【作者】 黄永宪；

【导师】 杨士勤； 田修波；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 博士

【摘要】 距地球表面200~700km的低地球轨道是航天飞机、空间站以及一些卫星的主要运行轨道。原子氧是低地球轨道氧化能力强,对航天器影响最为严重的因素之一,它能使航天器零部件性能衰减而导致服役失败,尤其是应用于热控涂层、太阳能电池帆板中的聚合物及其复合材料对原子氧特别敏感。本文针对如何提高聚合物Kapton表面抗原子氧侵蚀的性能,从性能评价设备的研制、聚合物表面等离子体注入鞘层动力学、防护涂层制备和原子氧侵蚀机理等方面展开研究,以提高卫星、空间站等航天器的在轨服役寿命。基于加速暴露试验的思路和电感耦合射频放电技术,研制了原子氧辐照地面模拟器。该模拟器可获得纯净的高密度氧等离子体,进行长时间持续辐照,且等效原子氧束流密度可连续调节。当射频功率为400W时,获得高达1016 atom/cm2·s的等效原子氧束流密度,相对低地球轨道的原子氧束流密度其加速暴露能力约为100倍。聚合物Kapton材料的质量损失随辐照时间的增加呈线性增加的趋势,其侵蚀表面的凹坑密度呈下降趋势。基于NASA长期飞行试验数据及其对原子氧与Kapton材料作用提出的假设,将原子氧侵蚀视为粒子输运过程,采用Monte Carlo模型,利用Visual C++编程进行了数值仿真,研究了原子氧环境及掺杂对侵蚀效应的影响和裂纹间侵蚀过程中的交互作用。低能量原子氧模拟器辐照所产生侵蚀腔体的深度较小而宽度较大。离子注入掺杂可以有效地提高Kapton表面抗原子氧侵蚀的性能。为进一步提高改性层的抗原子氧侵蚀性能,需要采用等离子体注入/沉积或复合膜层方法降低改性层中有机成分或碳的含量,同时提高膜层的致密度,降低膜层与原子氧反应的系数。裂纹所产生的原子氧侵蚀效应是巨大的,特别是膜层表面存在大量裂纹时,由于裂纹间原子氧侵蚀的交互作用,裂纹的危害性更强。当ANI发生转变时,原子氧侵蚀腔体底部出现“山丘”形貌,其倾斜度随原子氧攻角的增大而增大。相邻宽度大小不等的裂纹间原子氧侵蚀过程中存在交互作用,出现具有台阶的侵蚀腔体。由于聚合物导电性能较差以及表面电荷积聚所产生的电容效应致使其表面电位衰减,采用等离子体浸没离子注入对其表面改性,是非常困难的。利用等离子体动力学粒子模型,实时跟踪离子在等离子体鞘层中的运动形态及特性并进行统计分析。研究表明对于较薄的聚合物膜,电容效应影响较小,离子注入能量较高。而对于厚大聚合物材料,为消除电荷积聚、打火及施加偏压难的问题,栅网辅助等离子体浸没离子注入是非常理想的方法。研究了辅助栅网对二次电子发射的抑制作用和离子注入动力学行为。离子注入聚合物表面,其表面电位降低,在栅网与聚合物表面形成电场并抑制二次电子的发射。栅网对二次电子发射的抑制能力,随栅网间隙率的减小和高度的增加而增强。通过栅网上方鞘层内电场的加速作用,离子由栅网间隙穿过并以较高的能量注入聚合物表面。辅助栅网产生的阴影效应致使离子注入剂量不均,阴影效应可通过缩短脉宽,适当增大栅网的高度和间隙率来减弱。高能离子的注入在膜基界面处形成混合层,有利于提高膜基结合力和抵抗裂纹萌生和扩展的能力,但太高的注入能量使聚合物基体的光学透过率降低,吸收系数升高。射频辅助成膜有利于提高膜层的膜基结合力和改善光学性能。选择10kV负脉冲偏压和辅助射频功率200W的参数,通过铝/硅等离子体的注入/沉积制备了氧化铝/氧化硅复合涂层。该复合涂层具有良好的膜基结合力和抗裂纹萌生和扩展的能力,氧化硅膜的增透作用和硅等离子体的再注入作用Al2O3·SiO2新相形成,改善了氧化铝涂层的光学性能。原子氧辐照结果表明,复合涂层防护试样的质量损失不及Kapton基体质量损失的1/20,且光学性能稳定、表面仍较平整,表明等离子体注入/沉积技术可在热控涂层材料Kapton表面制备抗原子氧侵蚀性能优越的防护涂层。膜基界面的状态对膜基结合力尤为重要。采用X射线光电子能谱对原子氧侵蚀前后的氧化铝/氧化硅复合涂层进行了逐层剖析,重点研究了氧化硅与氧化铝和氧化铝与聚合物过渡区元素的结合状态。在氧化铝与聚合物和氧化硅与氧化铝交界面处分别形成了Al-O-C和Si-O-Al键,提高了膜基与膜层间的结合力。复合涂层中有机成分夹杂与原子氧反应生成易挥发的物质,形成缺陷为原子氧的侵蚀提供了条件。基于复合涂层原子氧的侵蚀行为,分析了原子氧侵蚀过程,主要包括原子氧与聚合物基体和有机成分夹杂及涂层的往复碰撞、原子氧吸附和向聚合物基体表面的扩散、原子氧与聚合物中的元素反应生成气体小分子并在聚合物表面下积聚形成气泡、气泡破裂气体溢出发生质量损失并反作用于防护涂层的复杂作用过程。更多还原

【Abstract】 In low Earth orbit (LEO), between 200 and 700km altitude, where space shuttles and International Space Station (ISS) fly, atomic oxygen (AO) is the dominant atmospheric constituent. AO is highly reactive and erodes many materials commonly used in spacecrafts. This may degrade the performance of spacecraft components, leading to service failure. Some polymeric materials such as those used in thermal control blankets, solar arrays, and lightweight composite structures are particularly susceptible to AO. In the thesis, some topics on the fabrication of AO ground simulation apparatus, simulation of AO erosion of protected Kapton, enhancement anti-AO erosion of Kapton, sheath dynamics during plasma immersion implantation, performance evaluation, fabrication and interaction with AO of protective coating are discussed. These serve for improving service lifetime of satellites, ISS and other space shuttle.A radio frequency inductively-coupled plasma (RF-ICP) apparatus has been developed to simulate the AO environment encountered in LEO. Owing to the novel design, the apparatus can achieve stable, long lasting operation, pure and high density oxygen plasma beam. Furthermore, the effective AO flux can be regulated with large scale. The equivalent effective AO flux can reach 1016 atom/cm2·s at RF power of 400 W. The equivalent AO is about 100 times than that in the LEO environment. The mass loss measured from the Kapton sample changes linearly with the exposure time while the density of the eroded holes becomes smaller. The erosion mechanism of the polymeric materials by AO is complex and involves initial reactions at the gas-surface interface as well as steady-state materials removal.The results of LDEF and the hypothesis of erosion model proposed by NASA are utilized. The program is developed with Monte Carlo model and compiled by VC++ to simulate the interaction of protected Kapton with AO. The effect of different AO environment, improvement of erosion resistance of Kapton by ion implantation, and interaction between cracks were focused on during AO erosion. Compare with the space erosion result, the erosion cavity with shallower depth and wider width was obtained during low-energy AO exposure. The ion implantation technique shows excellent promivement for the protection of Kapton for space applications. To reduce the composition of organic compound and carbon in the film, some methods are to use plasma implantation/deposition or multiple-laryer films. These can reduce the AO reaction probability of modified Kapton, and provide excellent protection from erosion by AO. Crack is harmful for resistant to AO attack. When some cracks take place interaction during exposing to AO, the harm of cracks is more serious. The bottom of erosion cavity shows“massif-like”shape, and the gradient grows with the increase of ANI. The numerical simulation results can provide a useful guide to develop new protective coatings for aerospace application.Plasma immersion ion implantation (PIII) of insulating materials is inherently difficult because the voltage across the sheath is reduced by the voltage drop across the insulator due to dielectric capacitance and charge accumulating on the insulator surface during the pulse. The spatio-temporal evolution of plasma sheath, energy and dose of ion implantation has been simulated by particle-in-cell (PIC) modeling. Statistical results can be achieved through scouting each ion in the plasma sheath. For the thinner polymer, the dielectric capacitance could be neglected, and the incident energy of ion is high. But treating thick insulating parts, mesh-assisted plasma immersion ion implantation is a promising technique for overcoming charging, arcing and the difficulty of controlling the surface potential of the target. The internal electric field in the cage automatically builds up once plasma implantation starts. This electric field substantially suppresses the emission of secondary electrons from the insulator surface. With decreasing the ratio of mesh space and increasing the height of mesh, the capability of the secondary electron suppression was enhanced. Ions were accelerated toward the mesh passing through the apertures and implanted into the insulating materials with high energy. But the mesh casts a shadow on the sample surface leading to potential non-uniformity dose. And the shadow effect can be weakened by shortening the pulse, increasing the height and space ratio of mesh.Alumina film has been fabricated on Kapton substrate by aluminum plasma ion implantation and deposition in an oxidizing ambient and the effects of the bias voltage, thickness and power of assistant RF on the film properties are investigated. Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS) reveal successful deposition of alumina films on the polymer surface. Our results indicate that higher bombardment energy may lead to higher crack resistance and better film adhesion by inducing more extensive ion mixing and recoil implantation. However, a higher bias degrades the optical properties of the films as indicated by the higher absorbance. The crack onset strain, the adhesion, the cohesion, and the optical transmittance are increased with RF-assisted plasma ion implantation and deposition due to an enhanced ion bombardment, thus forming network densification. And multiple-layer alumina/silica films were fabricated by implantation and deposition with Al/Si plasmas using metal vapor vacuum arc source with -10kV bias and 200W assistant RF power. The hybrid processes can fabricate thicker protective film with higher crack resistance and better film adhesion. The multiple-layer films enhanced the optical transmittance compared to the single alumina layer. This may be attributed to enhanced-transmittance ofsilica and formation of the new material Al2O3·SiO2. The properties of anti-AO erosion were evaluated in ground-based facility. The multiple-layer alumina/silica films demonstrated slight attenuation of reflectance and twentieth mass loss. The slight change of solar specular reflectance, surface morphology and little mass loss of samples with multiple-layer films show that the techgnique is an effective method to protect Kapton material which was applied to thermal control system of spacecrafts.Since the interface greatly influenced the adhesion, XPS is employed in a study of interaction and AO erosion mechanisms of silica/alumina films on Kapton. The results revealed that bonding between the ceramic and the polymer occurred primarily via Al-O-C bonds, and Si-O-Al bonds were formed in the interface between silica and alumina coatings. These interfacial bonds play an important role in the enhancement of adhesion. The significant carbon content in the multiple-layer films can be explained by the presence of organic component, and reacts with AO to form volatile fragments. The reaction of organic carbon is the main route for AO to be introduced into the multilayer film. The erosion process of multiple-layer alumina/silica films induced by AO involving the collision, diffusion, reaction, gas release, and retroaction to protective films is proposed.更多还原