A study of atomic oxygen interactions with protected silver surfaces

The durability of protected silver surfaces, proposed for space applications, in an atomic oxygen (ATOX) environment was evaluated. The protective coating consists of a thin aluminium oxide layer. Two types of ATOX simulation systems were used: (i) an RF oxygen plasma, and (ii) a laser detonation source that produces 5 eV ATOX. The effects of ATOX were studied by optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), combined with depth profile measurements. Total spectral reflectance was measured before and after ATOX exposure to evaluate the changes in optical properties. Severe degradation (visible even by the unaided eye) of the protective coating and the underlying silver layer resulted from exposure to the RF plasma environment at an ATOX fluence of about 1x10(19) atoms/cm(2). Samples, exposed to the 5 eV ATOX beam source demonstrated good stability and negligible changes in total reflectance after exposure to an ATOX flux of 4x10(20) atoms/cm(2). The enhanced erosion of the coating in the RF oxygen plasma may be explained by the presence of energetic ions, as well as by other reactive species, such as fluorine-containing contamination originating from the pump oil. It is suggested that the RF plasma environment is too severe for realistic simulation of the ATOX interaction with the protected silver surfaces. Based on the 5 eV ATOX exposure results, the protective coating was found to be suitable for potential LEO applications.