1. Coating design
① Absorption matching type. The starting point of this design is to increase absorption and reduce surface reflection. In this way, the absorbent must have a large p value and a small c value.
②Resonant type. The starting point of this design scheme is to select the appropriate electrical thickness at the required frequency and use the interference principle of the interface to reduce the reflection of electromagnetic waves. The bandwidth of the ideal resonance is narrow.
③Composite structure. Through the multi-layer design, the impedance gradient is used to reduce reflection and increase absorption, which can reduce the requirements for the electromagnetic parameters of the material, which is conducive to the decrease of the surface density of the material. Calculations show that for single-layer absorbing materials, the absorption matching principle or resonance principle is used alone, and it is difficult to meet the requirement that the reflectivity in the passband of two to three centimeters is not more than -4dB under the constraints of absorber performance and material thickness. Under non-ideal resonance, taking into account the characteristics of the coating surface that can allow greater reflection, the use of absorption matching, including the resonance effect, combined with the dispersion characteristics, can achieve the electrical performance indicators we require. According to the requirements of some specific parts of the aircraft, the thickness of the coating should not be greater than 0.5mm, and the number of layers should not exceed two. The double-layer electrical design results show that the advantages of the multilayer structure are not very obvious due to the limitation of thickness and number of layers. This has been verified in the material development process.
Although the electrical performance of the absorbing material is related to the electrical design skills, the decisive factor is the electromagnetic performance of the absorber itself. Therefore, while paying attention to the dispersion characteristics of the absorbent, we should also consider: ① high magnetic permeability; ② high dielectric constant; ③ good temperature characteristics; ④ physical properties such as the density, morphology, and particle size of the absorbent; ⑤ absorption The agent matches the adhesive. A large number of screening work of different types of absorbents show that the current types of absorbents with better comprehensive performance are still ferrites and iron powders, and our research focuses on iron powder absorbents.
The absorbing coating is mainly composed of absorbent and adhesive. The electrical properties are mainly determined by the electromagnetic properties absorbed, while the mechanical properties are mainly guaranteed by the adhesive. As we all know, the absorbing coating is a stealth material with special functions. Its color to base ratio, thickness and quality are far greater than those of ordinary aviation coatings. This makes the development of absorbing coatings for aircraft more difficult.
Adhesive system research should solve:
①The mechanical properties are close to the index under the condition of high color to base ratio;
②The electrical parameters matched with the absorbent;
③Good environmental adaptability and physical and chemical properties;
④Satisfying the requirements of the coating construction process In order to ensure that the coating performance meets the index requirements, after comprehensively analyzing the pros and cons of various adhesive systems, one or two adhesive systems are selected for modification, and a special microwave absorbing coating is developed. Adhesive.
Figure 3-7 is the partial enlarged DSC curve of 5# sample. The main mechanical performance test results of the absorbing coating system. The environmental resistance test of the coating system includes heat resistance, low temperature resistance, machine medium oil resistance, distilled water resistance, and salt resistance. Fog and heat resistance and other performance tests. Experimental results: There is no obvious change in the appearance, electrical properties and adhesion of the coating. The absorbing coating is an engineering and practical radar absorbing material with excellent comprehensive performance. Its single-layer structure, sprayability and good physical and chemical properties fully ensure the feasibility of material construction and the stability of material properties.