KEY CONCEPTS
• A new material known as a stainless-steel composite metal foam (S-S CMF) infused with an epoxy resin demonstrates potential for use in the leading edge of an airplane wing.
• The S-S CMF exhibited superior wear and abrasion resistance in tribology tests compared to the incumbent material, aluminum. 
• This metal foam combines the strength of metal with the cushionability of a foam to display superior high-speed impact resistance. 

The continuing objective of improving efficiency has been leading to the development of higher-performing, more value-added lubricants. Also involved in the process is the movement to identify new materials that are lighter yet retain the same valuable performance properties of the incumbent materials they are trying to replace. 

A good deal of attention has been paid to the replacement of traditional steel alloys in automobiles with non-ferrous alloys such as aluminum and high-strength, lower-weight steel alloys. But there are applications where aluminum itself also might be replaced.

An example is the leading edge of an aircraft wing. Afsaneh Rabiei, professor of mechanical and aerospace engineering at North Carolina State University in Raleigh, N.C., says, “The leading edge of an aircraft wing is the first part of the wing to contact air. This component in the wing must display excellent aerodynamics to enable the airplane to fly through the air efficiently.”

An analogy to the leading edge of an aircraft wing is the front bumper of an automobile. Rabiei says, “One of the challenges for both structures is how to deal with the level and types of contaminants that come into contact with them. In the case of the leading edge of an aircraft wing, many different types of debris can hit the wing including water, ice, sand and other abrasives and insects.”

These varying types of debris can impact the aerodynamic profile of the wing, which may adversely reduce the flight performance, safety and fuel efficiency of the airplane. The objective of a leading edge of an airplane wing is to maintain and establish laminar flow to maximize efficiency and performance.

Past attempts to protect the leading edge of aircraft wings involved the application of organic coatings. Rabiei says, “The problem with using these coatings is that they wear away and are not available during landings to protect the wing. In addition, frequent reapplication is necessary.”

Composite metal foams (CMFs) are materials that combine exceptional strength with light weight. In a previous TLT article (1), a metal matrix syntactic foam based on a magnesium alloy and the filler silicon carbide exhibits comparable strength to other aluminum and magnesium syntactic foams yet has a low enough density to float on water.

The reason for the light weight of this metal foam and other metal foams is that the major component is air. In 2003 Rabiei and her team created a new type of metal foam known as a stainless-steel composite metal foam (S-S CMF). She says, “This material combines the strength of the metal with cushionability of a foam to absorb the impact of various objects.”

The researchers have now evaluated the effectiveness of S-S CMF in the leading edge of aircraft wings. 

Infusion with two-part epoxy
The S-S CMF evaluated as a material for use in the leading edge of an aircraft wing was prepared by infusing S-S CMF with a two-part aromatic epoxy resin that contained a surface modifying agent known as aminopropyl-terminated siloxane. This latter material is known to improve the surface properties of epoxy resins. 

The S-S CMF is a porous network containing stainless steel metallic spheres embedded within a stainless-steel metallic matrix. The epoxy resin fills the voids within the S-S CMF leading to a material that has superior strength yet also has a density that is less than aluminum.”

Rabiei says, “Our approach produces a composite metal foam with the majority of its porosity content filled with epoxy. The S-S CMF exhibits the beneficial aspects of open- and closed-cell foams, a unique property associated with CMF only. This means that during service, the epoxy resin does not leave the matrix porosities enabling the epoxy fused S-S CMF to retain its physical and mechanical properties in various loading and environmental conditions.”

Figure 1 shows an image of the S-S CMF.


Figure 1. The epoxy infused stainless steel composite metal foam shown here has the potential to be used in the leading edge of an aircraft wing and can be considered to be ‘bubble wrap on steroids.’ (Figure courtesy of North Carolina State University.)

The researchers evaluated the S-S CMF versus the widely used aluminum aerospace alloy 6061-T6 in a series of evaluation tests. Surface durability was evaluated using the pin on disk procedure (ASTM G99) that is a technique utilized in the testing of lubricants. A second technique known as taber abrasion (ASTM D4060) tracked the evolution of wear along the material’s surface.

In both tribology tests, the epoxy infused S-S CMF demonstrated superior wear and abrasion resistance. Contact angle measurements were made to assess the hydrophobicity of the surface. Rabiei says, “The presence of the epoxy resin within the porosities of the S-S CMF increases the contact angle compared to aluminum meaning that there is greater resistance to water and ice buildup.”

Insect build up can lower wings’ performance during take-off and landing. A pneumatic delivery device (wind tunnel) delivered a stream of fruit flies against the epoxy infused S-S CMF materials. The S-S CMF exhibited a lower insect impact height and a smaller impact area than aluminum.

Rabiei says, “To mimic the natural environment encountered by an airplane, a micro-grit sand blasting test was conducted. The S-S CMF also displayed superior erosion resistance to aluminum.”

The overall test results demonstrate the potential for epoxy infused S-S CMF to be used as a superior material to aluminum in the leading edge of an airplane wing. Rabiei feels that there are many more applications where the S-S CMF can be used. She says, “We consider the composite metal foam to be ‘bubble wrap on steroids.’ This is a material that has exceptional high-speed impact resistance.”

Rabiei envisions that the CMF has potential to be converted into a lubricant carrier material through the application of a grease or liquid lubricant into its pores. Additional information can be found in a recent article (2) or by contacting Rabiei at arabiei@ncsu.edu. 

REFERENCES
1. Canter, N. (2015), “Lightweight metal matrix syntactic foam,” TLT, 71 (8), pp. 12-13.
2. Marx, J., Robbins, S., Grady, Z., Palmieri, F., Wohl, C. and Rabiei, A. (2019), “Polymer Infused Composite Metal Foam as a Potential Aircraft Leading Edge Material,” Applied Surface Science, click here.