, producing a “cloaking device” that hides the object from detection. But the “mechanical metamaterials” that Chris Spadaccini’s group at Lawrence Livermore National Laboratory and Nicholas Fang’s team at the Massachusetts Institute of Technology (MIT) are working on aim to avoid bending as much as possible—mechanical bending, that is. Instead, by causing forces to distribute only in stretching or compression modes along the struts of an octet truss, they have fabricated ultralight, ultrastiff materials from polymers, metals, and ceramics. These materials could have applications in automobiles and aircraft, among other applications, where lightweighting could help conserve fuel without giving up strength.
“I don’t know who coined the term ‘mechanical metamaterial’ or ‘structural metamaterial,’” Spadaccini said, “but we think it’s appropriate in that a metamaterial is something that has a unique property that is based on the structure and the layout of the material as opposed to its composition.”
The unique properties of what Spadaccini calls their “architected” material combine the lightness of an aerogel with a stiffness that is four orders of magnitude higher than that of a typical aerogel. The stiffness and density scale linearly over this range of magnitudes. In cellular materials found in nature, the scaling factor is usually a power of two (quadratic) or three (cubic) or higher, leading to deleterious nonlinear effects that result in a dramatic loss of stiffness with decreasing density. Natural materials tend to have more random pore sizes and distributions that lead to bending under stress.
“You really need to have more of those members within your material that are stretching or in compression as opposed to bending in order to move off that cubic relationship,” Spadaccini said. “But there aren’t many geometric configurations that you can conceive of where nearly all of the elements are in either tension or compression.”
In this case, the researchers chose one of the known geometric configurations that satisfies this requirement, the octet truss, for their experiments, as reported in the June 20 issue of Science (DOI: 10.1126/science.1252291; p. 1373). The unit cell of the octet truss has a regular octahedron at its core, with eight regular tetrahedra on its faces, which lead to a face-centered-cubic (fcc) structure. Using projection microstereolithography, they were able to produce highly ordered, nearly isotropic microlattices within these fcc, stretch-dominated architectures. Critical features in the 20 µm to 40 nm range created lots of void space for ultralightweight properties.
One key to their success was the ability to use projection microstereolithography to fabricate arbitrary, three-dimensional (3D) microscale structures. This additive manufacturing technique is based on building patterns of photocured polymer resins (HDDA or PEGDA) layer by layer. The pattern is generated slice by slice from a 3D CAD model, and projected through a liquid-crystal-on-silicon chip, which acts as a reconfigurable digital photomask, onto the surface of a bath of UV-curable resin. After a layer hardens, the sample is lowered in the polymer bath, new resin coats the surface, and the next layer of the 3D structure is projected and polymerized onto the layer beneath.
The result is an extended microlattice of octet truss unit cells made of solid polymer struts. By coating these struts with a nickel-phosphorus alloy through electroless nickel plating, and removing the polymer struts through thermal decomposition, a hollow-tube metallic Ni-P microlattice can be formed. Similarly, by depositing the ceramic Al2O3 by atomic layer deposition onto the polymer struts and removing the polymer, a hollow ceramic Al2O3 microlattice is formed. For yet another configuration, Al2O3 nanopowder is mixed in with the polymer before the process begins, resulting in a structure consisting of a polymer–ceramic hybrid core. Subsequent heat treatment decomposes the polymer and sinters the Al2O3 to form a solid ceramic microlattice.
In each case (polymer, metallic, hollow ceramic, solid ceramic), uniaxial compression studies yielded plots of relative compressive stiffness and relative compressive strength versus relative density. These plots showed the stiffness and strength to be linear functions of the relative density of the material for each of these stretch-dominated lattices. In contrast, a bend-dominated, solid polymer Kelvin foam made by the same process for comparison decreased in strength and stiffness by a power of two with decreasing density.
“We fabricated an ultrastiff, ultrastrong material that is primarily void space, which makes it very light weight,” Spadaccini said. “Then we took that a step further by combining additive micromanufacturing processes with nanoscale coating processes to give us a material that is about as light as an aerogel. The mechanical properties, relative to the material’s density, go through the roof.”