A team of researchers in U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have created the toughest ever ceramic by mimicking the structure of mother of pearl.
The team was guided by Robert Ritchie, who holds joint appointments with Berkeley Lab's Materials Sciences Division and the Materials Science and Engineering Department at the University of California, Berkeley
The researchers successfully emulated nature's toughening mechanisms in making ice-tempered alumina hybrids that are comparable to aluminum alloys.
The resulting ceramic was 300 times tougher than its constituent components and was made through the controlled freezing of water suspensions of alumina and addition of polymethylmethacrylate (PMMA).
"We believe these model materials can be used to identify key microstructural features that should guide the future synthesis of bio-inspired, yet non-biological, light-weight structural materials with unique strength and toughness." Said Ritchie.
Mother of pearl or Nacre, awesomely tough, is the inner lining of shells of abalone, mussels and other mollusks. Nacre is 95% aragonite (A hard but brittle carbonate mineral) and rest is made up of soft organic molecules but still it can be 3000 times more resistant to fracture than aragonite. Nacre's unusual strength comes from a structural architecture that varies over lengths of scale ranging from nanometers to micrometers.
Two years ago, Berkeley Lab researchers Tomsia and Saiz created a ceramic that was four times stronger than artificial bones through a processing technique, freeze-casting technique, that involved freezing of seawater resulting in scaffolding of pure ice thin layers. The resulted architecture roughly resembled that of nacre.
Berkeley Lab researchers Tomsia and Saiz created a ceramic that is four times stronger than artificial bones through a processing technique, freeze-casting technique, that involved freezing of seawater resulting in scaffolding of pure ice thin layers. The resulting architecture roughly resembles that of nacre.
"Since seawater can freeze like a layered material, we allowed nature to guide the process by which we were able to freeze-cast ceramics that mimicked nacre," said Tomsia when this research was reported.
The team fine tuned the freeze-casting technique and applied it to alumina/PMMA hybrid materials. To achieve the microstructure of nacre, the team first formed lamellae of ice by directional freezing that served as templates for the creation of the layered alumina scaffolds. After removal of ice, spaces between the alumina lamellae were filled with polymer.
"The key to material toughness is the ability to dissipate strain energy," says Ritchie. "Infiltrating the spaces between the alumina layers with polymer allows the hard alumina layers to slide (by a small amount) over one another when load is applied, thereby dissipating strain energy. The polymer acts as a lubricant, like the oil in an automobile engine."
The team has also fabricated nacre-like "brick-and-mortar" structures with high alumina content by collapsing the scaffolds in a perpendicular direction to the layers then sintering the resulting alumina "bricks" to promote brick densification and the formation of ceramic bridges between individual bricks.
"Using such techniques, we have made complex hierarchical architectures where we can refine the lamellae thickness, control their macroscopic orientation, manipulate the chemistry and roughness of the inter-lamellae interfaces, and generate a given density of inorganic bridges, all over a range of size-scales," stated Saiz.
The team is further investigating variations made using metals and polymer other than alumina/PMMA and different proportion of ceramic to polymer.
Says Ritchie, "The polymer is only capable of allowing things to slide past one another, not bear any load. Infiltrating the ceramic layers with metals would give us a lubricant that can also bear some of the load. This would improve strength as well as toughness of the composite."
Ritchie said that significant use of such future composite material would be in energy and transportation.
