Afsaneh Rabiei checks on metallic foam created in her lab. (Credit: Rabiei/North Carolina State University) The word foam doesn't necessarily convey strength, but seeing a bullet explode upon impact with composite metal foam certainly does. Researchers at North Carolina State University created a bubble-filled metal composite that combines the strength of steel with the airiness and heat-resistant properties of foam. In tests with armor-piercing bullets, a one-inch layer of the material stopped the projectile in its tracks while allowing only an 8-millimeter indentation on the other side. Afsaneh Rabiei, a professor of mechanical and aerospace engineering at North Carolina State University, claims her version of metallic foam is the strongest yet.
The Secret is the Bubbles
Foams are made mostly of air pockets — that's what makes them so light. They derive their strength from the composition and structure of the material that surrounds the air pockets, a steel alloy in this case. Rabiei has worked with composite metal foams (CMFs) for years to understand their unique properties and find the optimal configuration of materials and bubbles to maximize results. "If you look around, you will see a ton of porous materials in nature: wood, bone, leaves, a bird's wings, anything that you want to be lightweight or have cushionability, you'll see that it has air bubbles," says Rabiei. "It's a waste of material and energy to carry a solid bulk of materials with you, when you can actually use something much lighter and accomplish even more." [embed]https://www.youtube.com/watch?v=lWmFu-_54fI[/embed] In her latest study, published in the International Journal of Thermal Sciences, Rabiei describes two different methods of making her metal foam, both of which rely on hollow steel spheres to form the "bubbles." The first method casts a metal with a low melting point, like aluminum, around the spheres, creating a matrix of hollow spaces within. For metals with higher melting points, the spheres are packed in by a metal powder that is compressed in a process called sintering to form a solid.
It Can Take the Heat
But it all comes down to the bubbles, says Rabiei. By keeping the diameter of the spheres constant, she says, any stress will impact them equally, increasing the material's strength. The spherical shape of the bubbles confers strength as well, by distributing the force evenly around the structure. Where other metal foams buckle as their bubbles collapse, Rabiei's maintains its structure thanks to the uniform nature of the components. Because the spheres are filled with air, the material is also resistant to heat and heat-related expansion. In her latest experiment, she compared a block of solid stainless steel to the bubble-infused steel as both were heated by an 800-degree flame on one side. The CMF took twice as long to reach 800 degrees as the solid block, a result of the air-filled bubbles inside.
Putting CMFs to Work
The unique properties of CMFs make them ideal for use in body armor, as the bullet demonstration makes plain. Rabiei also sees uses for them in the space industry, where lightweight, durable materials are in high demand. Additionally, by adding tungsten and vanadium to the composite, Rabiei says her CMF blocks radiation, including gamma rays, X-rays and neutron radiation. This makes the material ideal for transporting nuclear waste, she says, and also increases its utility for space travel, where harmful radiation poses a danger to human health. In the future, Rabiei would like to test how well her metal foam stands up to vibrations, a property that could carry further benefits for vehicle construction dampen sound-proofing. Ultimately, her end game is to save a person's life. "If I can make one person walk out of an accident because of my materials, I will have accomplished my goal," she says.
