目的 在多孔陶瓷（SiO2f/SiO2）表面喷涂改性硅溶胶封闭涂层,分析涂层在200~650 ℃多循环状态下耐热性、疏水性、介电参数等性能的演变规律,从而探究封闭涂层的失效原因及失效路径,为涂层在长航时高温电磁窗表面的推广应用探明可能性。方法 采用空气喷涂法在石英陶瓷表面制备了一种陶瓷基用改性硅溶胶封闭涂层材料,以不同的温度在马弗炉中循环加热处理涂层样板,并采用水接触角测试仪、扫描电镜、矢量网络分析仪等表征涂层性能。结果 涂层在300 ℃以内可循环使用。400 ℃下,漆膜中Si—C键及C—H键开始断裂、分解,形成亲水通道,水接触角从初始的114°下降至96°,但仍然具备疏水性。500 ℃下,涂层经历1次热循环后水接触角下降至92°,并且随着温度和循环次数增加涂层形成裂纹并扩张。200~650 ℃范围内,涂层经历6次热循环后介电常数平均值为3.39,介电损耗角正切值平均值为0.003 6,与基材本体值基本相当（8~12 GHz）。结论 制备的改性硅溶胶封闭涂层具有良好的耐热性、疏水性、介电性能,涂层长时循环耐高温可达400 ℃,短时（1 h）耐高温可达500 ℃,涂层失效过程遵循热分解-鳞片化-裂纹出现-裂纹扩张的规律,鉴于涂层优异的综合性能,该涂层有望在长航时高温陶瓷天线罩表面使用。
Abstract
With the continuous development of modern equipment, hypersonic aircraft has become a key technology that countries are competing to develop. Under the high-speed flight, the radome surface faces serious aerodynamic heating problems. Therefore, radomes are mainly fabricated with ceramic materials which show excellent dielectric and high-temperature resistance properties, such as quartz ceramics, alumina ceramics, silicon nitride ceramic, etc. However, ceramic materials are prone to absorb water and moisture, which seriously affect the transmission performance of radomes. Therefore, it is usually necessary to seal the surface of ceramic radomes to prevent water from entering inside. Although there have been various sealing coatings applied in equipment, these coatings were inevitably failed as long as suffering high-temperature above 400 ℃ in several minutes. Based on this issue, a modified silica sol coating with excellent high-temperature resistance, hydrophobicity and dielectric properties was prepared and sprayed on the surface of porous ceramics materials (SiO2f/SiO2). By analyzing the evolution of heat resistance performance under multiple thermal cycles range from 200 ℃ to 650 ℃, as well as hydrophobicity and dielectric parameters of the coating, the failure mechanism and path of coatings were explored, and possibilities for promoting its application on surfaces of long-haul high-temperature electromagnetic windows were identified. The coating was characterized with a water contact angle tester, a Fourier infrared spectrometer, a scanning electron microscope and a vector network analyzer. Experimental results indicated that the water contact angle, infrared characteristic peaks, microscopic morphology, and hydrophobicity of the film did not change significantly after undergoing six thermal cycles below 300 ℃. When the temperature reached 400 ℃, the Si—C and C—H bonds in the film began to break and decomposed, causing a decrease in coating density and forming hydrophilic channels. As a result, the water contact angle decreased from an initial value of 114° to 96°. The water contact angle of the film dropped from 114° to 92° after one cycle at 500 ℃, and generated scales after six cycles, which meant that the coating film was failed. Cracks formed at 600 ℃ and continued to expand with the rise of temperature and thermal cycles. The reason was that stress appeared in various regions after coating decomposition, resulting in local bulges of the paint film, showing a scaly shape and gradually intensifying. When the temperature became higher, the decomposition of the film became worse and suffered greater stress, leading to the generation of cracking. The average dielectric constant of the coating between 8 and 12 GHz was measured at 3.39 with an average dielectric loss of 0.003 6 after six thermal cycles within 650 ℃, which was comparable to that of substrate material with a dielectric constant of 3.33 and a dielectric loss of 0.003 0. The coating exhibited long-term thermal resistance up to 400 ℃ and short-term thermal (1 hour) resistance up to 500 ℃. Microscopic characterization showed that the failure path at high temperature above 500 ℃ followed the step of thermal decomposition-scaling-crack generation-crack expansion. Considering the excellent performance under high temperature, the coating is potential for use on surfaces of long-haul high-temperature radomes.
关键词
陶瓷天线罩 /
封闭涂层 /
循环耐温 /
疏水性 /
介电参数 /
性能演变
Key words
ceramic radome /
sealing coating /
cyclic-thermal properties /
hydrophobicity /
dielectric parameter /
properties evolution
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基金
航空科学基金（202200180T2001）
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