Researchers Build Extremely Hard Metals From Nanoparticles

Brown University researchers have shown off a new way that allows them to build super-hard metal from nanoparticles. By smashing the tiny metal nanoparticles together, the researchers could customize grain structures and improve their mechanical and other properties. Metalworkers have long used methods to make metal harder such as bending, twisting, and pounding with hammers.

All of those methods are intended to make the metal grain structure fracture into smaller grains because the smaller the grain, the harder the metal. Brown University researchers recently published a paper outlining their breakthrough method for smashing individual metal nanoclusters together to form macro-scale chunks of solid metal.

Mechanical testing of the metals made using the technique shown they were up to four times harder than naturally occurring metal structures. Researchers on the project note that all other hardening methods are top-down ways to alter the grain structure. The problem with those methods is that it's very hard to control grain size. Using nanoparticle building blocks fused together when squeezed, the researchers end up with uniform grain sizes that can be precisely tuned for enhanced properties.

In their study, the team made centimeter-scale "coins" using nanoparticles of gold, silver, palladium, and other metals. Metals of that size could be used for making high-performance coating materials, electrodes, or thermoelectric generators. The team does believe their process could be scaled up to make super-hard metal coatings or larger industrial components.

One key to the process is a chemical treatment given to the nanoparticle building blocks. The team stripped away organic molecules called ligands, which typically coat nanoparticle building blocks and prevent metal-metal bonds from forming between particles. The ligands are stripped away chemically to allow the clusters to fuse with little pressure. The resulting coins were in the area of two to four times harder than standard metal. Electrical conduction and light reflectance were nearly identical to ordinary metals.