Design of hierarchical surface lattice microstructures with isotropic stiffness, strength and energy absorption

Surface lattice microstructures have garnered significant attention, however their efficient design with isotropic mechanical properties remains a considerable challenge. This study proposes a novel class of hierarchical surface lattice microstructures that simultaneously achieve isotropic stiffness, strength and energy absorption properties. The key idea involves embedding a set of second-level lattices within the solid regions of triply periodic minimal surface (TPMS) lattices, then realizing uniform properties by tuning the distribution of uniaxial stiffness at two levels. The effectiveness is demonstrated across both sheet-type and solid-type TPMS architectures. Furthermore, the micro-laser powder bed fusion technology is employed for high-precision fabrication of the proposed hierarchical design with two-level feature size ratio exceeding 500:1. Compression tests validate the isotropic stiffness, as well as the nearly-isotropic initial peak stresses and specific energy absorption characteristics of the hierarchical surface lattices. The maximum-to-minimum property ratios are notably reduced from 3.09, 2.17 and 2.03 for the TPMS counterparts to 1.00, 1.25 and 1.23 for the hierarchical designs, respectively. It is remarkable that the designed hierarchical surface lattices preserve geometric advantages of TPMS, including smooth surfaces, open-cell architectures, and configuration symmetry. Hence, they present a transformative design strategy for surface lattice microstructures in multifunctional applications.