Looking to sea urchins for stronger ceramic foams

Ceramics are highly resistant to heat, which makes them a favorite choice in managing the brutal thermal demands of high-speed vehicles that travel faster than the speed of sound. At those breakneck speeds, compressed air creates significant friction against the vehicle, resulting in a rapid rise in the heat it encounters.

Heat resistance may be the strength of ceramics, but damage tolerance is a weakness. A single pinpoint impact in a ceramic plate can result in a rapidly spreading crack that causes total structure failure. Ceramics become even less tolerant to damage when they are made porous for weight reduction; however, decreasing weight is a critical requirement for many structural applications, including high-speed vehicles.

The U.S. Air Force, one of the sponsors of Li's research, has long been interested in improving the mechanical performance of ceramic materials. In addition to receiving financial support from the Air Force Office of Scientific Research, Li's team also secured funds from the National Science Foundation.

These combined funds, received by the lab in 2018, have equipped researchers to explore new design principles embedded in the natural ceramic cellular solids formed by organisms such as sea urchins. A sea urchin's exoskeleton is one type of cellular solid, or "foam," so called because its microstructure is an assembly of open cells with solid edges or faces, packed together so that they fill space. The gaps between the cells make them porous, creating a material that can be more mechanically efficient than dense structures.

How to handle the damage like a sea urchin

"In this work, we think we found some of the key strategies that enable the sea urchin to be strong and tough while offering weight reduction with its porous microstructure," said Li. "This Nature Communications paper reports the results we found of what is hidden inside."

The spines of sea urchins are stiff, strong, and lightweight. These spines are made of a brittle mineral called calcium carbonate, which is similar to synthetic ceramics, but the urchin has a much higher tolerance for damage when receiving weight or force. Li's team tested this principle by pressing the spines mechanically, simulating the same kind of condition under which an engineering ceramic might need to endure. The sea urchin spines deformed gracefully under the force placed on them, in contrast to catastrophic failure of current synthetic ceramic cellular solids. This "graceful failure" behavior allows the sea urchin spines to withstand damage with significant energy absorption capability.

In the course of this research, Li's team uncovered some secrets that give the urchin its ability to hold together during mechanical loading.

Secrets of the deep

"There are a couple of secrets in the structural features of sea urchin spines. One is related to the connection of branches," said Li. "The second is the size of the pores."

Under a microscope, Li's team observed an architecture of interconnected short branches. A network of nodes hold these branches together, and one of the secrets to the urchin's damage tolerance is the balance between the number of nodes and branches. That number is precisely critical because nodes with too many connected branches will cause the structure to become more brittle and breakable. The nodes in the porous structure in sea urchin spines are connected to three branches on average, which means the network of branches will undergo bending-induced fracture instead of more catastrophic stretching-induced fracture.

The second secret lies in the size of the gaps, or pores, between branches. The team discovered that the gaps within the porous structure of sea urchin spines are just slightly smaller than the size of the branches. This means that once the branches fracture, they can be locked in place immediately by these smaller openings. Broken branches stack on top of one another on the pores, creating a dense region that is still able to sustain load.

Sea urchins also have a different surface morphology than synthetic ceramics. Manufactured cellular ceramics have many microscopic defects across their surfaces and internally, making these materials more susceptible to failure. This isn't the case with the sea urchin spine, which has an almost glasslike surface, smooth down to the nanometer scale. Defects are points from which damage can start, and a lack of defects means a lack of locations prone to failure.

Li demonstrated this idea with a piece of paper. "When you try to rip an undamaged piece of paper, the paper resists ripping. If you make a small tear at the side of the paper, however, the tear will continue from that damaged point."

With branches, pores, and a smooth surface in play, the lightweight sea urchin spines achieve high strength and damage tolerance by uniformly distributing the stress within the structure and absorbing energy more efficiently.

Making the next generation of ceramics

Having this knowledge, can we recreate the smoothness, lack of defects, and the specific branch and node structures needed to capitalize on the sea urchin's secrets? Right now, we can't, because the current methods of processing ceramics aren't quite there.

Ceramics made synthetically are typically formed in a two-step process. The first step is to create the shape, and the second is to fire the piece so that the ceramic hardens, which gives it the strength for which it is known. Potters follow this method when they create a pot and heat it in a kiln. Similar processes are also used for 3D-printed ceramics, where the 3D-printing step forms the shape and then subsequent firing is needed to produce final ceramic parts.

That firing, or sintering, step is the most problematic for recreating the sea urchin's microstructure because the sintering process leads to the formation of microscopic defects, rendering low strength.

"In my lab, we are also interested in how organisms such as sea urchins form these natural ceramic cellular solids," said Li. "Hopefully one day, we can not only integrate the material design principles to bio-inspired lightweight ceramic materials, but also the material processing strategies learned from natural systems."