Composite metal foam

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Composite Metal Foam is a novel branch of metal foam, made out of homogeneous air-filled spheres within a metallic matrix. This material outperforms traditional fully-solid materials in nearly every performance parameter and does so at a third of the weight. Developed at North Carolina State University in Raleigh, North Carolina, CMF has a history of successful testing programs conducted in civilian, military, and academic laboratories and is now entering full commercialization for global markets.

Composite Metal Foam

Composite Metal Foam (CMF) is a novel branch of metallic materials that was developed at North Carolina State University by the inventor, Dr. Afsaneh Rabiei. Unlike other metal foams that contain randomly sized air bubbles/cells separated from each other by cell walls of varying shapes and thicknesses, Composite Metal Foam is a cellular structure that consists of a repeating arrangement of prefabricated air-tight hollow metal spheres that are surrounded by a metallic matrix. These hollow metal spheres are arranged in a random loose packing density (something in-between a simple cubic and a body-centered cubic arrangement), ensuring homogeneity of air pockets/bubbles within the material and providing extraordinary energy absorption and strength.

CMF can be made from many different metals, alloys, or combinations, allowing for material refinement and optimization per specific use-case. In every combination of hollow metal spheres and matrix material, the final CMF product will weigh about 70% less than an equivalently sized, fully solid parent material.... A 100% steel CMF piece has a density about 2.7 g/cc (similar to that of aluminum) while a magnesium CMF piece can have a density below 1 g/cc, allowing it to float on water. Replacing a traditional metal (such as steel with a density of about 8 g/cc) with a steel CMF piece will not only lower the weight of the product, improving the fuel economy/efficiency of the structure, but will lower the raw material demand needed to produce it as well, reducing its environmental impact and production cost accordingly. CMF’s defining characteristic is its high strength to density ratio. In comparison to solid metals and other metal foams under compressive loads, CMF is significantly stronger per weight and per volume

Composite Metal Foam is a proprietary material patented by Advanced Materials Manufacturing in Raleigh, North Carolina.

Material Properties

Energy absorption

Composite Metal Foam materials are world-renowned for their ability to absorb impact energy. As forces are applied, the porosities within the CMF collapse to absorb the energy. As the force increases, the pressure of trapped air inside the airtight hollow spheres increases as well, pushing back against the applied force. The faster and more powerful the force is when applied to the CMF, the higher the energy absorption capacity that can be expected from CMF, preventing further deformation, a property that is only offered by CMF due to its air-tight porous structure. This property results in an energy absorbing capacity of CMF that is multiple orders of magnitude higher than that of its parent material. This novel lightweight material looks and works like a heavy-duty bubble wrap as the air pockets that are trapped within work to absorb energy in an impact.

Fire/extreme heat protection of CMF

The trapped air within the material allows CMF to act as a thermal barrier as well, slowing the transmission of heat from one side of the material to the other. This allows the material to work as a heat shield and can significantly increase the fire resistance of products. In a study of pool-fire testing of CMF, a ~5/8” thick steel CMF panel (15 mm) withstood temperatures of over 1,500 ℉ (827 ºC) for over 100 minutes. The highest temperature behind the panel registered as 698 ℉ (370 ºC) across the 100-minute test, well below the 800 ℉ (427 ºC) limit that defines failure, with only slight discoloration to the panel resulting from the test. For reference, a solid steel plate of equal dimensions failed the test in only 13 minutes

In a 2020 small scale torch-fire test, the steel CMF panel withstood temperatures of 2,200 ℉ (1,204 ºC) for over 30 minutes, never reaching the temperature of 800 ℉ (427 ºC) that defines failure. Again, compare this to the steel plate of the same dimensions that failed after only 4 minutes. Steel CMF once again passed the test successfully, with a large margin to spare proving the potential application of the material for protection against fire

These incredible test results are due in large part to the uniform distribution of trapped air within the metallic materials. Similar to traditional insulation made of small feathers or small fibers for the purpose of preventing the movement of air particles, the hollow metal spheres trapped within CMF prevent the movement of air particles inside this material. When air is stagnant, its thermal conductivity is roughly 0.0564 (W/m-ºC) at 500 ºC compared to stainless steel at roughly 16.8 (W/m-ºC) at 500 ºC or aluminum at roughly 210 (W/m-ºC) at 500 ºC. Stainless steel CMF has shown a thermal conductivity value of roughly 5 (W/m-ºC) at 500 ºC, 3.36 times lower than traditional steel, while aluminum CMF displayed a thermal conductivity of only 30.3 (W/m-ºC) at 500 ºC, 7 times lower than a block of solid aluminum.

The air pockets within the material also reduce the amount of thermal expansion that takes place while heating. Compared to a block of stainless steel, a block of stainless-steel CMF expands roughly 5 times less when exposed to the same conditions. In places where high heat is experienced but tight tolerances must be maintained, steel CMF would be the perfect material, especially if the piece being replaced is made out of aluminum. Say, for example, a piece of solid aluminum was to be replaced with a piece of stainless-steel CMF. In this case, the new CMF product would weigh the same as the original solid aluminum part but the fire resistance would be 42 times greater as the original aluminum part has a thermal conductivity of about 210 W/m-ºC while the stainless-steel CMF has a thermal conductivity of about 5 W/m-ºC. This does not even consider the other advantages of using CMF instead of traditional material such as increased impact resistance, radiation blocking, lower thermal expansion, and vibration/sound absorption. Overall, CMF displays thermal resistance at levels far exceeding most metallic materials used today, potentially resulting in significantly safer or higher performance products.

Radiation shielding

CMF has proven itself for use as a radiation shield. Producing CMF out of heavy elements will allow the material to block X-Rays and Gamma Rays while the gas trapped inside the spheres can be a light material such as nitrogen to protect against Neutron radiation. This material blocks all types of radiation while remaining relatively lightweight and maintaining a low density. Plus, CMF radiation shields can be created using aluminum or steel which are not toxic, like lead. Studies have shown that a block of aluminum-steel CMF (made with steel spheres in an aluminum matrix) is roughly 150% better at shielding from X-ray radiation compared to an identically sized block of aluminum. Meanwhile, steel CMF is roughly 275% better at shielding when compared to a solid block of steel. These traits allow CMF to be used in the transportation of radioactive materials or as parts of medical devices, etc. reducing the weight, increasing the performance, and increasing the overall safety of the operators, patients, and doctors.

CMF ballistic performance

Composite Armor including a thin ceramic faceplate, Composite Metal Foam center, and a thin rear catcher plate (aluminum or Kevlar) has been tested against various ballistic threats. CMF has demonstrated multi-hit performance, ensuring that the armor does not fail if it is struck many times. The hollow spheres crush and absorb the majority of the bullet’s energy (>65%) upon impact and small, singular ring cracks on the 10” x 10” ceramic face plates indicate that the energy is transferred to and absorbed by CMF, opening room for multi-shot capability.. It should be noted that other armors will stop projectiles as well however, other armors will have a large protrusion on the rear of the plate where the bullet was stopped. This large protrusion, called Back Face Deformation (BFD), can cause trauma to the individual wearing the armor. CMF stops the projectile with 4-5 times less BFD.

Puncture resistance of CMF core sandwich panels

A study conducted in 2020 determined that a sandwich panel created from steel CMF sandwiched in-between two thin steel plates with a total sandwich panel thickness of 1” can absorb the entire kinetic energy of over 14,500 Joules from a 1.25” steel ball traveling at over 1,000 miles per hour without penetration through the panel. This is more than 7 times higher than the energy from a bullet fired from an AK-47.

CMF properties under high-speed impact

The hollow metal spheres in Composite Metal Foam act as fine, sealed pockets of air uniformly distributed throughout the CMF structure that can dampen impact, blast-wave, fragments, and ballistic threats, and can protect against extreme heat, fire, vibration, and radiation. Under loading, the sealed pockets of air will resist the force as the air pressure builds within. (For a real-world example, imagine compressing a sealed water bottle. When the bottle is filled with air and sealed, it is difficult to compress but once the bottle is opened and the air is allowed to leak out, it is easy to compress. CMF is like the sealed bottle while other metal foams are like the open bottle). Once the pressure passes a critical point, it will burst through the steel spheres and the air will penetrate through the matrix to escape the material. The resistance to this escaping air provides CMF with its extraordinary energy absorption.

Other advantages of CMF

In addition to the aforementioned advantages made possible by Composite Metal Foam, CMF can also absorb vibrations/shocks/sound, can withstand millions of high loading cycles, mitigates shock from blasts and frags, isolates impact energies, and has the potential to shield against RF and EM interference. CMF accomplishes all of this while weighing a third of the bulk materials in use today. In a study published in 2018, experimental and modeling studies on the performance of bare CMFs panels (no front or back plate added) against Blast and Frags generated by high explosive incendiary (HEI) was reported: load equivalent to 206 g TNT (experiment) and 206 g TNT equivalent (modeling). Shock front statistics: velocity 2,075 mps (4,642 mph); peak overpressure: 4.3 Mpa; blast energy: 587 kJ. The results indicated that Aluminum plates with the same thickness and a similar weight to steel CMF panels have significant hole gouging and bulge out created by fragments, while CMFs show minimal damage against the same blast and frags. Aluminum plates transfer the blast and frag energy over the entire panel (not absorbing), while CMF absorbs all of the impact energy locally through collapsing of spheres, leaving the rest of the panel essentially unstressed

Comparison to other metal foams

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Composite Metal Foam has a strength to density ratio 5-6 times greater than the next metal foam competitor and has over 7 times higher energy absorption capacity compared to other metal foams. Many other structural foams have been created in the past but due to inherent flaws or inadequacies in their design or manufacturing process, none of these materials can match CMF in performance across multiple parameters. Other types of metal foam are created by injecting air bubbles into molten metal (like a bubble bath) or by introducing a foaming agent into the melt (like baking soda in cake batter). As the molten metal solidifies, the air bubbles create the metal foam. Unfortunately, both of these processes for production result in non-homogeneous foam structures with randomly arranged porosities made of varying sizes. This non-homogeneity reduces the performance of the material across multiple parameters. The mix of large and small air bubbles results in localized collapse bands in the material as it undergoes compression. The larger bubbles collapse first, resulting in a sudden load increase on the smaller bubbles, forcing them to collapse. This process occurs rapidly and causes the material to fail spectacularly when under compression, absorbing very little energy in the process. Composite Metal Foam is produced using prefabricated hollow metallic spheres of the same size within a metal matrix. These hollow metallic spheres are uniform in size, allowing the CMF to spread the compressive load evenly across the structure, while the added matrix further supports the cell walls, creating an air-tight porous structure with greater energy absorption capacity and greater overall strength.

Syntactic foams are another branch of structural foam that is created using ceramic hollow spheres in a metallic or polymeric matrix. In either case, the energy absorption and mechanical strength is drastically reduced due to the use of brittle ceramic hollow spheres. When this material is subjected to a high compressive load, the ceramic hollow spheres shatter and allow the air trapped inside to escape out of the material entirely. The brittle nature of the ceramic does not allow the material to retain air pockets within. Composite Metal Foam, on the other hand, is created with ductile metallic hollow spheres in a metallic matrix. When subjected to high compressive stresses, the metallic spheres do not shatter in a brittle nature in the way that the ceramic spheres do. Instead, the malleable nature of the metallic hollow spheres allows the air pockets within the material to compress without releasing the air out of the material. As the compressive stress imposed onto the material increases, these metallic spheres further deform, increasing the air pressure within the spheres and increasing the overall resistance of the material. This effect actually allows Composite Metal Foam to increase its energy absorbing capacity as more energy is applied to the material at a higher rate (such as high-speed impact, blast, or ballistic impacts).

Composite Metal Foam is manufactured using either a powder metallurgy or casting technique, depending on the desired matrix material. For example, if steel Composite Metal Foam is being created, a powder metallurgy technique is used to sinter (bond) hollow steel spheres to steel matrix particles. If a binary CMF such as aluminum-steel Composite Metal Foam is being created, a casting technique is used where a low melting point metal such as aluminum is cast around a higher melting point spheres such as steel hollow spheres. These processes are staples of manufacturing and have been refined through more than a century of innovation. Vast industry knowledge is available regarding these traditional manufacturing metal processes. Using the traditional manufacturing techniques, multiple CMF products can be made simultaneously, reducing the time and cost per product. This will allow the manufacturing of CMF at costs much lower than other techniques such as additive manufacturing, a technique that can produce metal foams that are not air-tight and as such, their performance cannot match that of CMF while their cost of production is an order of magnitude higher.

History of Composite Metal Foam

Materials found throughout nature show high strength with low density, increasing both their performance and efficiency.

• The human skull is made from porous bone, allowing for lightweight impact absorption, protecting the brain while allowing for maneuverability.

• Bird bones are lightweight yet strong to support flight. Bird beaks are cellular structures that allow for repeated impact and vibration while retaining a low weight.

• Still air prevents thermal transmission while the act of compressing air absorbs energy.

How do we learn from nature? Using porous metals with large surface areas and air trapped inside porosities, we can create lightweight products that protect against impact, absorb vibrations and sounds, stop blasts, ballistics, and frags, shield from radiation, EM, and RF, and protect against fire and extreme heat, allowing for the manufacturing of advanced and more efficient protective structures. Natural structures have served as the inspiration behind Composite Metal Foam. CMF materials have been developed by Dr. Afsaneh Rabiei over the past 20 years using military, civilian, and academic resources. Initiated in North Carolina State University laboratories, CMF is now transitioning from academia into industry. The company Advanced Materials Manufacturing has been created around this material and is attempting to commercialize CMF products.

External links

• https://advancemnm.com/

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