, an assistant professor in the mechanical engineering department at Virginia Tech, has unraveled a mystery in the porous microstructures of sea urchin exoskeletons that could lead to the creation of lightweight synthetic ceramics. His findings were published in Nature Communications on October 14.
Watch the video: https://video.vt.edu/id/1_x9ca7e8a
Ceramic is highly heat resistant, making it a preferred choice for handling the brutal thermal demands of high-speed vehicles that travel faster than the speed of sound. At these breakneck speeds, the compressed air creates significant friction against the vehicle, resulting in a rapid increase in the heat it encounters.
Heat resistance may be ceramic’s strength, but damage tolerance is a weakness. A single point impact in a ceramic plate can lead to a rapid crack propagation that causes total structural failure. Ceramic becomes even less tolerant of damage when made porous to reduce its weight; however, weight reduction is a critical requirement for many structural applications, including high-speed vehicles.
The US 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’s Office of Scientific Research, Li’s team also secured funding from the National Science Foundation.
These combined funds, received by the lab in 2018, allowed researchers to explore new design principles embedded in natural ceramic cellular solids formed by organisms such as sea urchins. A sea urchin’s exoskeleton is a type of cellular solid, or “foam,” so called because its microstructure is an assembly of open cells with solid edges or faces, packed together to fill space. The spaces between the cells make them porous, creating a material that can be more mechanically efficient than dense structures.
How to handle damage like a sea urchin
“In this work, we believe we have found some of the key strategies that allow the sea urchin to be strong and resilient while providing weight reduction through its porous microstructure,” Li said. “This Nature Communications article reports the results we found on what’s hidden inside.”
Sea urchin spines are stiff, strong and light. These spines are made of a fragile mineral called calcium carbonate, which is similar to synthetic ceramic, but the sea urchin has a much higher tolerance for damage when given weight or force. Li’s team tested this principle by mechanically pressing down on the spines, simulating the same type of condition an engineered ceramic might need to withstand. Sea urchin spines deformed gracefully under the force applied to them, unlike the catastrophic failure of today’s synthetic ceramic cellular solids. This “graceful failure” behavior allows sea urchin spines to resist damage with significant energy absorption capacity.
During this research, Li’s team discovered some secrets that give the sea urchin its ability to hold together under mechanical loading.
Secrets of the Deep
“There are some secrets in the structural characteristics of sea urchin spines. One is related to branch connection,” Li said. “The second is pore size.”
Under the microscope, Li’s team observed an architecture of interconnected short branches. A network of nodes holds these branches together, and one of the secrets of the sea urchin’s damage tolerance is the balance between the number of nodes and branches. This number is precisely critical because nodes with too many connected branches will make the structure more fragile and breakable. The nodes of the porous structure of sea urchin spines are connected with an average of three branches, which means that the network of branches will experience a bending-induced fracture instead of a more catastrophic stretch-induced fracture.
The second secret lies in the size of the spaces, or pores, between the branches. The team found that the gaps in the porous structure of sea urchin spines are just slightly smaller than the size of the branches. This means that once the branches are broken, they can be locked in place immediately through these smaller openings. The broken branches pile on top of each other over the pores, creating a dense region that is still able to support the load.
Sea urchins also have a different surface morphology than synthetic ceramics. Manufactured cellular ceramics have many microscopic defects on their surfaces and inside, making these materials more susceptible to failure. This is not the case with the backbone of the sea urchin, which has an almost glassy surface, smooth down to the nanometer scale. Faults are points at which damage can begin, and a lack of faults means a lack of failure-prone locations.
Li demonstrated this idea with a piece of paper. “When you try to tear an undamaged piece of paper, the paper resists tearing. If you make a small tear in the side of the paper, however, the tear will continue from that damaged point.
With limbs, pores, and a smooth surface in play, lightweight sea urchin spines achieve high strength and damage tolerance by evenly distributing stress throughout the structure and absorbing energy more efficiently.
Making the next generation of ceramics
Having this knowledge, can we recreate the smoothness, flawlessness, and specific branch and node structures needed to capitalize on the secrets of the sea urchin? At present, we cannot, because the current methods of processing ceramics are not quite there.
Synthetically made ceramics are usually 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, giving it the strength it is known for. 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 required to produce the final ceramic parts.
This 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, which makes the strength low.
“In my lab, we are also interested in how organisms such as sea urchins form these natural ceramic cellular solids,” Li said. materials to bio-inspired lightweight ceramic materials, but also material processing strategies learned from natural systems.”