CHEMICAL AND MORPHOLOGICAL-CHANGES ON SILVER SURFACES PRODUCED BY MICROWAVE GENERATED ATOMIC OXYGEN

Silver surfaces, useful in space applications because of their high conductivity and reflectivity, can be corroded by the gases and ultraviolet radiation present in the earth's upper atmosphere. At shuttle altitudes (200-700 km), the atmosphere mainly consists of neutral atomic oxygen whose flux is aboUt 10(15) atoms/cm2 sec. On exposure to such a high flux, silver surfaces are strongly oxidized, and after prolonged exposure, the oxide layer formed is flaked off. A real-life time characterization of the oxidation of silver in space is not practical. To understand this process, it is necessary to study it under controlled laboratory conditions. In the present work, an attempt has been made to examine and understand the influence of atomic oxygen on the amount of erosion/corrosion of silver surfaces. A special emphasis has been given to studying the formation of oxides of silver, when exposed to atomic oxygen. Thin films and high purity foils of metallic silver were exposed in a microwave powered atomic oxygen generator for various powers and time intervals. X-ray photoelectron spectroscopy and x-ray diffraction results indicate the presence of only Ag2O in all samples. Scanning electron microscopy investigations confirm that the oxidation process starts as soon as silver comes under the bombardment of atomic oxygen. The oxide layer swells on formation, and becomes saturated when the oxygen concentration reaches a critical limit of 35 (+/-5) at. %. Stresses and strains are developed due to volume expansion of the oxide layer, causing it to crack and spall away. This brings a fresh surface in contact with atomic oxygen and on further exposure, the process repeats itself.