What materials absorb radar waves? Technical breakdown of the coatings essential to the stealth capabilities of modern fighter jets.

In the world of air combat, stealth is not limited to an angular silhouette or the intensive use of composites. It also relies on an invisible but decisive element: radar-absorbing coatings. As radars become more accurate and ground-to-air defense systems evolve, military engineers have redoubled their efforts to reduce the radar signature of aircraft. This is achieved not only through design, but also through materials capable of trapping or dissipating the electromagnetic energy emitted by enemy radars.

Stealth aircraft such as the F-22 Raptor, Su-57, and B-21 Raider all rely on a combination of geometric design and sophisticated surface treatments. These materials—called RAM (Radar Absorbent Material)—are not interchangeable. Their effectiveness depends on the radar frequency, the angle of incidence of the waves, and the composition of the coating. Some absorb by conduction, others by resonance or dielectric loss.

This technical article explores families of RAM materials, their physical properties, the challenges of their industrial application, and the current limitations of these technologies in the race for air superiority.

A physical principle: absorbing electromagnetic energy

The functioning of a radar-absorbing coating is based on a simple principle: reducing the electromagnetic energy reflected back to the radar source. This energy normally bounces off the metal surface of a fighter jet, allowing it to be detected from several hundred kilometers away.

Absorbent materials aim to convert this energy into heat (through the Joule effect or dielectric losses) or disperse it, preventing the return of a clear echo. To achieve this, researchers use three main mechanisms:

1. Magnetic resonance: ferromagnetic particles embedded in a polymer generate an electromagnetic response that opposes that of the incident wave, reducing the reflected signal.
2. Dielectric losses: in specific composites, molecular polarization delays the electric field, thereby dissipating the incident energy.
3. Destructive reflection: certain layers are arranged to cause destructive interference of the reflected waves, particularly in multilayer materials.

In practice, RAMs must be effective over a frequency range of 2 GHz to 40 GHz, depending on the threats being considered. A conventional search radar can operate between 8 and 12 GHz (X band), while newer systems extend beyond 18 GHz (K band).

A variety of materials with targeted properties

There is no single coating, but rather a family of absorbing materials, each suited to specific frequency ranges or usage constraints (weight, flexibility, durability).

Filled polymers

The first operational RAMs were made of flexible polymers (elastomers, epoxy resins) filled with metal powders or ferrite particles. They offer good absorption in the X band but require thicknesses of between 3 and 10 mm, which adds weight. Their density ranges from 1.2 to 2.4 g/cm³. For a fighter with 20 m² of exposed surface area, this can represent up to 50 kg of coating.

Ferrite materials

Ferrites, such as nickel-zinc ferrite (NiZnFe₂O₄), are used in the form of tiles glued to certain parts of a stealth aircraft. They are effective in the VHF and UHF bands (long-range search radars). However, their high density (up to 5 g/cm³) and rigidity limit their use to non-deformable surfaces such as leading edges and air intakes.

Metamaterials

These artificial structures, arranged on a sub-millimeter scale, manipulate the electromagnetic field by creating negative permittivities and permeabilities. Their use remains experimental, but some military prototypes have reportedly achieved absorption rates of over 90% on specific frequencies with a thickness of less than 1 mm.

Adaptive coatings

More recent, these materials change their electromagnetic response depending on the incident frequency or temperature. Based on liquid crystal composites or active semiconductor layers, they could offer dynamic stealth. They are currently being tested on some experimental drones.

A complex application on fighter jets

Installing and maintaining RAM on a stealth aircraft is a major challenge. For example, the F-22 Raptor requires between 700 and 1,000 hours of RAM maintenance for every 100 hours of flight time. This represents a significant additional operating cost.

Fragility and costs

The materials are sensitive to moisture, thermal shock, and chemical attack. Prolonged exposure to kerosene, acid rain, or UV radiation rapidly degrades the coating’s effectiveness. Some coatings must be completely replaced after 6 to 12 months, generating estimated costs of between $250,000 and $500,000 per year for a single aircraft.

Aerodynamic compatibility

The coating must adapt to the curves of the aircraft without disrupting the boundary layer or generating turbulence. Microcracks measuring just a few microns in the RAM layer can be enough to increase the radar equivalent surface area (RESA) by a factor of 10.

Radar-transparent coatings

Conversely, certain areas such as radomes must remain transparent to onboard radar. These areas are not covered with RAM, but with specific composites that allow waves to pass through while limiting their external signature.

An ongoing technological race

No stealth technology is eternal. Advances in multistatic radars, passive networks, and detection intelligence require RAM to constantly adapt.

New developments

DARPA, Rostec and the CETC laboratory in China are investing in nanostructured composites capable of acting as thermal insulators, radar absorbers and infrared dissipators. These new-generation coatings aim for a thickness of less than 2 mm and an absorption rate of over 95% between 2 and 40 GHz.

Towards integrated infrared stealth

Future fighter jets will not only have to absorb radar waves, but also reduce their thermal signature. The integration of thermal RAM capable of masking the heat emitted by turbojet engines is being studied in programs such as the US NGAD and the European SCAF.

New threats

The arrival of quantum radars could eventually challenge the traditional principles of stealth. These radars, based on photonic entanglement, are still in the experimental stage but could theoretically detect a stealth aircraft despite its RAM.

Electromagnetic stealth currently relies as much on geometry as on the effectiveness of radar-absorbing coatings. Although these materials have proven their worth in modern conflicts, they require costly compromises and intensive maintenance. In a constant technological war, every gain in invisibility comes at a price in terms of complexity, weight, and logistics.

The evolution towards thinner, more stable, and more versatile materials will determine the future of stealth fighter aircraft programs in the face of increasingly adaptive and interconnected detection systems.

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