Engineering materials that are both strong and tough is still difficult. For example, if a material has increased strength, it unfortunately becomes much more brittle. Nature has overcome this problem with nacre, which boasts high strength and toughness thanks to its hierarchical structured consisting of layered hard aragonite platelets embedded in a soft organic biopolymer matrix. The aragonite is load bearing, which makes nacre strong, while the biopolymer both distributes load and dissipates energy as it deforms. Finally, surface nanoasperities in the aragonite platelets and mineral bridges between the platelets interlock the platelets and prevent single platelets from moving.

Although this architecture appears simple at a first glance, it has proved notoriously difficult to duplicate using traditional engineering processes. Until now, most approaches relied on a ceramic/polymer configuration – for example, Al2O3/PMMA lamellar composites that structurally resemble nacre or nacre-like CaCO3/PAA composites have been made using mesoscale assemble-and-mineralization techniques. Although these composites are tougher than their individual components, they are not very strong because they contain polymers.

Mimicking the hard/soft/hard architecture in nacre

Now, researchers Xiaodong Li and Yunya Zhang have exploited the oxygen-containing groups in graphene oxide to build Al/graphene/Al2O3 composites that mimic the hard/soft/hard architecture in nacre. Thanks to its ultrahigh hardness and outstanding chemical stability, Al2O3 nanoparticles are ideal for reinforcing polymer and metallic materials.

“Making metal-based composites often requires severe plastic deformation and high-temperature annealing,” explains Li. “These procedures inevitably damage the structure of any added graphene, annihilating potential increases in mechanical strength. This is the main reason for why previous graphene/metal composites had a lower strength and stiffness than theoretically expected.

Al2O3 acts as a co-reinforcing agent

“Instead of seeking a way to avoid damaging the included graphene we have instead put forward a smart manufacturing method that takes advantage of the reaction between graphene oxide and Al. We were able to effectively reduce graphene oxide into defective graphene and Al2O3, with the Al2O3 acting as a co-reinforcing agent that compensates for graphene’s degradation, so making the composite stronger.”

Nanoindentation and tensile testing by the researchers revealed that the difference in modulus between the Al layer and the Al2O3/graphene/Al2O3 layer was over 30%, while the ductility of the severely deformed Al matrix was only 10%. “What is more, Al2O3 asperities grew from the Al matrix, leading to intimate interfaces and geometrical interlocking between these layers, which helps to avoid large-scale delamination”, says Li. “The obvious mismatching of modulus and ductility between Al and Al2O3/graphene/Al2O3 layers as well as the close bonding between them makes the ceramic/graphene layers behave like hard bricks and the softer Al metal layers like the mortar, which is a very similar scenario to that seen in nacre,” he tells

Extending to other metals

The new composite might be used in applications in which weight and strength are both important, he adds. “Some examples include high-speed vehicles, where reduced weight could greatly reduce energy consumption. Another area is next-generation electronics that require strong, stiff and tough raw materials that also have a good thermal conductivity (which the new composite has, thanks to its graphene constituent).”

The researchers, reporting their work in Nano Letters DOI: 10.1021/acs.nanolett.7b03308, say that they would now like to optimize their Al/graphene/Al2O3 composite and look into ways of scaling up its production. “Secondly, we would like to extend the reinforcing mechanism we have seen in Al to other metals, such as Ni, Cu, Ti and Mg,” adds Li. “Finally, we will try to find out if these composites could have other possible applications in areas like energy harvesting and energy storage.”