Toughest ever ceramic that mimics mother of pearl created

December 6th, 2008 - 2:23 pm ICT by ANI  

Washington, Dec 6 (ANI): Mimicking the structure of mother of pearl, scientists with the U.S. Department of Energy’’s Lawrence Berkeley National Laboratory (Berkeley Lab), have created what could turn out to be the toughest ceramic ever produced.
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, led the research effort.
The researchers produced the novel ceramic via controlled freezing of suspensions in water of an aluminum oxide (alumina) and the addition of a well known polymer, polymethylmethacrylate (PMMA).
The resultant ceramics are 300 times tougher than their constituent components.
The roughness of the alumina/PMMA hybrid ceramic controls the strength of the interfaces, which is critical in determining the material’’s overall toughness as it affects the sliding process in the polymeric “mortar” layers.
“We have emulated nature’’s toughening mechanisms to make ice-templated alumina hybrids that are comparable in specific strength and toughness to aluminum alloys. 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, the inner lining of the shells of abalone, mussels and certain other mollusks, is popular for both its iridescent beauty and its amazing toughness. Nacre is 95-percent aragonite, a hard but brittle calcium carbonate mineral, with the rest of it made up of soft organic molecules. Yet nacre can be 3,000 times (in energy terms) more resistant to fracture than aragonite.
Till date, no human-synthesized composite outperforms its constituent materials by such a wide margin. In the “brick-and-mortar” phase of the alumina/PMMA hybrid, aragonite “bricks” slide past each other to dissipate strain energy while the polymer “mortar” acts as a lubricant.
However, two years ago, the researchers found a way to improve the strength of bone substitutes through a processing technique that involved the freezing of seawater. This process yielded a ceramic that was four times stronger than artificial bone.
“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 a co-researcher on the project.
In this current research, the team refined the freeze-casting technique and applied it to alumina/PMMA hybrid materials to create large porous ceramic scaffolds that much more closely mirrored the complex hierarchical microstructure of nacre.
Besides making the lamellar scaffolds, the team was also able to fabricate nacre-like “brick-and-mortar” structures with very high alumina content.
And now Ritchie says that to create ceramic materials that are even tougher in the future, his team needs to improve the proportion of ceramic to polymer in their composites.
“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,” said Ritchie.
He claimed that such future composite materials would be lightweight and strong as well as tough, and could find important applications in energy and transportation.
The results of the research were reported in the latest issue of the journal Science, in a paper entitled: “Tough, bio-inspired hybrid materials.” (ANI)

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