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Design and Preparation of Wave-Transmitting Reinforcing Coatings for Multifunctional Ceramic Composite Antenna Windows

For obtaining multifunctional ceramic composite antenna windows with integrated function including low density, wave-transmission and thermal insulation, the design and preparation of reinforcing coatings on ceramic matrix was discussed in detail in this work. With quartz glass chopped fiber as the major constituent for matrix, borosilicate glass was selected as the main phase for reinforcing coatings, and functional filler was added to match the coefficient of thermal expansion between the two components. According to the performance testing of the as-prepared samples, content of B2O3 in SiO2/B2O3 binary system and sintering temperature showed a marked impact on the microstructure, density and thermal behaviour of the ceramics. Densified coatings were prepared at 1160°C after sintering for 1h with 20% B2O3 in borosilicate glass, which bond well with the substrate and have radiation coefficient of 0.85 and linear expansion coefficient of (1.77∼2.39)×10-6/°C (200∼1000°C). Stronger matrix, better ablation resistance and dielectric properties are expected to be integrated in the obtained sample pieces, which can be a good candidate as effective thermal insulation materials for aerospace aircrafts at high flight Ma.


Introduction
Rigid thermal insulation materials with ceramic fibers as raw materials, has excellent thermal insulation performance [1], wave permeability [2,3], anti-erosion [4] and shape retention, as well as superior mechanical properties at high temperature [5], which has become a good candidate for multifunctional ceramic composite antenna windows integrating low density, wave-transmission and thermal insulation qualities. The service temperature of these materials is mainly in intermediate region from 600 to 1260 o C [6]. In order to avoid being destroyed by high-speed airflow scour and reduce the incoming heat, densified coatings with good anti-erosion properties, high radiation coefficient and excellent thermal shock resistance are usually applied on the surface of the ceramic matrix [7].
Borosilicate glass is known to be a kind of material system with heat-resistance, low expansion, good thermal shock resistance and erosion resistance [8]. Moreover, borosilicate glass shows good chemical and physical compatibility with ceramic matrix using silica fibers as main components [9]. Therefore, high-performance borosilicate glass coatings were designed and prepared in this work, through systematically investigating the effects of chemical component and sintering temperature on the phase composition and microstructure of borosilicate glass coatings. The expansion coefficient, dielectric property and radiation coefficient of the obtained coatings were also characterized as well.

Materials and Methods
Coating slurries were prepared by ball-milling raw materials for 30min with ratios listed in Table 1. Spray finishing was employed to coat the slurries on the surface of the multifunctional ceramic matrix, which was then dried and sintered above 1000 o C for 1h to obtain coating samples for further characterization with scanning electron microscope (SEM). To investigate the effect of sintering temperature on the microstructure of coatings, SEM characterization was carried out with samples sintered with slurries in No.3 ratio for 1h at 1100 o C, 1130 o C and 1160 o C respectively. Besides, back-scattering electron (BSE) imaging, expansion coefficient, dielectric properties and radiation coefficient were also tested for coating samples sintered at 1160 o C for 1h in No.3 ratio. Figure 1 shows the microstructure of coatings sintered at 1160 o C for 1h, with slurries in different components. As can be seen from section images shown in Figure 1a, 1c and 1e, melting degree increases and internal porosity decrease with the increasing amount of B 2 O 3 from 10% to 20% in slurries, indicating the increasing density of sintered coatings. Specifically, fine particles in slurries become to melting and joining with each other, while many interconnected holes can be found in Figure 1a. When B 2 O 3 increases to 15%, particles in slurries gradually fused and grow up to form larger glass phase, with less interconnected holes owing to the reduction of intergranular space, as shown in Figure 1b. While the amount of B 2 O 3 is up to 20%, continuous glass phase is obtained with almost no intergranular space and few isolated pores in Figure 1c.

Effect of slurry components on the microstructure and phase composition of sintered coatings
As for surface images shown in Figure 1b, 1d and 1f, continuous fused glass phase is form gradually with the increase of B 2 O 3 in slurries. Specifically, particles begin to melting and joining in Figure 1b with more space, which is further reduced to form larger fused area in Figure 1d. A nearly continuous glass phase is obtained with few isolated pit areas in Figure 1f Figure 2 shows the microstructure of coatings sintered with slurries in No.3 ratio for 1h at 1100 o C, 1130 o C and 1160 o C respectively. As can be indicated from Figure 2, the melting degree of the coatings increases with decreasing pores and the density is gradually increasing, as the increase of sintering temperature. It is interesting to find that, the sintering temperature has similar effect on the microstructure of the coatings, which is indicated in detail in section images (Figure 2a, 2c, 2e) as well as surface images (Figure 2b, 2d, 2f).

Effect of sintering temperature on the microstructure of the coatings
The binary phase diagram of SiO 2 -B 2 O 3 [10] with 20% B 2 O 3 was consulted to further confirm the phase composition of the coatings sintered at different temperature. At 1100 o C, the coating is composed of more solid phase of molten quartz and less liquid phase of borosilicate glass. At 1130 o C, the coating is composed of less molten quartz solid phase and more borosilicate glass liquid phase. At 1160 o C, the coating is composed of single borosilicate glass liquid phase. In brief, densified coatings   Figure 3 shows the SEM and BSE images of typical coating samples prepared at optimized conditions mentioned above. As seen from Figure 3a, continuous densified structure is formed in the section of the coatings, with few isolated closed pores. Some pits and semienclosed pores are found on the surface of the coatings, as shown in Figure 3c. According to corresponding BSE images in Figure 3b and 3d, relatively brighter particles as circled in red are identified as Y 2 O 3 , which are uniformly distributed in the section and surface of the coatings. To further confirm the component analysis, Energy Dispersion Spectrum (EDS) in different areas in the coating samples was collected and the results are shown in Figure 4 and Table 2.   Based on the results of EDS, it can be seen that the elements in typical coating samples include B, O, Si and Al, indicating the generation of aluminum borosilicate glass with low aluminum content. Y is found to be accumulated in both selected area 1 and 2, while no Y is observed in selected area 3. The main substances in area 3 are SiO 2 , B 2 O 3 , and a small amount of Al 2 O 3 , which is supposed to be aluminum borosilicate glass. That is, filler Y 2 O 3 particles are evenly dispersed in the coated borosilicate aluminum glass independently and do not react with SiO 2 , B 2 O 3 and Al 2 O 3 to form glass solid solution.

The matching of expansion coefficient between coating and matrix
Thermal matching plays key effect on the high-temperature behavior of the composite multifunctional materials. Therefore, the expansion coefficient of matrix and coatings is measured from 200 o C to 1000 o C, which is demonstrated in Figure 5. The coatings present larger expansion coefficient than matrix with maximum and minimum difference of 1.5×10 -6 /K -1 and 0.4×10 -6 /K -1 in the whole testing range, respectively. The relatively small difference in the expansion coefficient of matrix and coatings means the fewer cracks possibly appeared during the period of their service, and the better thermal insulation properties obtained in this work.

Conclusions
Dense coatings have been obtained in this work, with raw materials containing 20% B 2 O 3 by sintering slurries at 1160 o C for 1h. Filler Y 2 O 3 particles are evenly dispersed in the coated borosilicate aluminum glass independently, which are supposed to increase the porosity of the coatings and correspondingly provide stronger resistance against thermal shock. Moreover, the relatively small difference in the expansion coefficient of matrix and coatings indicates better thermal match and thermal insulation properties of the as-prepared sample pieces, which can be a good candidate as effective thermal insulation materials for aerospace aircrafts at high flight Ma.