STUDY OF THE PHASES FORMED DURING THE GROWTH OF THICK COATINGS WITH COARSE GRAINED TIN

The microstructural and morphological properties of the as-prepared tin (Sn) coating on ferrous substrates affects the lifetime as well as the applicability of the coating itself. In order to improve our knowledge of this system, strips of steel St-37 were coated with Sn in the form of gas-atomized powder, with an average diameter of 110±10 μm, using plasma spraying. The plasma gas was an Ar-He mixture and the current intensity was 650 A. The as-casted coating was characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD) and conventional transmission electron microscopy (CTEM). It was deduced that the coating is characterized by high integrity, although certain pores were detected far from the Fe-Sn interface. However they refer to less than 1% of the total volume. Nevertheless the examination with SEM and XRD proved that the coating is mainly composed of pure Sn. However, interdiffusion takes place between the two phases (Fe-Sn) at their interface as deduced by CTEM. As a result, certain Fe-Sn phases are formed, which seem to be scattered in the coating close to the interface in the form of small inclusions.