Fabrication Defect-Insensitive Minimal Surface-Based 3D Metamaterial at Nanoscale

Material scientists have been striving to break the conventional relationships between mutually exclusive materials’ properties such as density-strength and to fill the white area in the materials’ property space. One effective way to explore the empty space is harnessing nanoarchitectures to develop lightweight and strong artificial materials not only by the architectural designs but also by embedding the benefits of nanoscale effects of materials into hierarchical nanostructures. Generally, in fabrication aspects of those nanoarchitectures, multi-step procedures are required including fabrication of architecture frames and material conversions. During these complicated procedures, fabrication defects are almost inevitable, especially when we considering mass production too. Therefore, at some point, the design of architecture itself is demanded to be defect insensitive with high tolerance of fabrication defects.<br/>For that purpose, we make the best use of 3D minimal surface structure to the design of nanoarchitectures. Since the minimal surface is a local minima of surface area under given constraints by its definition, it is energetically beneficial and efficient in the use of limited resources. Moreover, 3D minimal surface, so called Triply Periodic Minimal Surface (TPMS), exhibits smooth continuous surface without any self-intersections or singularities in all three dimensions avoiding the stress concentrations. And one important characteristic of them is that Gaussian curvature is negative everywhere on the surface by definition. This negative Gaussian curvature plays a key role to superior load carrying capacity and defect insensitivity which will be discussed in this study. Because of these fascinating features of TPMS structures, many engineers have been trying to use them for architected materials in macro- and microscale to achieve the best mechanical performance. However, many of the experimental works barely reach to the level of their initially designed performance due to the fabrication defects. Especially, for brittle ceramic materials, not many experimental works have been investigated and the performance of them has been severely affected by the imperfections resulting to the steep slope on the density-strength Ashby plots.<br/>Therefore, in this study, we are suggesting nanoscale diamond-like 3D minimal surface shell made of brittle amorphous alumina by intertwining the advantages of TPMS structure and the nanoscale effect of the base material to overcome these limitations. Nanopatterning using near-diffraction patterns of laser with multi-phase masks enables us to fabricate the nanoscale TPMS structures with rapid large area production. In addition, it has been reported that the elastic body at nanoscale withstands high stress fields without catastrophic fracture so that to bring additional opportunities for the inelastic deformation even in intrinsically brittle materials. This unexpected inelastic deformation of the ceramic body makes the TPMS structure defect-insensitive. Experimental data from monotonic and cyclic micropillar compressive tests reveals that our nanoarchitecture exhibits damping behavior, high strength and low scaling exponent of 1.07 in density-strength chart even with severe geometric defects. This new minimal surface nanoarchitecture will contribute to creating superior engineering ceramics with capability of mass production through its high defect tolerance.