Site competition in surface segregation

Observations have been made, by Auger electron spectroscopy, of site competition between tin, sulphur and calcium co-segregating to the free surface of otherwise pure iron in the temperature range 600–800°C. The correlations between the segregation levels of the species are explained for sub-monolayer and multilayer tin adsorption, in terms of a simple model. In the absence of calcium the reduction in available adsorption sites for tin is a function of both the tin and sulphur coverages. With calcium present the formation of surface precipitates of calcium compounds is observed. Excellent agreement is obtained between the observed interrelated behaviour of the segregants and the predictions of the model, over a wide range of experimental conditions. A similar behaviour is expected, and must be allowed for, in all studies of segregation thermodynamics and kinetics of surfaces and interfaces exhibiting more than two components.