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Friction from initial failures allows microlattices to retain their performance

By Lauren Borja March 9, 2018
Friction from initial failures
Micrographs of tested microlattices with different types of architecture, in honeycomb (a), hexagonal (b), cubic (c), and tetrahedral (d) geometries after one cycle and in (e)-(h) after 20 cycles to a maximum displacement of 5 µm. Already after the first cycle, numerous fracture events within the different structures are visible (marked with arrows), mainly at the nodes. After 20 cycles, the extent of damage increased for all structures. Credit: Journal of Materials Research

Researchers at the Karlsruhe Institute of Technology (KIT) and the Swiss Federal Institute of Technology (ETH) in Zürich, led by Ruth Schwaiger, have explored how polymer and ceramic-polymer composite microlattices deform under compression. Their results, reported in a recent issue of the Journal of Materials Research, will help scientists engineer strong, low-density materials which could be used in aerospace applications.

Initially recognized for their superior strength, microlattices are now being designed with specific mechanical properties. The mechanical properties of a microlattice such as strength, elasticity, and ability to dissipate energy after impact are governed by its constituent materials and architecture. Some materials exhibit higher strengths at the microscale, which can increase a microlattice’s resistance to failure. By understanding various architectures, researchers can engineer microlattices to precise specifications.  

To understand how both material and architecture influence microlattice mechanical properties, Schwaiger and her colleagues fabricated a series of polymer microlattices using three-dimensional direct laser writing. Half of these microlattices were then coated with alumina using atomic layer deposition to create the ceramic-polymer composite microlattices. Schwaiger says that while this process was very time consuming, it allowed her group to precisely “make accurate, well-defined structures.” To investigate how architecture affects mechanical properties, microlattices with honeycomb, hexagonal, cubic, and tetrahedral subunits were fabricated. The different microlattices were then repeatedly compressed and their response observed using a scanning electron microscope.

For all the examined materials and architectures, the first compression cycle damaged many of the internal connections between trusses and the microlattices were not able to dissipate as much energy in subsequent compressions. Because many of the architectures appeared mostly intact after the first cycle, Schwaiger used simulations to confirm that the change in observed mechanical properties was due to numerous broken internal connections.  

Despite this initial degradation in performance, Schwaiger says that many microlattices “still [had some degree of] mechanical performance, even if some parts were not intact anymore.” Surprisingly, many of the different architectures were able to use friction to dissipate energy in later cycles. Because the broken struts rubbed against each other as the lattice compressed, this allowed both the polymer and ceramic-polymer microlattices to dissipate energy. In the case of the ceramic-polymer lattices, friction between two edges of a crack in the ceramic coating could also dissipate energy.

“It’s not just that when the structure fails, it does not work anymore,” Schwaiger says; “There are different mechanisms that kick in and the [structure] can continue to work.”

Different architectures responded differently to cyclic testing. Honeycomb microlattices dissipated the most energy, but they also experienced the most damage and change in strength over multiple compression cycles. On the other hand, tetrahedral architectures displayed the most consistent behavior and recovered the best after many cycles. “It’s an interesting concept that the damping is driven by the architecture and not the material itself,” says Tobias Schaedler of HRL Laboratories. Schaedler was not affiliated with the recent publication.

In the future, Schwaiger sees potential for more characterization of these microlattices. X-ray microscopy could be used to observe how the internal cell structure of the microlattices changes during deformation. This would allow researchers to more precisely quantify the nature and extent of the damage after the first compression.

Microlattices have received much attention as next-generation materials for aerospace applications or impact energy dissipation. These characterization studies help scientists understand how to use a microlattice’s architecture to their advantage, making it another “tunable degree of freedom” in materials design, according to Schaedler. According to Schwaiger, “tailored properties—like stiffness, deformability, thermal expansion—can be designed through the architecture.”

Read the abstract in the Journal of Materials Research.