Stronger than a speeding bullet, but lighter

New tests of nanostructured material could lead to better armor against everything from gunfire to micrometeorites
November 9, 2012

This electron-microscope image of a cross-section of a layered polymer shows the crater left by an impacting glass bead, and the deformation of the previously even, parallel lines of the layered structure as a result of the impact. In this test, the layered material was edge-on to the impact. Comparative tests showed that when the projectile hit head-on, the material was able to resist the impact much more effectively. (Credit: Thomas Lab, Rice University)

While traditional shields have been made of bulky materials such as steel, body armor made of lightweight material such as Kevlar has shown that thickness and weight are not necessary for absorbing the energy of impacts.

Now, a new study by researchers at MIT and Rice University has shown that even lighter materials may be capable of doing the job just as effectively.

The key is to use composites made of two or more materials whose stiffness and flexibility are structured in very specific ways — such as in alternating layers just a few nanometers thick. The research team produced miniature high-speed projectiles and measured the effects they had on the impact-absorbing material.

The results of the research are reported in the journal Nature Communications, in a paper co-authored by former postdoc Jae-Hwang Lee, now a research scientist at Rice; postdoc Markus Retsch; graduate student Jonathan Singer; Edwin Thomas, a former MIT professor who is now at Rice; graduate student David Veysset; former graduate student Gagan Saini; former postdoc Thomas Pezeril, now on the faculty at Université du Maine, in Le Mans, France; and chemistry professor Keith Nelson. The experimental work was conducted at MIT’s Institute for Soldier Nanotechnologies.



The researchers developed a self-assembling polymer with a layer-cake structure: rubbery layers, which provide resilience, alternating with glassy layers, which provide strength. They then developed a method for shooting glass beads at the material at high speed by using a laser pulse to rapidly evaporate a layer of material just below its surface.

Though the beads were tiny — just millionths of a meter in diameter — they were still hundreds of times larger than the layers of the polymer they impacted: big enough to simulate impacts by larger objects, such as bullets, but small enough so the effects of the impacts could be studied in detail using an electron microscope.

Seeing the layers

Structured polymer composites have previously been tested for possible impact-protection applications. But nobody had found a way to study exactly how they work — so there was no way to systematically search for improved combinations of materials.

The new techniques developed by the MIT and Rice researchers could provide such a method. Their work could accelerate progress on materials for applications in body and vehicle armor; shielding to protect satellites from micrometeorite impacts; and coatings for jet engine turbine blades to protect from high-speed impacts by sand or ice particles.

The methods the team developed for producing laboratory-scale high-speed impacts, and for measuring the impacts’ effects in a precise way, “can be an extremely useful quantitative tool for the development of protective nanomaterials,” says Jae-Hwang Lee, now a research scientist at Rice, the lead author of the paper, who did much of this research while in MIT’s Department of Materials Science and Engineering. “Our work presents some valuable insights to understand the contribution” of the nanoscale structure to the way such materials absorb an impact, he says.

Because the layered material has such a predictable, ordered structure, the effects of the impacts are easily quantified by observing distortions in cross-section. “If you want to test out how ordered systems will behave,” Singer says, “this is the perfect structure for testing.”

The work was supported by the U.S. Army Research Office.