Finding the Materials Harder than Diamond: Macroscale and Microscale Studies

Triply periodic minimal surfaces (TPMSs) are currently promising models for novel metamaterials that can be produced by 3D printing. It was previously shown how features of crystal microstructures can be transferred to architected materials, providing lightweight, damage-tolerant designs with desired strength and toughness (Pham, M.-S.; Liu, C.; Todd, I.; Lertthanasarn, J. Damage-Tolerant Architected Materials https://doi.org/10.1038/s41586-018-0850-3). This work uses a simple method to produce TMPS architectures via a polycrystal-like approach. We show that among incoherent, coherent polycrystalline-like cellular metamaterials those with twin grain boundaries, only the latter can provide effectively increased isotropy (up to 3.61 times), while maintaining mechanical properties. We have found that face-centered cubic (FCC) grain packing in metamaterials is superior in Young's and shear moduli and isotropy compared to random grain arrangement, while grain boundary strengthening is essential for toughness. We demonstrate how our crystal-inspired methodology can design metamaterials exceeding the diamond-type structure in Young's and shear moduli, isotropy, and toughness. Extreme twinning of a diamond metamaterial has produced a metamaterial similar to fullerite with Young's and shear moduli, strength, and isotropy superior to the diamond-type metamaterial lattice. This is in line with the existing reports on fullerite outperforming diamond at the microscale (Kvashnina, Y. A.; Kvashnin, A. G.; Chernozatonskii, L. A.; Sorokin, P. B. Fullerite-Based Nanocomposites with Ultrahigh Stiffness. Theoretical Investigation. Carbon N Y 2017, 115, 546-549. https://doi.org/10.1016/j.carbon.2017.01.028). We then studied the hypervelocity ballistic impact of a bullet on the twinned diamond at the microscopic scale applying molecular dynamics (MD). The twin diamond appeared to outperform a singular crystalline diamond in toughness. Our results would contribute to designing high-performance metamaterials both at macro- and miscroscale.