Mother-of-Pearl
Two different approaches to the creation of materials that could be described as artificial nacre – nacre being that super strong substance produced in nature by some mollusks and something of a Holy Grail pursued by materials scientists – have recently been announced.
First some background on why nacre is so strong, courtesy of the Asian Institute of Gemological Sciences:
Under microscope, nacre is astonishingly orderly. Its layers of complexity, from large to small scale, give it what engineers call a hierarchical structure. A microscopic cross-section looks like brickwork, with flat, hexagonal tablets of a crystalline, calcium carbonate mineral stacked in neat layers. Mortaring them is a flexible, protein-rich gum originally secreted by the shellfish.
Paul Podsiadlo, a protege of renowned Univ. of Michigan material scientist Nicholas Kotov, in late November won the Collegiate Inventors Competition and its $15,000 prize for a new ceramic material he calls “plastic steel.”
Paul Podsiadlo
Podsiadlo created transparent sheets of the new material using clay nanotubes and assembling them together in thin sheets. By adding hundreds of layer, Podsiadlo formed a material that resembles seashell in both strength and appearance
Kotov, who pioneered much of the science behind Podsiadlo’s application, said he hopes the plastic steel will be widely used.
“These composites can be applied in biomedical devices, bone replacements for injuries, military applications such as personal protection, microelectromechanical devices and energy generation and storage,” Kotov said.
Along these same lines, the CIC website reported that
Podsiadlo looks forward to the broad impact his innovation could have, especially in the military, aviation, medical, and energy sectors. He envisions his structure being used for anything from body armor to biomedical coatings. In fact, research for the project was initially funded by the U.S. Defense Department and the National Institutes of Health.
Then on Friday, the Dec. 5, 2008 issue of Science published a study by a group of researchers at the Lawrence Berkeley National Lab reporting that they, too, have been able to create a nacre-like material, according to a LBNL release, “may well be the toughest ceramic ever produced.”
The LBNL group, Robert Ritchie, Etienne Munch, Max Launey, Daan Hein Alsem, Eduardo Saiz and Antoni Tomsia, employed controlled freezing of aqueous suspensions alumina and polymethylmethacrylate (PMMA). They claim they were able to produce ceramics “300 times tougher than their constituent components.”
“We have emulated nature’s toughening mechanisms to make ice-templated alumina hybrids that are comparable in specific strength and toughness to aluminum alloys,” says Ritchie. “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.”
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.
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.
The group built on earlier work to develop a strong, bone-like material that utilized the properties of freezing of seawater to form a ceramic that was four times stronger than artificial bone. Seawater freezes in small layers and traps impurities (that are expelled during the freezing process) between the layers of ice.
“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.
Ritchie and the others used a similar technique focusing on alumunia and PMMA to recreate the layer microstructure of nacre. The layering in natural nacre allows mother-of-pearl to disapate stress and dimish the effects of small cracks.
“The key to material toughness is the ability to dissipate strain energy. 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,” said Ritchie.
The BNL group’s next step is to ramp up the material’s load-bearing abilities by increasing the content of alumina and using a different polymer with improved characteristics.
“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.
Here is a bonus video about the BNL work: