Researchers develop new ultralight, ultrastiff 3-D printed materials

MIT and Lawrence Livermore use additive manufacturing to produce 3-D lattice metamaterials that are 100 times stiffer than previously reported materials.

Researchers at the Lawrence Livermore National Laboratory (LLNL, Livermore, Calif., USA) and Massachusetts Institute of Technology (MIT, Cambridge, Mass., USA) have developed a material which they claim is the same weight and density as aerogel — an ultralight material referred to as “frozen smoke” — but with 10,000 times the stiffness. They believe this material could have a profound impact on the aerospace and automotive industries as well as other applications where lightweight, high-stiffness and high-strength materials are needed.

Produced using additive micro-manufacturing processes, the researchers’ findings are published in a June 20 article in the journal Science titled "Ultralight, Ultrastiff Mechanical Metamaterials." Described as micro-architected metamaterials — artificial materials that have properties not found in nature —  they maintain a nearly constant stiffness per unit mass density, even at ultralow density. The research team explained that most lightweight cellular materials have mechanical properties that degrade substantially with reduced density because their structural elements are more likely to bend under applied load. These  metamaterials, however, exhibit ultrastiff properties across more than three orders of magnitude in density. The researchers believe that materials with these properties could someday be used to develop parts and components for aircraft, automobiles and space vehicles.

"These lightweight materials can withstand a load of at least 160,000 times their own weight," said LLNL Engineer Xiaoyu "Rayne" Zheng, lead author of the article in Science. "The key to this ultrahigh stiffness is that all the micro-structural elements in this material are designed to be over constrained and do not bend under applied load." According to the research team's findings, this observed high stiffness is shown to be true with multiple constituent materials such as polymers, metals and ceramics.

"Our micro-architected materials have properties that are governed by their geometric layout at the microscale, as opposed to chemical composition," said LLNL Engineer Chris Spadaccini, co-author and leader of the joint research team. "We fabricated these materials with projection micro-stereolithography." This additive micro-manufacturing process involves using a micro-mirror display chip to create high-fidelity 3-D parts one layer at a time from photosensitive materials. It allows rapid generation of materials with complex 3-D micro-scale geometries that are otherwise challenging or impossible to fabricate.

"Now we can print a stiff and resilient material using a desktop machine," said MIT professor and key collaborator Nicholas Fang. "This allows us to rapidly make many sample pieces and see how they behave mechanically." The team was able to build microlattice structures out of polymers, metals and ceramics. In one example, they used polymer to fabricate microlattices which were then coated with a thin-film of metal ranging from 200 to 500 nanometers thick. The polymer core was then thermally removed, leaving hollow-tube metal struts, and a resulting ultralight metal lattice materials.

"We have fabricated an extreme, lightweight material by making these thin-film hollow tubes," said Spadaccini, who also leads LLNL's Center for Engineered Materials, Manufacturing and Optimization. "But it was all enabled by the original polymer template structure." The team repeated the process with polymer mircolattices, but instead of coating it with metal, ceramic was used to produce a thin-film coating about 50 nanometers thick. The density of this ceramic micro-architected material is similar to aerogel, which is among the lightest materials in the world. However, Spadaccini said that due to the ceramic microlattice’s micro-architected layout, “it performs with four orders of magnitude higher stiffness than aerogel at a comparable density."

The team produced a third ultrastiff micro-architected material using a slightly different process, which they began by loading a polymer with ceramic nanoparticles to build a polymer-ceramic hybrid microlattice. The polymer was removed thermally, allowing the ceramic particles to densify into a solid. The new solid ceramic material also showed similarly extreme strength and stiffness properties.

The LLNL-MIT teams' new materials are 100 times stiffer than other ultra-lightweight lattice materials previously reported in academic journals. The project was funded by the U.S. Defense Advanced Research Projects Agency (DARPA) and Lawrence Livermore's Laboratory Directed Research and Development (LDRD).