目的 针对多孔Si3N4陶瓷因易吸潮导致介电性能劣化的问题,开发一种兼具优异防潮性能、良好结合强度及高抗热震性的复合涂层,以提升多孔Si3N4陶瓷在极端高温透波环境下的应用可靠性。方法 基于BaO-Al2O3-SiO2（BAS）三元相图,通过结构设计、压缩空气喷涂和低温烧结,在多孔Si3N4陶瓷表面制备了BAS玻璃-钡长石/Si3N4复合涂层,系统研究了不同熔融温度（1 300 ℃和1 400 ℃）的BAS玻璃、烧结温度以及添加5%（质量分数）Yb2O3对涂层的微观结构、防潮性能（防水率）、涂层与基体结合强度以及抗热震性能的影响规律。结果 添加5%（质量分数）Yb2O3可显著降低BAS玻璃熔体黏度,促进其在烧结过程中的横向流平和纵向渗透,有效提高了涂层的致密度及其与基体的结合强度;采用熔融温度为1 300 ℃的BAS玻璃,在1 390 ℃烧结温度下制备的涂层综合性能最优：涂层致密化良好,防水率高达97.14%,与基体的结合强度达到12.4 MPa;该涂层在经历ΔT=1 100 ℃的5次循环热震测试后,防水率仍能保持在91.01%,展现出优异的抗热震性能。结论 本研究在多孔Si3N4陶瓷表面制备了高性能的BAS玻璃-钡长石/ Si3N4复合涂层;Yb2O3的引入优化了BAS玻璃的烧结行为,显著提升了涂层的致密性、结合强度和抗热震性;特别是基于1 300 ℃熔融BAS玻璃制备的涂层,在1 390 ℃实现致密化,其优异的防潮性能和良好的结合强度表明该复合涂层能有效阻隔湿气侵蚀并承受极端温度冲击,为多孔Si3N4陶瓷在高温、高湿等恶劣透波环境下的长期稳定应用提供了可靠的技术途径。
Abstract
The work aims to fabricate a sealing and moisture-proof coating on the surface of porous silicon nitride (Si3N4) ceramics via the compressed air spraying method combined with pressureless sintering technology, so as to address the critical technical challenge that porous Si3N4 ceramic radomes are prone to moisture absorption, which subsequently leads to a drastic degradation in their wave-transparent performance—a key issue that has long restricted the reliable application of such ceramics in humid environments.
The initial structural design of the coating developed in this work adopts a two-layer configuration, consisting of an intermediate layer and an outer layer. Specifically, the intermediate layer is composed of 50wt.% Si3N4 and 50wt.% BaAl2Si2O8. It is important to note that BaAl2Si2O8 is formed through high-temperature solid-state reactions and phase transformations involving BAS (BaO-Al2O3-SiO2) glass, barium aluminate (BaAl2O4), and strontium aluminate (SrAl2O4). A small quantity of SrAl2O4 is intentionally incorporated into the coating, and its primary function is to facilitate the phase transition from hexagonal BaAl2Si2O8 to monoclinic BaAl2Si2O8. This phase transition is crucial as it helps reduce the thermal expansion coefficient of the coating, thereby safeguarding the ability of the coating to withstand high temperatures and preventing thermal mismatch-induced damage between the coating and the ceramic matrix.
The outermost layer of the coating is constructed by spraying a pure BAS glass layer onto the intermediate layer. During the sintering process, under the combined effects of capillary force and gravitational force, the BAS glass undergoes softening, followed by lateral leveling (spreading horizontally across the surface) and vertical infiltration (penetrating downward into the porous structure). Meanwhile, the intermediate layer acts as an effective barrier to restrict excessive penetration of the BAS glass into the porous Si3N4 matrix. This synergistic mechanism achieves two essential goals simultaneously: on one hand, it ensures that the coating can be sintered into a dense structure at relatively low temperatures, avoiding the risk of over-sintering that might damage the matrix; on the other hand, it realizes efficient sealing and moisture-proofing effects without significantly altering the porosity of the porous Si3N4 matrix—porosity that is critical for maintaining the matrix's inherent mechanical and wave-transparent properties.
A systematic and comprehensive investigation is conducted to explore the effect laws of three key experimental parameters on the overall performance of the coating. These parameters include BAS glasses at different melting temperatures (1 300 ℃ and 1 400 ℃), varying sintering temperatures, and the addition of 5wt.% ytterbium oxide (Yb2O3). The performance indicators evaluated in this work cover the microstructure and moisture-proof performance of the coating, bonding strength between the coating and the matrix, and thermal shock resistance.
The experimental results yield several significant findings. Firstly, the addition of 5wt.% Yb2O3 can significantly reduce the viscosity of the BAS glass melt. This reduction in viscosity enhances the fluidity of the glass during sintering, thereby promoting its lateral leveling and vertical infiltration. Consequently, the density of the coating is effectively improved, and the bonding strength between the coating and the matrix is notably enhanced. Secondly, among all the prepared coating samples, the one fabricated with BAS glass at a melting temperature of 1 300 °C and sintered at 1 390 ℃ exhibits the optimal comprehensive performance. This sample achieves excellent densification, a water resistance rate as high as 97.14%, and a bonding strength with the matrix reaching 12.4 MPa. Thirdly, even after undergoing 5 cycles of thermal shock testing at a temperature difference (ΔT) of 1 100 ℃, the coating still maintains a water resistance rate of 91.01%. This remarkable thermal shock resistance is primarily attributed to the crack self-healing mechanism of the coating during the thermal shock process. Two main factors contribute to this mechanism: on one hand, Si3N4 in the coating and matrix is oxidized to form silicon dioxide (SiO2), which can fill small cracks and block moisture channels; on the other hand, the creep behavior of the BAS glass generates compressive stress on the coating surface, inhibiting the propagation of existing cracks and preventing the formation of new ones. Additionally, after the thermal shock tests, a large number of cracks are observed in the porous Si3N4 matrix due to the matrix's relatively low thermal shock resistance. However, no delamination phenomenon between the coating and the matrix is detected. This observation further confirms that the coating and the matrix possess high bonding strength and strong resistance to thermal stress, enabling the coating to remain intact even when the matrix sustains damage.
This study not only provides a reliable technical approach for the long-term stable application of porous Si3N4 ceramics in harsh wave-transparent environments characterized by high temperature and high humidity but also lays a solid theoretical and experimental foundation for the low-temperature sintering of Si3N4/BaAl2Si2O8 composite materials.
关键词
封孔涂层 /
BAS玻璃 /
氮化硅 /
钡长石 /
低温烧结 /
结合强度 /
抗热震性
Key words
pore-sealing coating /
BAS glass /
silicon nitride /
celsian /
low-temperature sintering /
bonding strength /
thermal shock resistance
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基金
山西省重点研发计划项目（2022ZDYF027）; 晋中学院博士专项资金资助项目（23E00618）
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