Materials with a seashell-like microstructure resist fracturing
Source: “Tough, Bio-Inspired Hybrid Materials”
Robert Ritchie et al.
Science 322: 1516-1520
Results: A composite ceramic whose microscale structure mimics that of nacre, or abalone shell, is the toughest (that is, the most resistant to fracturing) ever made. Composed of microscale bricks of an aluminum oxide ceramic cushioned by a polymer filling, it has properties comparable to those of aluminum alloys and is twice as tough as the best structural ceramics.
Why it matters: Ceramics are lightweight and strong, but when pushed past their limits, they fail catastrophically–fracturing rather than bending, as materials such as steel would. That has limited their use as structural materials. The new materials, which exceed the toughness of nacre, could replace heavier structural materials in vehicles, improving fuel efficiency. They could also do double duty as insulation and structural support for buildings.
Methods: Researchers at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory created the material with the help of directional freezing of ice, a technique that one of them, Antoni Tomsia, refined. The researchers mixed aluminum oxide with water and froze the mixture by drawing the heat out from one side, which caused the ice to form distinct shapes. The ice served as a template, producing multiple layers of long, thin crystals of aluminum oxide, with microscopic bridges of the ceramic between the layers. After removing the water, the researchers crushed the aluminum oxide into tiny bricklike structures. Then they added a polymer “mortar” (polymethyl methacrylate) that created a cushion between the brittle bricks. The composite material is 300 times tougher than either constituent alone.
Next steps: The structure of the ceramic very closely mimics that of nacre, but nacre’s structural elements are on the order of nanometers, not micrometers. By making the bricks smaller and closer together, the Berkeley researchers hope to achieve a tougher material. They are also exploring ways to replace the polymer with other materials, in order to increase the ceramic’s tolerance of high temperatures.