Formation of Co–Si intermetallics in bulk diffusion couples. Part I. Growth kinetics and mobilities of species in the silicide phases

Diffusion couples, in which one single-phased layer of Co-silicide is growing from its saturated adjacent phases, were employed to study diffusion properties of the Co–Si intermetallics over the temperature range 914–1217°C. The position of the Kirkendall marker plane inside the reaction zones revealed that in this temperature interval Co is by far the fastest diffusing element in the Co2Si-intermetallic, the intrinsic diffusivities of the components are practically equal in the cobalt disilicide and that Si is virtually the only mobile species in the monosilicide CoSi. The concept of integrated diffusion coefficient is used to describe the growth kinetics of the intermetallic compounds. The integrated diffusion coefficient in an intermetallic is related to the tracer diffusivities of the components and the thermodynamic stability of the phases involved in the interaction. The tracer diffusion coefficients of the elements in the CoSi2- and CoSi-phases were obtained at various temperatures in the temperature range studied. The results are consistent with the customary Arrhenius relationship.