The Institute for Soldier Nanotechnologies (ISN), made up of the MIT, Caltech, ETH Zurich and the US Army Research Lab, has used 3D printing technology at the nanoscale to form a material that is reportedly more effective at stopping a projectile than Kevlar or steel.
Thinner than a single human hair, the material is made from tiny carbon struts that form interconnected tetrakaidecahedrons – structures with 14 faces – that are fabricated via two-photon lithography.
According to the team, the nano-architected material could potentially replace kevlar for a wide array of bulletproof protective gear used by the armed forces.
According to Julia Greer, Materials Scientist at Caltech, “the knowledge from this work could provide design principles for ultra-lightweight impact-resistant materials for use in efficient armor materials, protective coating, and blast-resistant shields desirable in defense and space applications.”
Engineering material structures at the nanoscale
Nano-architected materials have a structure that is designed at the nanometer scale, enabling scientists to engineer virtually any imaginable 3D shape with desirable properties. While the strength of nano-architected materials has been previously studied under compression and tension, the ISN team sought to explore how such a material might survive high-speed impacts.
The material developed by the ISN consists of interconnected tetrakaidecahedrons made up of carbon struts that are arranged via two-photon lithography. Greer’s team has been exploring the capabilities of two-photon lithography in printing nanoscale 3D printed objects since 2018.
The tetrakaidecahedron structure was first proposed by Lord Kelvin in the 19th century as theoretically one of the most efficient structures possible for filling space with duplicates of itself.
A light-sensitive photoresist forms the basis of the nano-architected material, conforming its shape based on light exposure from the lasers during the two-photon lithography process. During this process, a tightly focused laser is traced within the photoresist in three dimensions, solidifying the material until the full structure is printed. The printed structures are then pyrolyzed via burning in a furnace at extremely high temperatures to convert the polymer to pyrolytic carbon.
Two versions of the ultra-thin material were created with different densities and blasted with microparticles of 14-micron diameters at speeds of between 40 and 1,100 meters per second. For reference, the speed of sound is 340 meters per second. The denser version of the material was found to be more resilient to the blasts, with the microparticles embedding themselves in the material rather than tearing through, as would be the case with either fully dense polymers or carbon sheets of the same thickness.
The carbon struts immediately surrounding the microparticle were observed to crumple while the overall structure remained intact. According to the ISN team, pound for pound the nano-architected material outperformed steel by more than 100 percent, and Kevlar composites by more than 70 percent.
“Historically, this geometry appears in energy-mitigating foams,” said Carlos Portela, Assistant Professor of Mechanical Engineering at MIT and lead author of the paper. “While carbon is normally brittle, the arrangement and small sizes of the struts in the nano-architected material gives rise to a rubbery, bending-dominated architecture.
“We show the material can absorb a lot of energy because of this shock compaction mechanism of struts at the nanoscale versus something that’s fully dense and monolithic, not nano-architected.”
The ISN partners believe the developed material has the potential to replace Kevlar and steel for armor materials, protective coating, and blast-resistant shields used by soldiers in the armed forces. However, further development still needs to be undertaken before the material can be used in real-world applications.
Going forward, the researchers will seek to find ways to scale up the production of the material and explore how other nano-architected materials can hold up under high-speed impacts.
Further information on the nano-architected material can be found in the paper titled: “Supersonic Impact Resilience of Nanoarchitected Carbon”, published in the Nature Materials journal. The study was co-authored by C. Portela, B. Edwards, D. Veysset, Y. Sun, K. Nelson, D. Kochmann and J. Greer.
Nanoscale 3D printing
Thanks to the range of potential applications opened up by nanoscale 3D printing, scientists are increasingly looking to optimize the technology and develop new processes, materials, and applications.
For instance, researchers from the University of Dayton have developed an enhanced, cost-effective technique to 3D print nanoscale structures, known as Opto-Thermo-Mechanical (OTM) nano-printing. Utilizing low-cost laser beams, the technique is capable of printing at scales a thousand times smaller than a human hair.
Elsewhere, scientists from Fraunhofer IMM are developing a novel multi-photon lithography process to produce nanoscale metal 3D printed structures, and researchers at the National Institute of Standards and Technology (NIST) have been working on a new method to 3D print gels and soft materials at the nanoscale. According to NIST, the technique could allow for the creation of complex microscopic structures, such as flexible electrodes, biosensors, or soft micro-robots.
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Featured image shows the ISN has developed a nano-architected material that is reportedly more effective at stopping a projectile than Kevlar or steel. Photo via Caltech.