Strong as steel, but lighter than water
(Jens Bauer) Strong materials are heavy and light materials are weak. Strength and density, two materials properties of central relevance for engineering, are generally considered as strongly coupled. However, nature shows us how we may overcome long standing barriers on the search for light yet strong materials.
Containing several levels of hierarchical structuring from the macro- to the nano-scale, certain porous biological materials such as bone and wood remain strong despite being extremely light, even though their basic material of which they are composed is generally considered anything but strong. Light man-made materials such as technical foams on the other hand, attain only limited mechanical properties compared with corresponding bulk materials. Foams are structured randomly which is not weight efficient, with respect to strength. Cancellous bone and other natural cellular solids have an optimized architecture, designed adaptively to the loading situation. On the lowest level of hierarchy bone consists of nanometersize building blocks, additionally providing strongly enhanced material strength because of mechanical size effects.
Designing cellular materials with a specific microarchitecture allows one to exploit both structural advantageous mechanisms and size-dependent strengthening effects. Applying 3D direct laser writing (3D-DLW) microarchitected lightweight materials from ceramic-polymer composites have been fabricated (Fig.2) and mechanically characterized (Fig.3). Exceeding all technical and natural materials with a density below 1000 kg/m³ as well as most metallic alloys, ratios of strength-to-weight comparable to high-performance steels and technical ceramics are reached (Fig.1).
Fig.1 Hexagonal micro-truss structure (colorized SEM image). Compressive loads corresponding to 550 kg/cm² may be carried, at a density less than the half of liquid water.
left: Fig.2 Micro-truss structure from ceramic-polymer composite (colorized SEM image), fabricated using 3D-DLW and atomic layer deposition. The miniaturized, specifically designed architecture allows benefiting from both structural advantages and size-dependent material strengthening effects.
right: Fig.3 Deformed structure after uniaxial compression (colorized SEM image). Initial failure leads to a stackwise collapse.