Microbullet hits confirm graphene’s strength

Scientists at Rice University have developed a technique to fire microbullets at graphene sheets to show that multilayer graphene – 10 to 100 nm thick – may make excellent body armour.

Since the discovery of graphene in 2004, the material has been extensively studied for its electronic properties, static strength and stiffness, owing to its two-dimensional honeycomb structure.

However, this is the first study to measure its ability to absorb an impact. Jae-Hwang Lee, lead author of the study from the University of Massachusetts, said: “We are the first group to test graphene under high speeds comparable to actual bullet speeds.” The results suggest graphene can absorb ten times the amount of energy as steel before failing.

The study, published in Science, uses the energy from a laser to drive microbullets away from the opposite side of an absorbing gold surface at great speed. The team built a custom stage to line up multilayer graphene sheets. They then used a high-speed camera to capture microscopic images of the damages to the sheets. In every case, the 3.7-micron spheres punctured the graphene. But rather than a neat hole, the spheres left a fractured pattern of “petals” around the point of impact, indicating that graphene stretched before breaking.

“We started writing the paper about petals, but as we went along, it became evident that wasn’t really the story,” said Professor Edwin Thomas, Dean of Engineering at Rice University. “The bullet’s kinetic energy interacts with the graphene, pushes forward, stretches the film and is slowed down.”

The experiments revealed graphene to be a stretchy membrane that, in about 3 nanoseconds before puncture, distributes the stress of the bullet over a wide area defined by a shallow cone centred at the point of impact. “It’s a race,” Thomas said. “If the cone can move out at an appreciable velocity compared with the velocity of the projectile, the stress isn’t localised beneath the projectile.”

Ideally, there will be a lot of independent layers that are not too far apart, but not so close that they are touching as this would change the loading from tensile to compressive. Controlled layering of graphene sheets could lead to lightweight, energy-absorbing materials for use as body armour or for shielding spacecraft.

This technique could also be used to test many experimental materials. “This lets us develop rapid methodologies to test nanoscale materials and find promising candidates. We’re working to demonstrate to NASA and the military that these microscopic tests are relevant to macroscopic properties,” said Thomas.

Paper:

http://www.sciencemag.org/content/346/6213/1092.short#sthash.VhiR8Dah.dpuf

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