Stealth Materials

Many modern military aircraft incorporate some type of surface treatment that provides radar cross section reduction to thereby transform these aircraft into "low observable" or "stealth" airplanes. Generally, these treatments employ materials that absorb or conduct incident radar energy, and typically include adhesive bonding or spray-paint-like processes for material adherence. Where materials (e.g. caulks, paints, adhesives) requiring a wet application are used, inherent undesirable requirements include surface preparation, mixing, cure time, presence of volatiles and hazardous materials, use of personal protection devices, and acquisition of special application equipment. In addition to being quite inconvenient, the application of these materials requires an inordinate amount of time at both the manufacturing event and at any repair event in the field. Correspondingly, because of these time factors, labor expenses escalate significantly.

Electromagnetic radiation absorbent/shielding materials and structures are well-known. Such electromagnetic radiation absorbent/shielding materials and structures are commonly used in electromagnetic capability/electromagnetic interference (EMC/EMI) test cells to eliminate reflection and interference during testing. Electromagnetic radiation absorbent materials and structures are also utilized in electromagnetic anechoic chambers for testing high frequency radar, in antennas, and in Low Observable (LO) structures.

Two methods have been widely adopted in order to produce such absorbers. The first is to avoid a discrete change of impedance at the material surface by gradually varying the impedance. The removal of the discontinuity at the surface results in the microwave energy entering the absorbing medium without reflection. This transition from the impedance of free space to that of the bulk material is achieved by a geometric profile. The carbon-loaded foam pyramids used as the lining of anechoic chambers are typical of such an of absorber. To produce such absorbers, it is necessary to taper the material over distances that are large compared with the wavelength to be absorbed. Therefore, practical absorbers of this type giving greater than 20 dB absorption vary in thickness from a plausible 0.8 in. (2 cm) at 10 GHz and above to an implausible 6 ft (2 m) around 100 MHz and above.

The absorber performance improves with increasing thickness until the point is reached where all of the energy that enters the material is absorbed and only the front-face reflection is left. While this type of absorber is capable of producing a very high degree of absorption over a broad bandwidth, it is at the same time a relatively thick material.

The second method of absorber design produces much thinner [and thus more practical for aircraft] absorbing layers which are capable of producing good absorption (=25 dB) with restricted bandwidths. These materials achieve the absorption by a combination of attenuation within the material and destructive interference at the interface. The electromagnetic properties and the thickness of the layer are such that the initial reflected wave and the sum of the emergent rays from the multiple reflections within the material are equal in magnitude, and opposite in phase. The thickness of the layer is close to a quarter-wavelength at the frequency of operation, giving a 180° phase difference between the interface reflection and the emergent waves.

Radar absorbing material (RAM) currently in military and commercial use are typically composed of high concentrations of iron powders in a polymer matrix. These materials are both very heavy and very costly, two key limitations to their adoption for many applications. Various attempts to overcome these problems have involved the creation of artificial dielectrics, including ones based on conductive fiber-filled composites. While successful in many ways, these composites are beset by their own technical difficulties. Uniformity and consistency, critical attributes for a successful RAM, are difficult to achieve with fiber-filled composites as mixing and distribution of the fibers is opposed by the natural tendency of the fibers to clump. Also, there is not as great a cost reduction as expected with fiber-filled composites.

Typical radar absorbing material (RAM) coatings incorporate iron particles in a resin that is either spray painted on the surface of the vehicle or applied thereon in the form of decals. The iron particles can also be incorporated into a ceramic matrix material. In a good light weight specular RAM coating high attenuation level and broad frequency range are important. However, with such coatings peak attenuation band width decreases with decreasing frequency and causes attenuation at frequencies other than the peak attenuation frequencies to be less than 5 dB. One common technique to improve the broad band response of a specular RAM is to use multiple coatings separated by some kind of a band pass filter. However, such multiple layer absorbers have weak shear planes between layers, are expensive and, additionally, create field maintenance problems. A problem of both single and multiple coating is their high unit weight.

The performance of these coatings, particularly those using spherical particles, is dependent upon how closely the spheres are packed together. Thus the most efficient coating would be one approaching the density of solid iron with a minimum amount of resin included to electrically insulate the particles from one another. That is, the attenuation efficiency increases faster than the weight, so that a thinner coating with the same attenuation, can be used, providing an overall weight savings. Unfortunately, the particles, when produced, are of non-uniform diameter and not necessarily uniformly round.

Prior attempts to modify the radar cross-section associated with fasteners include the application of RAM material. Such a process begins with the initial installation of the fasteners. A RAM material is then applied to the surface of the panels and across the fastener heads. The RAM material may be a spray-on type of coating or may take the form of what is referred to as "caulk and tape" type of application. The RAM material is then allowed to cure as required. As a result, a RAM layer is formed which encapsulates the surface discontinuities present at the fastener-to-panel interface and the fastener tool recess. While this process improves the surface continuity characteristics, the application process is undesirably time and labor consumptive and requires specialized training. Further, such RAM materials must be removed to reveal the fastener driver tool recess before the fastener can be removed when it is desired to open the associated panel. Furthermore, upon subsequent closure of the panel, the entire RAM application process must be repeated.

Although various materials have been found to be suitable for use in electromagnetic absorbent/shielding structures, a problem that frequently arises concerns the treatment of gaps that are frequently formed by intermediate adjacent structural members, such as structural panels or coverings. In this regard, it is recognized that such gaps may contribute substantially to the undesirable reflection of electromagnetic radiation. Thus, in order to reduce the reflected by a gap, it is necessary to fill the gap with an electromagnetic radiation reflective material. To that end, namely, to mitigate electromagnetic radiation reflection from such gaps between adjacent electromagnetic radiation panels and the like, conventional methodology dictates the use of a conductive filler, which is typically known to comprise nickel-coated inclusions designed to produce a material with maximum DC conductivity.

While such contemporary conductive gap fillers have proven generally suitable for their intended use, the same nonetheless possess inherent deficiencies which tend to detract from their overall desirability. Such inherent deficiencies particularly detract from the usefulness of such gap fillers in the repair and maintenance of LO aircraft. Specifically, replacement of gap treatments for frequently removed/opened access doors and panels takes too long, dependent on cure time of caulks and tapes. Lack of performance in four areas also occurs, namely: (1) some caulks are not conductive enough, due either to less conductive fillers, or less volume % loading; (2) extension and elasticity at -67.degree. F. are too low; (3) resistance of gap fillers to aircraft fluids has been less than desired, often when using "accelerated" cures which are incomplete and thus susceptible to solvent-induced swell; and (4) adhesion and crack resistance are often low.

Most prior art conductive fillers fail to attain both properties of effective electric permittivity, on one hand, and resilient mechanical/material properties, on the other. The latter property is especially important when such gap fillers are utilized in LO aircraft maintenance due to the harsh environment to which such fillers will be subjected, which necessarily requires that such filler possess sufficient material durability and reliability.

Composite materials have a wide variety of commercial and industrial uses, ranging from aircraft and automobile to computer parts. Composite materials have many advantages which make them attractive to different industries. For instance, composite materials can reduce heat transfer, resist conduction of electricity, limit reflection of radar waves, are flexible but strong, and can be fairly easily formed into complex shapes during manufacturing. Some examples of commercial applications include the complex shapes of certain automobiles, airplanes, and boats which would be difficult to form with metal materials. Another important use of composite materials is the creation of stealth aircraft which minimize their radar cross section through the use of radar absorbing composite materials that form the majority of the aircraft's structure.

One example of an aircraft made largely from composite materials is the F-22 Raptor, the world's premier tactical aircraft, designed and manufactured by Lockheed Martin Tactical Aircraft Systems. The Raptor's composite parts are formed with flexible graphite fibers, called a ply, that are impregnated with epoxy or BMI resins which harden when subjected to the application of heat. The uncured plies are placed on tools, each tool corresponding to a composite part of the Raptor. Thus, when the graphite resin mixture hardens over the tool, the composite part is formed with the proper shape.

A number of production techniques are available for forming composite parts. Again, using the Raptor as an example, once the plies are placed over the tool, a vacuum bag is used to hold the plies securely to the tool during curing of the resin. The vacuum bag forces the material to the tool and prevents the formation of bubbles and other material deformities. The tools are then placed in an autoclave for heating. An autoclave is essentially a large oven with the ability to precisely control the thermal energy applied to tools during curing of composite parts. An autoclave operator can monitor and control the amount of thermal energy applied to the tools to maintain a predetermined heating rate of the composite parts.

A window member composed of a transparent resin or inorganic glass with a transparent conducting film such as gold or ITO (indium tin oxide) coated thereon, is used as an electromagnetic wave shield window for stealth aircraft. Applying such transparent conducting film enables, while maintaining transparency to visible radiation, both a radio wave stealth property which scatters radio waves in various directions so as not to be detected by radar, and an electromagnetic wave shield property which prevents harmful electromagnetic waves, except for visible radiation, from invasion into an aircraft.

However, in many cases, such window members for aircraft, especially windshields and canopies are bent at a large curvature and have a multiple curved surface shape, having a plurality of radius of curvature. Therefore there is a problem in that it is difficult to coat a transparent conducting film on a window member in a uniform thickness. There is yet another problem in that as the radio wave stealth property or the electromagnetic wave shield property is increased, the transparency to visible radiation is reduced. In particular, to achieve an adequate radio wave stealth property or electromagnetic wave shield property with a transparent conducting film, the transmissivity of visible radiation drops to 70% or lower, which causes a problem in that the view from the aircraft is darkened.

Furthermore, a window having at least one of a radio wave stealth property and a electromagnetic wave shield property, comprising a window member coated with a transparent conducting film, is exposed to a wide range variation in outside pressure and temperature during flight and the window member deforms. However, the transparent conducting film, especially ceramic transparent conducting film such as ITO does not deform sufficiently to follow the deformation of the window material. Therefore the transparent conducting film might crack even with relatively little deformation, which can occur during an actual flight. To prevent such cracking, especially ceramic transparent conducting film such as ITO is coated on the inner side of the windows, which deform relatively less.