Shrinking the dimensions of material structures can give rise to remarkable properties. For example, the mechanical hardness of nanocrystals of certain metals—such as copper—can be much greater than that of the same material in bulk. Chinese researchers now have found another example of this phenomenon—in the superelastic response of nanometre scale springs of silicon nitride.

The mechanical properties of materials are determined not only by the strength and coordination of the bonds between its atoms but also by the presence and movement of defects within its structure. An everyday demonstration of this occurs when a steel paper clip is bent back and forth repeatedly. Such bending generates a cascade of defects in the paperclip's atomic structure, causing it to become weaker and weaker until it eventually breaks.

In a structures that is just a few tens of nanometres wide, however, it is much more difficult for defects to become trapped or multiply and much easier for the atoms of its crystal structure to rearrange so that they can be repaired. Moreover, it is much easier to grow such structures without defects in the first place.

Exploitation of such effects is of particular interest in the production of ceramics. Although most ceramics exhibit a high mechanical strength to weight ratio, are robust against rapid changes in temperature and resistant to chemical erosion, they have one serious flaw — they are brittle, which causes them to fail under large tensile stress.

Fig. 1: Si3N4 Microcoils.

To try to improve the elastic properties of ceramics, Chuanbao Cao and colleagues1 from the Research Center of Materials Science at the Beijing Institute of Technology grew micro-springs of silicon nitride—an industrially important and widely used ceramic material. Remarkably, they find that when the springs were pulled to almost their maximum length— where they looked more like wires than coils — and then allowed to relax, they recovered their original shape. Moreover, unlike conventional steel springs, which would be irreversibly deformed if stretched in the same way, they found that their silicon nitride springs could be extended and relaxed many times without failure (Fig.1).