The application of surface science to fatigue: The role of surface chemistry and surface modification in fatigue crack initiation in silver single crystals

The role of surface chemistry in the initiation of fatigue cracks was studied for silver single crystals. Fatigue tests up to crack initiation were carried out on pure silver specimens and specimens surface-alloyed with gold. The ambient used was oxygen. Concurrent investigations of the effect of gold on oxygen adsorption on Ag(111) surfaces were carried out using X-ray photoelectron spectroscopy. A strong correlation was observed between the number of cycles to crack initiation and the reactivity of slip steps exposed by the fatigue process as well as the rate of oxygen adsorptio on the exposed slip steps. It was also found that oxygen adsorbed on the slip steps was transported into subsurface regions during fatigue. When the same test was performed in bromine, subsurface transport of bromine was not observed; yet the fatigue life up to crack initiation was found to be the same as that in oxygen. The absence of bromine transport is likely due to its larger atomic size. Based on these studies, we conclude that the dominant mechanism for accelerating fatigue crack initiation in an active environment is the reduction of dislocation reversibility by strong adsorption of ambient species on exposed slip steps.