Atomic oxygen induced degradation of MGF2 anti-reflective coatings

Spacecraft orbiting below about 1200 km are exposed to a high flux of atomic oxygen (AO) that can severely degrade surface materials. An Electron Cyclotron Resonance (ECR) based system was used to simulate LEO conditions. The ECR source was characterized by Kapton dosimetry and in-situ four point probe silver film resistively measurements. The VUV output was measured by a calibrated VUV spectrometer. Ceria (CeO2) doped quartz (CMG) were used to compare and contrast the degradation of a thin MgF2 AR coating on CMG. The effect of temperature and VUV radiation in enhancing/reducing the degradation was also studied. Unexpectedly severe degradation in the form of optical transmission loss was observed upon exposure to high AO fluences with and without VUV exposure. We will show that oxygen penetrates the MgF2 films to depths in excess of 20nm and there is clear oxygen bonding to the structure.