Microscopic carbon pillars are stronger than diamond yet elastic

Researchers have constructed microscopic carbon structures that boast the best combination of strength and elasticity of any material studied to date (Nat. Nanotechnol. 2019, DOI: 10.1038/s41565-019-0486-y). The tiny carbon pillars are stronger by weight than most existing materials and can bounce back after being pressed to half their size. The columns are also lightweight and heat resistant, making them ideal for use in aerospace components, engines, and nuclear reactor parts.

Engineers could use these microscopic structures as building blocks for macroscopic materials, says Julia R. Greer, a materials scientist at the California Institute of Technology. With the pillars’ unique combination of properties, “you could build robust parts for any extreme thermal mechanical environment,” she says.

The words strong and rubbery do not typically go together for materials. Strong materials are usually hard, while elastic materials that can recover their original shape after being deformed are pliable. Combining strength and elasticity has been difficult for materials scientists. Researchers have had some success by making 3-D aerogels of carbon and graphene. These porous materials are superelastic and light but not very strong.

To try to pair these seemingly contradicting properties, Greer, Xiaoyan Li of Tsinghua University, Huajian Gao of Brown University, and their colleagues picked another form of carbon known as pyrolytic carbon. It is made by heating hydrocarbon polymers to high temperatures in a vacuum to create a rigid carbon structure. Pyrolytic carbon is used in rocket parts and laboratory furnaces, but it’s not very elastic.

The researchers found that microstructures of pyrolytic carbon defied their expectations. The team made 0.7–12.7 µm wide polymer pillars using a 3-D printing technique called two-photon lithography and then heated the structures to 900 °C to yield pyrolytic carbon micropillars.

The micropillars have a compressive strength of 13.7 GPa, which by weight is higher than that of polycrystalline diamonds used in cutting tools. Narrower pillars were more rubbery, the team found. The researchers could squish pillars less than 2.3 µm in diameter to about half their height and watch as the structures fully spring back without any cracks. Wider pillars could withstand about half that compression before shattering.

Electron microscopy and electron spectroscopy revealed that the pillars were made mostly of diamond-like bonds between carbon, but the structures had tiny pockets of 1 nm long curled fragments of graphene. “When we apply pressure to this material, the graphene layers in these local spots are able to slip and pass one another,” Greer says. This movement of the graphene sheets allows the material to deform and bounce back despite being hard and strong.

“This study is an incredible example of materials by design, creating new hierarchical nanoarchitectured materials for optimized mechanical properties,” says Markus J. Buehler, a civil and environmental engineer at the Massachusetts Institute of Technology. Making a material that boasts such high compressive strength “presents an exciting new accomplishment.”