"Super-hard" metals forged in new nanoparticle manufacturing process

Researchers at Brown University have developed a new way to make super-hard metals. The team made nanoparticle “building blocks” that could be fused together under moderate pressure, thanks to a chemical treatment.

A material’s hardness specifically describes how well it resists being scratched or bent out of shape by force or pressure. In the case of metals, it’s usually determined by the size of the microscopic grains that make it up – the smaller the grain, the harder the metal.

Normally a metal is made harder using macroscopic manufacturing methods like hammering, bending or twisting. But in the new study, the team started from the “bottom up,” resulting in much harder metals.

“Hammering and other hardening methods are all top-down ways of altering grain structure, and it’s very hard to control the grain size you end up with,” says Ou Chen, corresponding author of the study. “What we’ve done is create nanoparticle building blocks that fuse together when you squeeze them. This way we can have uniform grain sizes that can be precisely tuned for enhanced properties.”

The problem, the team says, is that the surface of metals are usually covered in organic molecules called ligands, which can stop metal particles bonding strongly. The researchers developed a chemical treatment that can remove these ligands, leaving the metal nanoparticles free to fuse together easier through a pressure-sintering process.

Using this method, the researchers made rough “coins” out of nanoparticles of different metals, such as gold, silver and palladium. In tests they proved to be much harder than usual, with the gold coins becoming up to four times harder. Other physical properties remained more or less unchanged.

In another test, the researchers made a metallic glass using their new technique. Like the glasses we’re more familiar with, these materials have amorphous crystalline structures, which can make them easier to mold and potentially stronger than regular metals.

“Making metallic glass from a single component is notoriously hard to do, so most metallic glasses are alloys,” says Chen. “But we were able to start with amorphous palladium nanoparticles and use our technique to make a palladium metallic glass.”

At the current scale of centimeters, the researchers say this process could be used to make super-hard coatings, electrodes or other metal components. But it should also be relatively simple to scale up to larger items too, according to the team, because current industrial equipment can handle the pressures used.

The research was published in the journal Chem.

Source: Brown University