论文总字数：41132字

摘 要

随着航空航天技术的迅速发展，可重复使用空天往返飞行器（RLV）的飞行速度和在轨时间不断提升，这使得飞行器的服役环境更加恶劣，对其表面热防护系统提出了更高的要求。本文针对飞行器表面可重复使用热防护系统的应用需求，开展了内部为碳纤维增强碳基气凝胶隔热基体材料、表面为MoSi₂基高发射率抗氧化涂层的防隔热一体化热防护材料的制备与性能研究。

采用前驱体转化法，通过溶胶-凝胶法以及超临界干燥技术得到碳纤维增强的有机气凝胶前驱体，并先后在800 °C和1550 °C下进行热处理，得到碳纤维增强的碳基气凝胶基体材料。采用高温熔融结合水冷急淬的方法制备了铝硼硅酸盐玻璃，并以此为粘结剂，以MoSi₂为辐射剂，以SiB₆为烧结助剂，采用料浆刷涂与固相包埋快速热处理相结合的工艺在基体材料表面制备了耐高温、高发射率抗氧化涂层。为了缓解涂层与基体之间热膨胀系数的不匹配的问题，同时增强涂层与基体的结合强度，通过梯度化结构设计在高MoSi₂含量的外表面涂层与基体材料之间引入了低MoSi₂含量的过渡涂层。

气凝胶前驱体在热处理过程中生成的SiC相主要以β-SiC形式存在，残余的碳在高温下转变为石墨。所制备的铝硼硅酸盐玻璃具有良好的耐温性能，从室温至1200 °C的升温过程中失重率不足5%。铝硼硅酸盐玻璃中桥氧键的含量达93%，为高温下形成高粘度熔融玻璃层提供了保证。所制备涂层表面除了原始组分外，还存在少量氧化产物。热处理温度提高，涂层表面物相组成不变，只是各物相的含量发生变化。所制备的MoSi₂基涂层具有良好的热辐射性能，在300~2500 nm波长范围内的总发射率均在0.8以上，在1270~1967 nm波长范围内的总发射率均在0.84以上。其中在1200 °C下制备的MoSi₂含量为30 wt%的涂层发射率最高，在1270~1967 nm波长范围内的总发射率可达0.9017。随着涂层热处理温度的提高，涂层的发射率逐渐下降。通过“V型凹槽模型”阐明了涂层的发射率和表面粗糙度之间的关系，表面粗糙度增加，促进了涂层对电磁波的吸收，使得涂层吸收率提高，发射率也随之提高。在1200 °C下分别对不同组分、不同热处理温度涂层的静态抗氧化性能进行了测试。氧化后涂层表面的玻璃相含量提高，表面物相主要由SiO₂、MoO₂和Mo等组成。高温下高粘度的铝硼硅酸盐玻璃以及MoSi₂、SiB₆氧化生成的SiO₂保护层能够很好得起到阻隔氧气向基体内部扩散的作用。在所有组分中，在1200 °C下制备的MoSi₂含量为20 wt%的涂层的抗氧化性能最差，经氧化后涂层表面出现明显的孔洞。在1400 °C下制备的MoSi₂含量为40 wt%的涂层的抗氧化性能最佳。这主要得益于合适的涂层组分配比使其兼具高的发射率和适宜的粘度的玻璃相。

关键词：碳基气凝胶 热防护 涂层 高发射率 抗氧化

Preparation and properties of MoSi₂-based coatings on carbon fiber reinforced carbon-based aerogel

Abstract

With the rapid development of aerospace technology, the high speed and on-orbit time of reusable re-entry launch vehicles (RLVs) are constantly improved, making the service environment of vehicles more severe, which puts forward higher requirements for its surface thermal protection system. In this paper, the preparation and performance of the integrated thermal protection material with carbon fiber reinforced carbon-based aerogel matrix material and MoSi₂-based high emissivity anti-oxidation coatings were focused to meet the demand of reusable thermal protection system on the surface of vehicle.

The carbon fiber reinforced organic aerogel precursor was obtained by sol-gel method and supercritical drying technology. And then, two-step heat treatments were conducted at 800 °C and 1550 °C successively to obtain carbon fiber reinforced carbon-based aerogel. The alumina-borosilicate glass was prepared by water quenching method. The anti-oxidant coatings with high temperature resistance and high emissivity were prepared on the surface of carbon fiber reinforced carbon-based aerogel by slurry brushing combined with embedding sintering process. The coating system uses MoSi₂ as emittance agent, alumina-borosilicate glass as binder, SiB₆ as sintering additive. In order to achieve matched thermal expansion coefficient and more stable bonding interface between the coating and the matrix material, the transition coating with low MoSi₂ content was introduced between the outer surface coating with high MoSi₂ content and the matrix material.

The SiC phase generated during heat treatment mainly exists in the form of β-SiC, and the residual carbon was transformed into graphite at high temperature. The as-prepared alumina-borosilicate glass has good temperature resistance, whose weight loss was less than 5% during the heating process from room temperature to 1200 °C. The content of bridging oxygen bond in alumina-borosilicate glass reaches 93%, which guarantees the formation of high viscosity molten glass layer at high temperature. Besides the original composition, there are a few of oxidation products on the surface of the as-prepared coatings. With the increase of heat-treatment temperature, the phase composition of coating surface remains unchanged, but the content of each phase changes. The as-prepared MoSi₂-based coatings have good thermal radiation properties, whose total emissivity are more than 0.8 in the wavelength range of 300~2500 nm and more than 0.84 in the wavelength range of 1270~1967 nm. Among them, the coating prepared at 1200 °C with 30 wt% MoSi₂ content has the highest emissivity, which is as high as 0.9017 in the wavelength range of 1270~1967 nm. With the increase of heat-treatment temperature, the emissivity of coatings decreases gradually. The “V-groove model” was used to clarify the relationship between the emissivity and surface roughness of the coatings. The increase of surface roughness promotes the absorption of electromagnetic wave, which increases the emissivity of coatings. The static oxidation resistance of coatings with different composition and preparation temperature was tested at 1200 °C, respectively. After oxidation, the glass phase content on the surface of coatings is increased, and the surface phase is mainly composed of SiO₂, MoO₂ and Mo. The high viscosity alumina-borosilicate glass and SiO₂ protective layer generated by the oxidation of MoSi₂ and SiB₆ can effectively prevent the diffusion of oxygen into the matrix. Among all the coatings, the coating prepared at 1200 °C with 20 wt% MoSi₂ content shows the worst anti-oxidant performance, and an obvious hole is observed on the surface of the coating after oxidation. The coating prepared at 1400 °C with 40 wt% MoSi₂ content shows the best anti-oxidant performance, which mainly due to the suitable coating composition makes the glass phase have both high emissivity and suitable viscosity.

Key Words: Carbon-based aerogel; Thermal protection; Coating; High emissivity; Oxidation resistance

目 录

摘要 I

ABSTRACT III

第一章 绪论 1

1.1 引言 1

1.2 耐高温低热导率气凝胶材料 1

1.2.1 气凝胶材料的隔热机理 2

1.2.2 氧化物气凝胶材料 2

1.2.3 碳/碳化物气凝胶材料 3

1.3 高发射率涂层 5

1.3.1 过渡金属基高发射率涂层 5

1.3.2 稀土掺杂高发射率涂层 5

1.3.3 难熔金属硅化物基高发射率涂层 6

1.4 研究目的与内容 7

1.4.1 研究目的 7

1.4.2 研究内容 8

第二章 实验材料及方法 9

2.1 实验原料及设备 9

2.1.1 实验原料 9

2.1.2 实验设备 9

2.2 实验方法 10

2.2.1 碳纤维增强C-SiC-B₄C气凝胶的制备 10

2.2.2 铝硼硅酸盐玻璃粘结剂的制备 11

2.2.3 MoSi₂基抗氧化涂层的制备 12

2.3 测试表征方法 13

2.3.1 FT-IR光谱分析 13

2.3.2 热重-差热分析 13

2.3.3 孔结构分析 13

2.3.4 X射线衍射分析 14

2.3.5 X射线光电子能谱分析 14

2.3.6 微观形貌分析 14

2.3.7 气凝胶基体材料的热导率测试 14

2.3.8 涂层的发射率测试 14

2.3.9 涂层的恒温静态抗氧化性能测试 15

第三章 碳纤维增强C-SiC-B₄C气凝胶表面MoSi₂基涂层研究 16

3.1 气凝胶基体材料的组成结构表征 16

3.1.1 气凝胶的X射线衍射分析 16

3.1.2 气凝胶的红外光谱分析 17

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