WATER-INDUCED DEGRADATION OF CHROMIUM FLUORIDE FILMS

As an optical film, chromium fluoride is an important material because it has good transparency in short wavelength region. However, it was found that chromium fluoride film degrades at humid condition. Therefo re, we investigated the degradation phenomena of chromium fluoride fil ms under 100% humid condition. The degraded samples were analyzed by s everal thin film analyzing techniques. With these results, we proposed a possible degradation mechanism consisting of multi-step reactions. First, H2O molecules are permitted to the porous structure in the film s to percolate through numerous microchannels and microvoids. Then, th e water molecules react with the chromium fluoride film to make hydrox ide bonding producing HF gas. HF gas evolves and gathers at some spots , and dome structure was developed. Chromium oxide precipitates around HF gas gathered dome. After time passes, HF gas evolution occurs foll owed by explosion of the dome structure, and crater-like feature remai ns as a result around the dome. It was found that the chromium oxide r ing is left around the feature. In the whole process, cracking also oc curred due to the molecular volume difference between CrFx and degrade d part. (C) 1998 Elsevier Science S.A. All rights reserved.