WANG Tongtong, ZHANG Pingping, SHAO Changtao, WEI Qihong, CUI Haozhe,

WANG Hongsheng, LUAN Qiang, LIU Ruixiang

(Shandong Industrial Ceramics Research and Design Institute Co., Ltd., Zibo 255000, Shandong, China)

Extended abstract:[Background and purposes] Fiber-reinforced ceramic matrix wave-transmitting materials have become the main selection for aircraft radar radome & antenna windows. With the rapid development of guidance technology, the design, materials and preparation of multi-functional integrated radome & antenna windows, such as wave transmission, broadband and stealth performance, have become one of the bottlenecks that limit the development of weapons and equipment. With single medium radome & antenna windows, it is unable to meet the above requirements. Frequency selection surface (FSS) can regulate electromagnetic waves effectively, thereby achieving the above-mentioned purpose. However, the preparation of FSS radome & antenna window materials serving with high temperature (≥800 ℃) based on fiber-reinforced ceramic matrix composites mainly faces two major technical problems, which are (i) preparation of conductive coating with high temperature resistance and high bond strength and (ii) high-precision FSS structure processing for complex curved surfaces. The latter can be solved by the laser etching and other methods, while the former has exposed the surface fibers of the material and broken the matrix due to mechanical processing. When the metal layer is coated directly, it is difficult to create a strong bond between the two layers. Polysilazane (PSZ) is one of the important ceramic precursors with the characteristics of in-situ ceramic transformation. It has rarely been reported in the use of surface reinforcement material for fiber-reinforced ceramic-based wave-transmissive materials. In this paper, SiO_2f/SiO₂ ceramic matrix composite was studied, whereas the PSZ precursor was used to perform surface strengthening treatment, which then coated with a high-temperature resistant metal coating to prepare Pt-SiO_2f/SiO₂ composites. The preparation and performance of the composites were studied, aiming to propose a method to enhance the binding force of metal-ceramic matrix composites effectively and explore the preparation method of new ceramic-based FSS radome & antenna window materials.[Methods] The SiO_2f/SiO₂ composite was prepared by using liquid phase penetration combined with sol-gel methods, while the preform had a needle punched structure. The first round of the immersion was liquid infiltration, while the second round was sol-gel method, using silica sol with mass fraction of 25% and 40%. The PSZ precursor was mixed with self-made ceramic fillers and sprayed onto surface of the SiO_2f/SiO₂ composite. After that, the Pt slurry was sprayed and sintered at different temperatures. The internal quality of the composites was analyzed by using the Carl Zeiss Metrotom 1500 CT scanning system. Microstructure and morphology of the composites were characterized using Thermo Fisher Apero S scanning electron microscopy. Macroscopic morphology of the sample was analyzed by using Japanese KEYENCE VK-X2000 laser confocal microscope. The TG analysis on the sample using the German FTA449 F5 synchronous thermal analyzer. The EDX 600PLUS film thickness gauge and FT-341 four probe tester were used to measure the thickness and conductivity of metal coatings. A quartz lamp radiation heating system was used to assess temperature resistance of the composites, while the adhesion of the coating was evaluated by using the grid cut method (GB/T 9286-2021).[Results] Density of the SiO_2f/SiO₂ composite material is 1.73 g·cm⁻³, in which the internal voids of the preform are effectively filled with SiO₂ matrix. Volume defect rate of the composite material is only 0.08%. The exposed fibers and fracture of the matrix occurred due to the mechanical processing. The PSZ precursor undergoes ceramic transformation after 600 ℃. With increasing content of self-made ceramic filler, the PSZ precursor can form a fully encapsulated porous interface layer on the SiO_2f/SiO₂ composite materials. When the ceramic filler content is greater than 20%, defects, such as pits and cracks, will be present in the PSZ interface layer. As PSZ interface layer was subjected to 1000 ℃ for 300 s with quartz lamp heat treatment, it bonded well with the SiO_2f/SiO₂ substrate. With increasing sintering temperature, square resistance and conductivity of the Pt coating decreases and increases, respectively, over the same thickness range. When the coating thickness varies from 20 μm to 30 μm, the sheet resistance is (30±5) mΩ·□⁻¹, while the electrical conductivity reaches 106 S·m⁻¹. The Pt-coating prepared at sintering temperatures of 850–950 ℃ can withstand quartz lamp thermal assessment at 1000 ℃ for 300 s, while the adhesion remained at the highest-level 0. Finally, FSS periodic patterns can be obtained by using the laser processing on the surface of Pt-SiO_2f/SiO₂ composite materials.[Conclusions] The PSZ precursor significantly improved the condition of composites and bonding strength of Pt-coating, thereby realizing the application in frequency selective surface (FSS) radome & antenna windows. The optimal temperature for the Pt-coating is in the range of 850–900 ℃. When the coating thickness varies from 20 μm to 30 μm, the sheet resistance is (30±5) mΩ·□⁻¹, while the electrical conductivity reaches 106 S·m⁻¹. The Pt-coating adhesion remained at the highest-level 0, when subjected to 1000 ℃ for 300 s of quartz lamp thermal treatment, while the electrical conductivity remained unchanged. Moreover, the Pt-SiO_2f/SiO₂ composites had a high laser machinability.

Key words: ceramic composites; surface metallization; PSZ precursor; Pt-coating