Simulation of Atomic oxygen erosion of polymer materials: comparison between at ground experimental results and the spherical thermal spike model

摘要

The spacecraft surfaces in Low Earth Orbit (LEO) undergo atmosphere interactions that produce a variety of effects, which aren't normally significant for short duration missions (several months) but represent a great threat for vehicles designed for a long term missions (several years). At an altitude between 200 and 700 km - a region where the Space Shuttle, the International Space Station and many other satellites orbit - the neutral atmosphere consists of O_2, N_2, Ar, He, H and atomic oxygen (AO), which is the predominant constituent (~ 80%). The density of AO is ~ 2 × 10~7 and ~ 1.5 × 10~9 atoms/cm~3 at about 300 km, for minimum and maximum solar activity, respectively. AO flux is almost 10~(12) (600 km) to 10~（15） (200 km) atoms/ cm~2 s The average thermal velocity of the gas molecules at LEO altitudes is ~ 0.4 km/s and the collision energy produced by their impact with spacecraft surfaces is very low to start any surfaces reaction. However, when front surfaces (ram direction) of spacecrafts is orbiting in LEO at a velocity of about 7-8 km/s, the impingement kinetic energy between the spacecraft surfaces and oxygen atoms is approximatively 4.5-5 eV. Polymeric films, such as Kapton, Mylar and polyuretane undergo drastic degradation by atomic oxygen in LEO. In the present work the results obtained by the atomic oxygen erosion of polyimide samples in a plasma-type ground-based simulation facility are reported. The atomic oxygen simulation tests were done using an end-Hall ion source. The vacuum chamber in which was mounted both the ion source and the sample holder dome, was evacuated by a cryogenic pump up to 2.4-10~(-5) Pa. The samples bombarded by the oxygen were a sheets of Polyimide Kapton HN with a size of 5 cm × 5 cm and thick 50 μm. The erosion depth was measured by a surface profilometer and the morphology of the samples were observed by a electron secondary emission Scanning Electron Mycroscopy. The physical processes of the oxygen collision with the Kapton surface were simulated by the SRIM (The Stopping and Range of Ions in Matter) Montecarlo code. Presuming that most part of the energy transferred from the incident oxygen to the polymer was released by phonons (heat), the erosion phenomena was explained in terms of the Spherical Thermal Spike. A comparison between the experimental erosion depth and those calculated by the model will be performed.