Mechanically Robust Atomic Oxygen‐Resistant Coatings Capable of Autonomously Healing Damage in Low Earth Orbit Space Environment

Polymeric materials used in spacecraft require to be protected with an atomic oxygen (AO)‐resistant layer because AO can degrade these polymers when spacecraft serves in low earth orbit (LEO) environment. However, mechanical damage on AO‐resistant coatings can expose the underlying polymers to AO erosion, shortening their service life. In this study, the fabrication of durable AO‐resistant coatings that are capable of autonomously healing mechanical damage under LEO environment is presented. The self‐healing AO‐resistant coatings are comprised of 2‐ureido‐4[1H]‐pyrimidinone (UPy)‐functionalized polyhedral oligomeric silsesquioxane (POSS) (denoted as UPy‐POSS) that forms hydrogen‐bonded three‐dimensional supramolecular polymers. The UPy‐POSS supramolecular polymers can be conveniently deposited on polyimides by a hot pressing process. The UPy‐POSS polymeric coatings are mechanically robust, thermally stable, and transparent and have a strong adhesion toward polyimides to endure repeated bending/unbending treatments and thermal cycling. The UPy‐POSS polymeric coatings exhibit excellent AO attack resistance because of the formation of epidermal SiO₂ layer after AO exposure. Due to the reversibility of the quadruple hydrogen bonds between UPy motifs, the UPy‐POSS polymeric coatings can rapidly heal mechanical damage such as cracks at 80 °C or under LEO environment to restore their original AO‐resistant function.     

空间材料的原子氧侵蚀理论和预测模型

低地轨道环境中的原子氧对航天器材料的侵蚀导致材料的性能变坏甚至失效，原子氧的侵蚀机理和防护技术是当前空间环境效应研究的热点．在对原子氧效应机理已有理解的基础上，准确预测空间材料在低地轨道环境中由原子氧引起的侵蚀效应，可对设计者在工程选材和飞行器设计提供帮助。本文综述了近年来发展的原子氧与空间材料相互作用的理论模型和侵蚀速率预测模型，并对各种模型进行了分析，也指出了关于原子氧效应的研究重点．     

溶胶-凝胶法制备抗原子氧涂层及性质研究

采用有机无机复合的溶胶-凝胶方法,合成了用于聚合物表面保护层的前驱溶液.采用旋转涂覆方法,将前驱溶液涂覆于聚酰亚胺基片上,制成了表面涂层.将所制得的相应涂层分别进行原子氧暴露试验,其表面形貌的变化通过扫描电镜观测.结果表明,在原子氧辐照下,无保护的聚酰亚胺表面已非常粗糙,与辐照前的光滑表面形成显著的对比.而涂覆有保护层的表面,原子氧辐照前后却没有变化.这表明使用这种有机无机复合的溶液涂层具有很好的抗原子氧能力.     

Perhydropolysilazane derived silica coating protecting Kapton from atomic oxygen attack

By using surface sol-gel method with perhydropolysilazane (PHPS) as a precursor, a silica coating was prepared on a Kapton substrate as an atomic oxygen (AO) protective coating. The AO exposure tests were conducted in a ground-based simulator. It is found that the erosion yield of Kapton decreases by about three orders of magnitude after the superficial application of the coating. After AO exposure, the surface of the coating is smooth and uniform, no surface shrinkage induced cracks or undercutting erosion are observed. This is because that during AO exposure the PHPS is oxidized directly to form SiO₂ without through intermediate reaction processes, the surface shrinkage and cracking tendency are prohibited. Meanwhile, this PHPS derived silica coating also presents self-healing effect due to the oxidation of free Si. Compared with other kinds of silica or organic polymer coatings, this PHPS derived silica coating exhibits a superior AO erosion resistance.     

Investigation of Surface Reaction and Degradation Mechanism of Kapton during Atomic Oxygen Exposure

The erosion behavior of Kapton when exposed to atomic oxygen (AO) environment in the ground-based simulation facility was studied. The chemical and physical changes of sample surfaces after exposed to AO fluxes were investigated by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results indicated that Kapton underwent dramatically degradation, including much mass loss and change of surface morphologies; vacuum outgassing effect of Kapton was the key factor for initial mass loss in the course of atomic oxygen beam exposures. XPS analysis showed that the carbonyl group in Kapton reacted with oxygen atoms to generate CO2, then CO2 desorbed from Kapton surface. In addition, PMDA in the polyimide structure degraded due to the reaction with atomic oxygen of 5 eV.     

The stability of polymers in low earth orbit

Most organic polymers and some on polymeric materials such as silver or pigments suffer appreciable surface damage in the form of erosion or oxidation when exposed to the low earth orbit (LEO) environment encountered on shuttle flights. Surfaces facing the direction of flight suffer the most damage because of ram effects. Typical thickness losses for plastic films (eg. epoxy, Kapton, Mylar) can be 5 – 10μm. The aggressive chemical nature of the LEO environment (including atomic oxygen and O⁺ ions), aided by the high collision energies of the gases with forward facing surfaces, are thought to be responsible for the erosion and oxidation.     

Effects of atomic oxygen exposure on the tribological performance of ZrO2-reinforced polyimide nanocomposites for low earth orbit space applications

The effects of atomic oxygen exposure on pure polyimide and nano-ZrO₂ reinforced polyimide composites were investigated in a ground-based simulation facility. The experimental results indicated that the surface structure of both pure polyimide and ZrO₂/polyimide composites were destroyed by atomic oxygen attack, but the addition of nano-ZrO₂ particles in polyimide could obviously decrease the mass loss, which showed that ZrO₂ could enhance the atomic oxygen resistance. The results of ZrO₂/polyimide composites before and after atomic oxygen exposure showed that atomic oxygen irradiation aggravated the friction and wear of the ZrO₂/polyimide composites. The wear mechanism was mainly abrasive particles wear arising from the ZrO₂-rich layer on the surface of composites. The ZrO₂/polyimide composites with 1 wt% nano-ZrO₂ owns the lowest varying rate of the friction coefficient and wear rate before and after atomic oxygen exposure, which showed stable friction and wear properties and was expected to become a kind of potential tribological materials for practical spacecraft designation.     

Investigation on sol–gel boehmite-AlOOH films on Kapton and their erosion resistance to atomic oxygen

To improve the atomic oxygen (AO) erosion resistance of Kapton, boehmite-AlOOH films were deposited on it by sol–gel method and AO exposure experiments were performed in a ground-based AO simulator. The results indicate that the AlOOH-coated samples show an improved AO resistance and their erosion yield is one order of magnitude less than that of pristine Kapton. Furthermore, the AlOOH-coated Kapton remained optically stable under AO exposure. The AlOOH film structure before and after AO exposure was analyzed by scanning electronic microscope, atomic force microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. It was found that after AO exposure, the film structure tends to transfer from an octahedral coordination for AlOOH to an octahedral and tetrahedral mixed-coordination for γ-Al₂O₃. This implies that a more stable γ-Al₂O₃ structure could be formed in AlOOH film during AO exposure. The AO erosion mechanism of the coated Kapton was discussed.     

原子氧环境中聚酰亚胺的质量变化和侵蚀机制

用石英晶体微天平(QCM)原位监测并研究了聚酰亚胺薄膜在地面原子氧模拟装置中暴露时的质量变化.结果表明,聚酰亚胺薄膜在较低的原子氧束流通量暴露的初期,试样的质量先增加后降低,质量的降低与暴露的时间成正比.在高原子氧束流通量暴露的初期,试样质量的增加不明显,甚至一开始就发生稳态氧化失重.实验数据拟合的结果表明,原子氧对聚合物造成的侵蚀主要发生在有氧原子吸附的表面.质量的增加是由于较低的原子氧通量没有能完全氧化聚合物的表面.原子氧对聚合物材料的侵蚀机制服从Langmuir吸附理论.     

有机硅微胶囊-有机硅树脂复合涂层对空间 K apton的原子氧防护

将采用水相分离法制备的以明胶为囊壁、有机硅为囊芯的微胶囊与有机硅树脂乳液在一定条件下混合,在聚酰亚胺薄膜(Kapton)基材表面制备出含有机硅树脂的微胶囊-有机硅复合涂层,并将所制备的涂层分别进行原子氧暴露试验。结果表明,原子氧对Kapton侵蚀严重,质量损失严重,由原来光滑平整的表面变为凹凸不平的地毯状,太阳光吸收率变化值Δα为0. 272。微胶囊2有机硅复合涂层对Kapton基体优良的保护作用,使试样的质量损失和剥蚀率明显下降,且Δα仅为 0.071。尤其是微胶囊与有机硅质量比为1∶5的涂层,质量损失为Kapton试样的2.3%。     

O型圈的原子氧剥蚀效应实验研究

通过对O型圈的原子氧暴露实验研究 ,发现经过原子氧作用后 ,O型圈的表面粗糙度和质量损失率均随原子氧等效累积通量的增大而增大     

Protection of polymer from atomic-oxygen erosion using Al2O3 atomic layer deposition coatings

Thin films of Al₂O₃ grown using atomic layer deposition (ALD) techniques can protect polymers from erosion by oxygen atoms. To quantify this protection, polyimide substrates with the same chemical repeat unit as Kapton^® were applied to quartz crystal microbalance (QCM) sensors. Al₂O₃ ALD films with varying thicknesses were grown on the polyimide substrates. The ALD-coated polyimide materials were then exposed to a hyperthermal atomic-oxygen beam. The mass loss versus oxygen-atom exposure time was measured in situ by the QCM. Al₂O₃ ALD film thicknesses of ∼ 35 Å were found to protect the polymer from erosion.     

氨丙基倍半硅氧烷/有机硅杂化涂层抗原子氧侵蚀

为了提高航天器用有机硅涂层的抗原子氧侵蚀性能, 用氨丙基倍半硅氧烷交联固化环氧有机硅树脂, 在聚酰亚胺基材上制备出杂化涂层, 并对杂化涂层进行了地面原子氧暴露试验, 分析了试验前后杂化涂层表面形貌、 化学成分和化学结构的变化。结果表明, 氨丙基倍半硅氧烷阻止了有机硅涂层中微裂纹的产生, 避免了“淘蚀”现象, 材料质量损失明显下降。在原子氧暴露过程中, APOSS分子中的O和Si从低的结合能态慢慢向高的结合能态(氧化态)转变, 在表面生成了SiO₂保护层, 阻止了原子氧对底层材料的进一步侵蚀。      

Atomic oxygen resistant phosphorus-containing polyimides for LEO environment

In this study, a series of polyimides had been prepared from bis-(4-aminophenoxyl) phenoxyl phosphine oxide (p-DAPO₄) with corresponding dianhydrides via two-stage polycondensation method. The inherent viscosities of the polyamide acid were in range of 0.43–0.92 dL/g, and atomic oxygen (AO) exposure experiment was conducted in a ground-based atomic oxygen effects simulation facility with the filament charge and bound of magnetic field to determine the AO erosion-resistant properties of the polyimide films. Field emission scanning electron microscopy (FE-SEM), attenuated total-reflection Fourier transform infrared spectrometer and X-ray photoelectron spectrometer were employed to characterize the change on the surface of films after AO exposure, and the mass loss of some phosphorus-containing polyimide films reduced to about 20% that of Kapton^? HN film as AO fluence of 4.14 × 10²⁰ atoms/cm². The morphologies of heave residues of the polyimide films acting as a barrier to further erosion could be obtained from FE-SEM. The results indicated that a phosphate-type layer was left on the surface of phosphorus-containing polyimide films after AO exposure.     

Effect of low earth orbit atomic oxygen on spacecraft materials

This review attempts to bring together the published data and analysis related to the effect of low earth orbit (LEO) atomic oxygen (AO) interaction with spacecraft materials. The basic interaction mechanism of AO with spacecraft materials and quantification of its effect on materials performance are briefly discussed. After providing a list of materials susceptible to the LEO environment, the paper focuses on the degradation mechanism of various spacecraft materials. Particular emphasis is given to the protective mechanisms for AO-susceptible materials and development of AO-resistant materials for long-term LEO spacecraft applications. Ground-simulation testing requirements and their present status are reviewed briefly. The need for further research is emphasized.     

Effects of Vacuum Ultraviolet Radiation on Atomic Oxygen Erosion of Polysiloxane/SiO2 Hybrid Coatings

Polysiloxane/SiO2 hybrid coatings have been prepared on Kapton films by a sol-gel process.The erosion resistance of polysiloxane/SiO2(20 wt pct) coating was evaluated by exposure tests of vacuum ultraviolet radiation(VUV) and atomic oxygen beam(AO) in a ground-based simulation facility.The experimental results indicate that this coating exhibits better AO resistance than pure polysiloxane coating.The erosion yield(Ey) of the polysiloxane/SiO2(20 wt pct) hybrid coating is about 10-27 cm3/atom,being one or tw...     

激光源原子氧对Kapton薄膜和Kapton／Al薄膜二次表面镜性能的影响

在模拟空间环境原子氧暴露条件下,采用激光源原子氧对热控涂层材料Kapton薄膜、Kapton/Al薄膜二次表面镜进行了不同剂量的暴露试验。研究了这两种材料的质量损失、表面形貌随原子氧暴露剂量的变化关系,以及Kapton薄膜的光谱透过率、Kapton/Al薄膜二次表面镜的光谱反射率和太阳吸收比Δαs随原子氧暴露剂量的演化规律。结果表明:两种材料的质量损失随原子氧暴露剂量的增加呈线性增大;原子氧暴露后,试样表面呈"地毯"状形貌,且随暴露剂量的增加粗糙度变大;Kapton薄膜的光谱透过率、Kapton/Al薄膜二次表面镜的光谱反射率随原子氧暴露剂量的增加而降低,Kapton/Al薄膜二次表面镜的太阳吸收比Δαs随暴露剂量的增加而增大。最后对Kapton薄膜的存在寿命和Kapton/Al薄膜二次表面镜绝热平面的平衡温度进行了预测。     

(3-Glycidoxypropyl)-terminated silsesquioxane impact on nanomechanical properties of polyimide coatings exposed to atomic oxygen

Based on (3-glycidoxypropyl)silsesquioxane (GSSO) derived from the hydrolytic condensation of (3-glycidoxypropyl)trimethoxysilane (GPMS), GSSO-containing polyimide hybrid films were prepared using the sol-gel process and spin coating. Nanoindentation tests were carried out to study the influence of GPMS in the hybrid films on nanomechanical properties before and after exposure to atomic oxygen (AO) environments. The results show that hybrid films are three to four times harder than the usual plastic substrates. Compared with the unexposed AO samples, the elastic modulus (E) and the hardness (H) of the pristine Kapton AO-exposed samples significantly decreased, whereas the hardness of AO-exposed hybrid films increased slightly. This difference in nanomechanical properties is attributed to the chemical and the microstructural sample changes.     

原子氧防护涂层技术研究

原子氧是低地球轨道主要的残余气体粒子,其化学性质非常活泼,是极强的氧化剂,会对空间材料的性能产生严重的影响。介绍了低地球轨道的原子氧环境及原子氧防护方法;对有机、无机和复合原子氧防护涂层的优缺点、研究进展情况以及空间应用情况进行了详尽的阐述;最后分析了原子氧防护的发展方向。     

Effects of low earth orbit environments on atomic oxygen undercutting of spacecraft polymer films

Precise prediction on atomic oxygen undercutting by numerical simulation technique plays an importance role for long lifetime spacecraft design. A Monte Carlo mathematical model is presented to predict the undercutting process interaction between atomic oxygen and polyimide films of spacecraft in low earth orbital degree. In the meantime, the physical undercutting processes is described by tracing transportation particle approach on the basis of statistics. Simulated results showed that undercutting profiles with breaker patterns are in good agreement with flight experimental data for 43° and 28.5° orbit angle, as well as complicated effect factors are discussed in detail. Larger atomic oxygen fluence is favorable for producing more depth and width undercutting profiles and maximum depth is always larger than maximum width. Mass loss increased with the initial impact reaction probability increasing and decreased with the thermal assimilation coefficient going down. Especially for bigger orbit angle, the difference between depth and width are reduced due to the decrement of depth and the increment of width.