Perfecting bullet-proof glass from the nanoscale up
Material not only stops the bullets, but also remains clear. Could this be the future of glass?
By MATTHEW M. BURKE | STARS AND STRIPES Published: April 16, 2013
Rice University scientist Ned Thomas held up a polyurethane disc with three bullets perfectly suspended inside.
The material had not only stopped the bullets but also remained clear; there were no cracks and the material had even sealed the bullets’ entry-holes behind them.
The disc, made of a complex polyurethane material, could be the future for ballistic windshields, body armor and aerospace materials.
“People think the bullets are cast inside there,” said Thomas, who heads the research team as the dean of Rice’s engineering school. “Polyurethanes are pretty amazing materials.”
Using the material in Humvees and MRAPs is still at least five years away — too late for the wars in Iraq and Afghanistan —but it was born out of those conflicts and the research dollars that flowed as a result, said Thomas, whose researchers are working on the project with the Massachusetts Institute of Technology’s Institute for Soldier Nanotechnologies.
As IED attacks increased dramatically in Iraq in 2004 and 2005, people took notice that trucks and Humvees whose undercarriage had been sprayed by a corrosion-proofing polyurethane material seemed to fare better in blast events, Thomas said. However, there seemed to be no answer for chipping and cracking ballistic windshields, or for fuel trucks that spouted dangerous leaks after being shot by insurgents.
At the time, Thomas, a former captain in the U.S. Army Corps of Engineers, was working at the Institute for Soldier Nanotechnologies, which he founded in 2002, to develop better protective materials to safely lighten the load for troops in body armor and gear.
Polyurethane — the stuff used to make thousands of everyday items from garden hoses to skateboard wheels — was viewed as a potential answer. Researchers hoped a material could be engineered with hard and soft, glassy and rubbery, layers that could dissipate the strain of a projectile and be used to improve body armor, to seal fuel trucks so they won’t leak after being shot, to build stronger windshields, and even to save the lives of troops whose vehicles rollover roadside bombs.
Backed by the U.S. Army Research Office, Thomas and his team began to tinker, eventually discovering a complex multiblock copolymer polyurethane that was successful in stopping bullets in ballistics tests by melting in front of the projectile, stopping it and sealing the holes behind it.
“There’s no macroscopic damage; the material hasn’t failed; it hasn’t cracked,” Thomas said in an October news release from Rice. “You can still see through it. This would be a great ballistic windshield material.”
So far, his peers agree.
“I think their work on the complex multiblock copolymers might actually be ground-breaking,” said K.T. Ramesh, director of the Johns Hopkins’ Extreme Materials Institute. “This could indeed be the future of windshields for some kinds of vehicles.”
But Thomas and his team were not satisfied. They wanted to know more. Why had the material worked the way it did? They hoped that further study could lead to even greater and more helpful materials.
“Theoretically, no one understood why this particular kind of material – which has nanoscale features of glassy and rubbery domains – would be so good at dissipating energy,” he said. “We want to find out why this polyurethane works the way it does.”
But there were issues with the cost and time – issues that plague all chemists and scientists who study materials, Thomas said. When chemists come up with a new material, they only produce a small amount. It can be expensive and time consuming to make enough of a material to test, then one must bring it to a range, perform ballistics testing, only to find out it didn’t work or a variable needs to be altered, and go back to the drawing board.
To solve that problem, Thomas developed a miniaturized test method to shoot high-velocity silica spheres into the microscopic layers of a new material he created that was similar to the bullet-stopping material. Under an electron microscope, it looked like corduroy so disruption patterns could be viewed easily, Thomas said.
This method saved time, money, and the testing could be done in a lab instead of at the range. When he moved to Rice University in Texas about a year ago, Thomas continued the research.
“After the impact we can go in and cross-section the structure and see how deep the bullet got, and see what happened to these nice parallel layers,” Thomas said. “They tell the story of the evolution of penetration of the projectile and help us understand what mechanisms, at the nanoscale, may be taking place in order for this to be such a great, high-performance, lightweight protection material.”
Now they could do thousands of tests per week, testing an array of materials, Thomas said. They could now see if a material was scratch resistant, if it swelled when it came into contact with diesel fuel, and if adding a bit of clay to the mixture would make it stronger.
Ramesh said that testing materials on a nanoscale is not groundbreaking, but few have done it well. He said the team’s work was “outstanding” and could lead to the development of better armor.
“This is an advance in the fundamental science that is needed before one can develop better protective systems, and so I’m very pleased to see the nature and quality of the results presented here,” he said. “In my opinion, it is always the case that better materials and systems are produced more quickly and in a more robust manner when these manufacturing processes are buttressed by an understanding at the fundamental level.”
However, Ramesh said that while the results are exciting, there are some disadvantages and even dangers to testing on the nanoscale level. Results observed on a nano-scale don’t always extrapolate to the macro-scale.
“That is certainly the case here: we can learn some things that are very useful with respect to ballistic experiments,” he said, “but we have to be very cautious about interpreting these results in terms of macro-scale phenomena.”
Thomas said that the tests on the nanoscale can lead to macro-scale testing when there is promise. He hopes to test other lightweight, nanostructured materials like boron nitride, carbon nanotube-reinforced composites and graphite and graphene-based materials, accelerating the design of lab-engineered materials with precise controls over their micro-structures to have specific benefits depending on the platform it will be used in, whether it be body armor, jet engine turbine blades, the canopy of an F-18, spacecraft or satellites.
He said that with the wars winding down, there has to be other uses for the materials or the research dollars will undoubtedly dry up.
“These are interesting materials that are not expensive and seem to be performing in an extraordinary way,” he said. “There’s way more research to do.”
An electron microscope image of a silica sphere fired into a block copolymer shows how the glassy and rubbery layers contorted as they dissipated the strain from the projectile. Directly in front of the sphere, heat from the shot melted the copolymer into a homogeneous liquid.
Thomas Lab/Rice University