Scanning electron microscope image of a nanoscale diamond needle being deformed by a nanoindenter.
Scanning electron microscope image of a nanoscale diamond needle being deformed by a nanoindenter.
Simulation of nanoscale diamond needle being deformed by a nanoindenter.
Simulation of nanoscale diamond needle being deformed by a nanoindenter.

Diamond maybe the hardest natural material, but it is brittle and deforms little before breaking catastrophically. Now, however, researchers have shown that tiny nanoscale diamond needles can bend and deform reversibly, like bristles on a brush, before breaking [Banerjee et al., Science 360 (2018) 300].

The tiny diamond needles are created using a combination of chemical vapor deposition and plasma-induced etching. The team from City University of Hong Kong, Massachusetts Institute of Technology, Institute for Basic Science, Ulsan National Institute of Science and Technology, and Nanyang Technological University used a nanoindenter to repeatedly push the needles until they break.

The 300-nm-long needles instantaneously bounce back to their original positions when deformed, the researchers found. Remarkably, the single-crystal nanoscale needles can withstand tensile strains up to 9% before breaking, which is very close to the theoretical limit for diamond. Usually, it is very difficult to achieve a material’s maximum theoretical strength in either tension or compression because defects tend to lead to brittle fracture long before the chemical bonds holding the material together come apart.

“Theoretically, an ideal diamond lattice should attain more than 10% strain before it fails but, in practice, even a tenth of that limit has been hard to achieve,” says Yang Lu of City University of Hong Kong, who led the effort with Ming Dao, Wenjun Zhang, and Subra Suresh. “In bulk diamond, however, stress concentration at defect sites causes the material to fail at a much lower strain.”

The researchers believe that the greatly improved strength, which reaches a maximum tensile stress of 89–98 GPa, and deformability of the diamond needles is down to a lack of those defects. Compared with bulk diamond, the needles have very small volumes and comparatively smooth surfaces. The absence of defects makes it more difficult for surface cracks to proliferate and lead to failure.

“We believe that the mechanical properties (i.e. high tensile deformation) stems from the structural properties (i.e. lack of defects in the bulk and on the surface of the material),” says Lu. “There is no other cause that is more appealing.”

Shrinking diamond to the nanoscale offers a means of engineering elasticity into this ultrahard material, the researchers believe. The combination of nanoscale diamond’s remarkable mechanical properties, together with its band structure, could make the material attractive for bioimaging and biosensing, drug delivery, nanomechanical resonators, data storage, and optoelectronic devices.

“Diamond is a great material for biomedical applications because of its chemical inertness and affinity to drugs,” points out Lu. “Reliable mechanical properties are a prerequisite (more or less) for all device applications, and our work ensures that. ‘Deformable diamond’ is no longer an oxymoron, giving the material unprecedented potential for applications wider than gemstones and cutting tools.”

Javier Llorca of IMDEA Materials Institute and Polytechnic University of Madrid in Spain believes that the results are interesting and novel.

This is the first time that these high strengths have been measured in diamond, supported by ab initio simulations, and they are similar to those found in graphene and CNTs,” he comments. “It would be interesting to apply the same methodology to other materials and see whether such large strains can also be attained. Large changes in the electronic structure of materials when deformed elastically up to 10% may lead to dramatic changes in electronic, magnetic, and catalytic properties.”

“Demonstrating the ability of diamond, which is generally known as the stiffest solid, to bend captures imagination,” adds Yury Gogotsi of Drexel University. “Although the ability to bend and show close-to-theoretical strength is fairly common for microscale filaments of many strong brittle solids, I have not seen such a degree of elastic deformation and resistance to shear in diamond previously. This is new and very exciting.”

The research provides a quantitative insight into deformation of diamond and demonstrates the opportunities for this material in applications where a combination of strength, wear resistance, and high elasticity is advantageous, such as flexible micro-electrodes and atomic force microscope probes, he suggests.

This article was originally published in Nano Today 21 (2018) 1-2.