Researchers Engineer Nanoparticles To Create Super-Hard Metals

Traditionally, metallurgists have made metal harder through various processes - bending, twisting, passing it through rollers, or by hammering it - that work by breaking up the metal's grain structure. A new method has found a way to manage these small particles to create harder metals.

Researchers from Brown University have found a way to work on these metallic grain structures at the nanometer scale. Their report, "Bulk Grain-Boundary Materials from Nanocrystals, is published in the journal Chem, January 22.

Manufacturing Harder Metals

After conducting mechanical testing on the metals made from the novel method, the resulting work is up to four times harder than naturally occurring metal structures. This was achieved by smashing individual metal nanoclusters, which are then used to form harder and larger scale pieces of solid metal.

"Hammering and other hardening methods are all top-down ways of altering grain structure, and it's tough to control the grain size you end up with," said Ou Chen, corresponding author in the study and a chemistry assistant professor at Brown. He explains that their work creates "nanoparticle building blocks" that are then squeezed to fuse to create uniform grain sizes. With these microscopic pieces of metal, researchers can now control them precisely to achieve improved properties.

In their study, researchers created centimeter-scale "coins" from metallic nanoparticles - gold, palladium, silver, and more. Items at the size of these coins already have applications like electrodes, thermoelectric generators that convert heat flux into electricity, and coating materials. Researchers also believe that the method used to create these metals at the centimeter scale could be scaled even further for industrial parts of super-hard metal coating materials.

Chemically Altering Metal Molecules

According to Chen, the key is in the chemical treatment of the nanoparticle grains. They used ligands - organic molecules that bond to a central metallic atom and prevents metal to metal bonding. They were able to strip these ligands allowing the clusters to bond together with pressure, resulting in substantially harder metal coins. For example, the gold coins created in the study were two to four times harder than their naturally-occurring counterparts.

Additionally, the optical properties of their gold coins were "fascinating," noting a color change when the nanoparticles were squeezed to create the metal coins. He attributes it to a phenomenon called the plasmonic effect, which describes the interaction between free electrons and light. Chen notes that gold nanoparticles are actually purplish-black but turn into the bright gold color upon applying pressure. Referring to the change in color, Chen added that they knew they created bulk gold.

In theory, the technique used in the study can be used in any metal. Researchers demonstrated this idea by creating metallic glass: amorphous (non-crystalline) metals used in creating thin pipes with high corrosion resistance and tensile strength.

"Making metallic glass from a single component is notoriously hard to do, so most metallic glasses are alloys," Chen added. Starting with amorphous palladium nanoparticles, researchers were able to create a palladium metallic glass.