Gas adsorption on thin films of chromium studied by internal stress measurements

The effect of O 2, H 2O, CO, H 2, N 2 and CH 4 adsorption on the internal stress of evaporated chromium films was investigated under high vacuum conditions. While the stress in chromium films is not altered during exposure to H 2, CH 4 and N 2 distinct stress changes are found when chromium films are exposed to H 2O, CO and O 2. The adsorption of H 2O and CO on the metal film gives rise to a compressive stress, because the surface tension of the metal film is lowered owing to saturation of free surface valences at the film and the grain boundary surfaces. A different behaviour is observed for the O 2Cr interaction. On exposure of the film to O 2 a slight compressive stress is observed, which on further exposure changes to a tensile stress. While the compressive stress is again believed to be due to the formation of an adsorption layer, the subsequent tensile stress is interpreted as indicating the formation of a protective layer of bulk chromium oxide. The thickness of this protective layer depends on the O 2 exposure pressure. The chromium oxide layer is impermeable to O 2 and CO, but not to H 2O.