Making materials that are stronger yet lighter (less dense) is an on-going challenge for materials scientists.
It’s hard to make a strong, durable material without adding too much bulk. Even the current ‘strong but light’ materials such as aluminium alloys still have a considerable density – greater than 1000 kg/m3 (the density of water) – and are not suitable for some applications.
Using 3D printing to mimic cellular properties
Inspired by naturally light and strong materials, a team of German researchers is using 3D laser lithography (a type of very accurate, very small-scale 3D printing) to engineer a new class of alumina-coated composite materials mimicking the cellular properties of bone and wood. And while they might not be ready to print out building or aircraft parts (mainly because the technology doesn’t exist yet to allow them to do so), the microstructures they have made are less dense than water but as strong as some steels.
Lead author on the research, Dr Jens Bauer, Karlsruhe Institute of Technology, explains that natural cellular materials such as bone and wood “have an optimised architecture and their basic material is hierarchically structured, actually consisting of nanometre-size building blocks, providing enhanced material strength because of size-dependent effects”.
This means that, by mimicking the organised cellular architecture of bone and wood at the nanoscale using polymers, the researchers are able to use a ‘mechanical size effect’, whereby overall strength increases as the size decreases.
Microarchitecture on a nanoscale
The researchers made a variety of nanometre-sized honeycomb-like and truss-reinforced structures with a designed, rather than random, microarchitecture and then coated them with varying thicknesses of alumina using a technique called atomic layer deposition. They then tested each of the structures with compression loads to see when and how they would fail along the spectrum from buckling to fracturing. They found that, with a growing layer thickness of alumina, the failure mechanism changes from buckling to brittle fracture but that buckling always occurs at lower stresses than material failure, with the uncoated polymer obviously buckling first under a load.
Honeycomb structures show the highest strength
The various truss structures, ranging from simple open cube and hexagon structures to partially and fully diagonally braced triangular structures, all outperformed existing technical foam materials. However, it was the optimised honeycomb designs that achieved strength-to-weight ratios comparable to those of technical ceramics and high-strength steels, with a closed hexagonal alumina-coated (50 nm thick) honeycomb structure showing the highest strength of the designs tested.
This honeycomb structure was able to withstand compressive forces up to 280 MPa at 810 kg/m3, exceeding all known natural and engineered materials with densities less than water, the researchers report.
Jens Bauer, Stefan Hengsbach, Iwiza Tesari, Ruth Schwaiger and Oliver Kraft. (2014). High-strength cellular ceramic composites with 3D microarchitecture, PNAS 2014 111 (7) 2453-2458; published online ahead of print 3 February 2014, doi:10.1073/pnas.1315147111