Synergistic effects of atomic oxygen and UV radiation on carbon/carbon plates at different attitude positions

Atomic oxygen (AtOx) is a major component of the space environment between 200 and 800 km (LEO-low Earth orbit region) and is the principal source of erosion for exposed aerospace structures. The damage to surface materials is proportional to the AtOx fluence, which depends on altitude, exposure time, orbital inclination, and solar activity, and it is caused by the formation of volatile oxides which do not adhere to the surface; furthermore, the mass loss may also be worsened by UV radiation, which increases the chemical degradation of the exposed material. Carbon/carbon (C/C) is an advanced ceramic composite that is frequently found as a base component of thermal protection systems (TPS), rocket nozzles, or other spacecraft subsystems. In this work, a simulation of the AtOx/UV synergistic effects on C/C plates exposed at different attitude positions were carried out by experimental tests performed at the Aerospace Systems Laboratory (LSA-Sapienza University of Rome) by means of an Atomic Oxygen OS-Prey RF plasma source, which also included a high-power UV-ray generator. The present experimental plan was built on the activity developed during recent years at LSA concerning the study of C/C materials for protecting aerospace structures from thermal shock in re-entry missions. The tests were conceived by considering a fixed time of exposure with a base fluence of 7.6 x 1019 n.s./cm2, as evaluated from the erosion of the reference samples exposed to AtOx flux at a normal incidence; the simulation of the different attitude positions was then analyzed, also considering the simultaneous effect of UV radiation. The results of the aging ground test suggest the following: (i) C/C oxidation in LEO must be taken into full consideration in the TPS design with reference to protective coating solutions, (ii) the LEO environment simulation is closely related to AtOx/UV combined irradiation, as well as to the spacecraft's in-orbit attitude.