A New Superhard Material
Superhard materials like diamond are useful for industrial cutting tools but are expensive, so researchers have been searching for alternatives. Unfortunately, hard materials are often brittle. Now Shanmin Wang of the Southern University of Science and Technology in China and his colleagues have created a superhard but nonbrittle material by adding vacancies to the atomic lattice [1]. Wang says that the technique could be applied to other materials.
The mechanical properties of metals and alloys depend heavily on defects in their crystal lattices called dislocations. These are lines along which a plane of atoms comes to a sudden end. When a crystal is stressed, atomic bonds break and reform in a way that can cause a dislocation to move perpendicular to the planes. This ability for dislocations to “slide” allows metals to deform without breaking, but it also reduces a material’s rigidity and hardness. Treatments that prevent dislocation sliding can increase hardness but also brittleness.
Like similar ceramics, tungsten diboride has a 3D covalent-bonding network that ordinarily prevents large-scale dislocations from forming. The material is predicted to qualify as superhard (hardness above 40 gigapascals [GPa]), but in hardness tests its brittleness leads to cracks before high stress is reached. Wang and his colleagues used high-pressure and high-temperature techniques to substitute rhenium atoms for some tungsten atoms. Two tungstens were typically replaced by a rhenium and a vacancy because rhenium has an extra electron, and the number of valence electrons a crystal can tolerate is limited. These orderly vacancies opened channels for dislocations to form and slide, which prevented cracking and engendered hardness as high as 50 GPa. The new material also has high thermal stability, a useful property for machine tools.
–David Ehrenstein
David Ehrenstein is a Senior Editor for Physics Magazine.
References
- C. Gu et al., “Strengthening tungsten diboride toward a superhard material by ordered vacancy pairs,” Phys. Rev. Lett. 136, 106101 (2026).