Sheath Dynamics During Plasma Immersion Implantation/Deposition And Anti-Atomic-Oxygen Erosion Effect Of Kapton

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.