Suppressed Size Effect in Nanopillars with Hierarchical Microstructures Enabled by Nanoscale Additive Manufacturing

WX Zhang and Z Li and RQ Dang and TT Tran and RA Gallivan and HJ Gao and JR Greer, NANO LETTERS, 23, 8162-8170 (2023).

DOI: 10.1021/acs.nanolett.3c02309

Studies on mechanical size effects in nanosized metals unanimously highlight both intrinsic microstructures and extrinsic dimensions for understanding size-dependent properties, commonly focusing on strengths of uniform microstructures, e.g., single-crystalline/nanocrystalline and nanoporous, as a function of pillar diameters, D. We developed a hydrogel infusion-based additive manufacturing (AM) technique using twophoton lithography to produce metals in prescribed 3D-shapes with similar to 100 nm feature resolution. We demonstrate hierarchical microstructures of as-AM-fabricated Ni nanopillars (D similar to 130-330 nm) to be nanoporous and nanocrystalline, with d similar to 30-50 nm nanograins subtending each ligament in bamboo-like arrangements and pores with critical dimensions comparable to d. In situ nanocompression experiments unveil their yield strengths, sigma, to be similar to 1-3 GPa, above single-crystalline/nanocrystalline counterparts in the D range, a weak size dependence, sigma alpha D-0.2, and localized-to- homogenized transition in deformation modes mediated by nanoporosity, uncovered by molecular dynamics simulations. This work highlights hierarchical microstructures on mechanical response in nanosized metals and suggests small-scale engineering opportunities through AM-enabled microstructures.

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