_TECHNOLOGY Materials

_The Breaking Point

A new finding expands scientists' understanding of how layered materials handle pressure.

_Michel Barsoum

Barsoum is a distinguished professor in the College of Engineering’s Department of Materials Science and Engineering.

_Mitra Taheri

Taheri is the Hoeganaes associate professor in the Department of Materials Science and Engineering.

Bend, but don’t break. It’s an approach to life, and also the way materials in nature are inclined to function.

New findings from Drexel researchers indicate that contrary to common understanding of how layered materials — everything from sedimentary rocks to atomic layers of graphite — behave when compressed, they actually form a series of ripples as they deform.

“Dislocation theory — in which the operative deformation micromechanism is a defect known as a dislocation — is very well established and has been spectacularly successful in our understanding of the deformation of metals,” says Garritt J. Tucker, who was an assistant professor in the College of Engineering’s Department of Materials Science and Engineering when the study was published. “But it never really accurately accounted for the rippling and kink band formation observed in most layered solids.”

Ripplocation

Ripplocation, a rippling of internal atomic layers, is observed in a ceramic material when loaded parallel to the layers.

A 2015 paper published by a group at the Massachusetts Institute of Technology suggested a new deformation micromechanism — best described as an atomic-scale ripple — that they dubbed a “ripplocation.”

Inspired by the paper, Michel Barsoum, a distinguished professor in the Department of Materials Science and Engineering in the College of Engineering, and his team set about proving that ripplocation exists not only in near-surface layers of two-dimensional materials, as the MIT paper had suggested, but also throughout the internal layers of all layered materials. Through careful examination of computer models of compressed graphitic atomic layers, the researchers saw deformation consistent with ripplocation.

“We now have evidence for a new defect in solids; in other words, we basically doubled the deformation micromechanisms known,” says Mitra Taheri, the Hoeganaes associate professor in the Department of Materials Science and Engineering.

The findings apply to most layered materials, including, quite possibly, geologic formations, says Barsoum.