Curiosities: Is anything harder than diamond?

People have made that claim, but the verdict is not in, says Don Stone, a professor of materials science and engineering at the University of Wisconsin–Madison. “Scientists even debate how best to measure extreme hardness,” Stone says.

One common standard, called Meyer hardness (the load applied divided by the area being loaded), correlates well with the more-familiar scratch hardness. As far as is known, diamond has the highest Meyer hardness, and it scratches any other material.

But that doesn’t mean scientists aren’t trying to find something that diamond won’t scratch. Stone says that materials scientists are continually seeking new ways of synthesizing exotic, super-hard materials starting with nitrides, borides and carbides.

Hardness results from resistance of atoms to shearing past each other. “High shear strength is associated with covalent bonding, high bond energy and high melting temperature,” Stone says. “The bonds between carbon atoms in diamond fulfill all of these requirements.”

What is the point of making something harder than diamond? “Partly it’s the claim to fame, but there are plenty of uses for super-hard materials,” says Stone, adding that diamond has long been used for coating computer discs to help prevent damage from head crashes.

“I believe someone will eventually find a material that is harder than diamond, if they haven’t already,” says Stone, “but the experiments are not yet conclusive.”

In any case, who better to discuss hardness than Professor Stone?