Composite metal foam with a ruler for scale.
Photo credit: Afsaneh Rabiei

Lightweight steel/epoxy resin composite metal foam (CMF) aircraft wing leading-edge material repels water and insects better than aluminum, research from North Carolina State University (NC State) shows.

“We call our hybrid material infused CMF,” says Afsaneh Rabiei, corresponding author of a paper on the work and professor of mechanical and aerospace engineering at NC State. “While infused CMF is about the same weight as aluminum, it is tougher and has other characteristics that make it more appealing from a flight performance, safety, and fuel efficiency standpoint.”

CMF consists of hollow, metallic spheres of stainless steel or titanium embedded in a matrix of steel, aluminum, or metallic alloys. Researchers tested CMF with spheres and matrix made of steel. Previous work showed the metal foam can resist high temperatures, withstand .50 caliber rounds, and block blast pressure from high-explosive incendiary ammunition.

Infused CMF is made by immersing the steel-steel CMF in a hydrophobic epoxy resin and using vacuum forces to pull the resin into the hollow spheres and smaller pores within the steel matrix, filling about 88% of the CMF’s pores.

Researchers tested infused CMF and aerospace-grade aluminum to see how they performed in contact angle, which determines how quickly water streams off of a material; insect adhesion, or how well bug parts stick to the material; and particle wear, or how well the material withstands erosion. These factors all affect an aircraft wing leading edge’s performance.

Contact angle measures how well water beads up on a surface. The lower a material’s contact angle, the more water clings to the surface, which on aircraft wings, can affect aircraft performance. Researchers found that infused CMF had a contact angle 130% higher than aluminum.

Insect adhesion is measured by the maximum height and surface area of insect residue on a material. Infused CMF outperformed aluminum – with 60% of maximum height and 30% of the surface area.

Erosion. Grit blast experiments simulated the wear that occurs on aircraft wings in flight. Researchers found that while grit blast increased surface roughness for infused CMF, it still fared better than aluminum. At its worst, infused CMF still had a contact angle 50% higher than aluminum. Infused CMF retained its properties through erosion and wear, which indicates it would give leading-edge wing components a longer lifetime – and reduce the costs associated with maintenance and replacement.

Rabiei says results suggest that infused CMF performs better at the same weight or that using different materials for the matrix or spheres can perform as well as conventional aluminum at a fraction of the weight. She concludes, “Either way, you’re improving performance and fuel efficiency.”

The research was done with support from NASA, under grant number NNX17AD67A.