Elsevier

Extreme Mechanics Letters

Volume 2, March 2015, Pages 7-14
Extreme Mechanics Letters

Ultra-strong architected Cu meso-lattices

https://doi.org/10.1016/j.eml.2015.01.006Get rights and content

Abstract

3-dimensional solid Cu octet meso-lattices with characteristic features on the micron-scale were fabricated and mechanically tested under uniaxial compression. These architected cellular materials were fabricated by a three-step process: (1) direct laser writing of the lattice pattern into a polymer template, (2) electroplating of Cu into the template, and (3) removal of the polymer matrix. The microstructure of the electroplated Cu mainly consists of polycrystalline grains with average diameters of 2μm such that cross-sections of lattice beams mostly consist of a single grain. We discovered that the compressive yield strengths of the open-cell Cu meso-lattices can exceed the yield strength of monolithic bulk Cu as measured from a Cu thin film made with identical conditions. Meso-lattices with relative density of 0.8 had a strength of 332 MPa, which surpassed the bulk yield strength by 80%. This is diametrically opposite to predictions from structural mechanics theory, which states that strength scales linearly with relative density for the octet structure. We attribute the ability of solid Cu meso-lattices to attain such high strengths to the “smaller is stronger” size effect present in single crystalline metals with sub-micron dimensions. This work demonstrates the use and proliferation of the size-dependent strengthening unique to nanostructures in an architected structural material.

Introduction

A decade of intense study has shown the universality of the “smaller is stronger” size effect in single crystalline metals with sub-micron dimensions subjected to uniaxial deformation  [1], [2]. Single crystalline metallic samples with micro- to nanoscale dimensions often have strengths enhanced by orders of magnitude relative to the equivalent bulk-scale materials because plasticity is carried out via nucleation of dislocations from single-arm sources and/or surfaces rather than by dislocation multiplication alone [3], [4]. Cylindrical micro- and nanopillars have served as the typical geometry for studying plasticity in nanocrystals because they experience a well-defined stress state upon uniaxial loading. Fewer efforts have been dedicated to exploring the “smaller is stronger” paradigm under complex structural geometries or stress states.

We show that size-dependent strengthening can be used in a hierarchical structural material by designing and building a porous meso-lattice out of microscale structural elements such that a high free surface to volume ratio is preserved and lattice beams behave essentially like interconnected single crystalline micropillars  [5], [6]. The use of this materials size effect has been previously demonstrated in nanoporous foams, in which structural strength is determined by ligament size, in additional to relative density and material properties  [7], [8]. Although porous foams are appropriate for many low density structural applications, the topologies of ordered architected cellular solids can be designed to optimize properties like strength, stiffness, and fracture toughness per density at every relevant scale—from microstructural features to individual truss members to unit cells to the overall structure  [9], [10]. The use of nanoscale beams in cellular solids enables utilizing the material size effect to enhance the mechanical strength of the structure by providing another tunable parameter, characteristic lattice member size, in addition to geometrical attributes. In this paradigm of bulk-material processing, the mechanical properties can surpass those attainable by the existing processing routes because it enables independent optimization of structural topology, material chemistry and microstructure.

Ordered architected structures are ideal for lightweight structural applications because they have a superior strength per weight ratio compared with disordered porous foams  [5], [9]. In particular, the octet structure, a lattice with face centered cubic arrangement of nodes, is predicted to have a yield strength that scales linearly with relative density, ρ̄, because structural deformation is accommodated by stretching of the lattice members [11]. Open-cell foams whose beams deform by bending have strengths that scale as ρ̄3/2, which means that the octet-truss and other stretching-dominated geometries are a better choice for lightweight structural applications. Such stretching-dominated 3-dimensional ordered architectures have been notoriously challenging to fabricate, especially on the microscale, where sample size-effects are expected to emerge. Mechanically robust 3D metallic lattices and sandwich structures have been formed at the macroscale through investment casting, weaving metal fibers and by folding and welding stacks of 2D metal lattices  [12], [13]. Hollow Ni and NiP micro-lattices with features like the tube wall thickness, lattice beam length, and sample dimensions extending from the nano- to the millimeter scale were formed by the electroless plating of metal onto a 3-dimensional photolithographed polymer scaffold which was subsequently removed  [14], [15]. The structural topology of these micro-trusses is limited by the polymer waveguide fabrication technique to produce bending-dominated geometries only. Hollow metallic and ceramic nano-lattices have also been fabricated through atomic layer deposition and RF sputtering onto a solid polymer lattice fabricated by 2-photon lithography [16], [17], [18]. These hollow nanostructured lattices are extremely mechanically robust considering their lightweight nature, but are limited in their stiffness and strength compared to lattices with solid beams.

Section snippets

Materials and methods

In this work, we describe fabrication and mechanical compression experiments on solid Cu lattices with octet structural topology and micron-sized beam thicknesses, in which lattice members deform similarly to single crystalline micropillars. Solid metallic meso-lattices were fabricated by electroplating Cu into a polymer template with pores defined via direct laser writing, and subsequent removal of the polymer (Fig. 1). This process extends the method of Gansel et al. to 3D interconnected

Results

Uniaxial compression experiments were carried out in the G200 XP Nanoindenter (Agilent) using a 120 μm-diameter diamond flat punch tip at a prescribed strain rate of 0.0025s1. Load–displacement data from compression experiments were converted to engineering stress–strain curves by normalizing by sample height and cross-sectional area. The stress–strain plots obtained from compression tests has features typical of cellular solids compression such as linear-elastic, plateau, and densification

Discussion

Comparison of the yield strengths of the meso-lattices and of the electroplated Cu thin film revealed that meso-lattices with ρ̄>0.5 and 6 μm unit cell size were stronger than the bulk yield strength, with the densest structures (ρ̄0.8) having a strength 1.8 times higher than the strength of bulk Cu (Fig. 4). This finding is remarkable because introducing porosity into a bulk material should lower its strength, because less material within the same volume supports the same load in the case of

Conclusions

In summary, we developed a fabrication process for monolithic 3-dimensional Cu lattices with structural (unit cell size) and microstructural (grain size) features on the micron-scale. Compression experiments on meso-lattices with ρ̄>0.5 and 6μm unit cell size revealed strengths that were 1.8 times higher than that of monolithic bulk Cu. We deduce that the single crystalline regions in the lattice beams exhibit the “smaller is stronger” size effect which elevates the overall structural strength

Acknowledgments

XWG would like to thank the National Defense Science and Engineering Graduate Fellowship for financial support during her Ph.D. studies. JRG gratefully acknowledges the financial support of National Science Foundation grants DMR-1204864 and CMMI-1234364. Thanks to Nigel Clark for help with electroplating and to Lucas Meza and Lauren Montemayor for helpful discussions and computer design of mechanical structures. We are grateful to the Kavli Nanoscience Institute and staff for cleanroom

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