Plain bearing shells are used for both conrod bearings and main bearings. They are mostly designed as thinwalled composite bearing shells. Bearings made of two materials have a steel back onto which the bearing metal, typically aluminium with tin and copper additives, is plated. In the case of bearings made of three materials, the bearing metal is copper with additives of lead and tin cast onto the steel back or applied by the sinter-rolling-sinter method. A nickel dam (diffusion seal) separates the bearing metal and the galvanic sliding layer.