Space environment exposure effects on ceramic coating for thermal protection systems

Carbon/carbon (C/C) structures are widely considered for space thermal protection systems (TPSs), being able to withstand critical reentry conditions thanks to an excellent thermal stability. In low-Earth-orbit long-time missions, however, the TPS effectiveness is significantly lowered due to the surfaces' prolonged staying within the harsh space environment. In particular, the detrimental oxidation due to thermal cycles and atomic oxygen (AtOx) exposure greatly affects the TPS integrity, thus leaving the spacecraft's outer surface less protected during reentry. A great effort is thus made for testing solutions based on advanced coatings aimed at preserving TPS materials from oxidation. In the present work, ceramic coatings are evaluated by measurements of the thermal expansion coefficient and AtOx erosion rate. The specimens tested are C/C substrates on which commercial refractory varnishes are applied. Silicon carbide and aluminum and zirconium oxides are the basic ceramic constituents, and the effect of silica nanoparticles' inclusion on the coating performances is also evaluated. A phenomenological modeling is then introduced to approach the relationship between erosion mechanism due to AtOx impact and surface energy variation induced by thermal cycles; such analysis highlights the importance of considering the combined action of different aging factors, in order to achieve a reliable interpretation of the space environment's effect on spacecraft structures and subsystems.