Study of the structure and morphology of plasma-sprayed tin coating

The structure and the morphology of a tin (Sn) coating deposited on carbon steel St-37 with plasma spray are characterized with Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Conventional Transmission Electron Microscopy (CTEM) and microhardness measurements. The microhardness of the as-cast coating is of the same order of magnitude with the hardness of the Sn-based “white” alloys. Its porosity is very low, as the SEM observation of the cross-section showed, and as a result it is rather unlikely to affect its tribological properties. Furthermore the coating is mainly composed of pure Sn, while a tin oxide film is likely to cover its surface. Nevertheless the XRD revealed that the coating is stress-free and composed of tetragonal tin, while CTEM showed that interdiffusion of Fe and Sn takes place at the interface between the coating and the substrate and Fe–Sn phases are formed. This phenomenon ensures good adhesion of the coating to the underlying steel.