The microstructure and precipitation behaviour of superalloys with thermally insulating coating systems

The Ni-base solid solution- and carbide hardening alloy NiCr22Co12Mo9 is us ed preferentially for cladding the combustion chambers of stationary gas- a nd aircraft turbines. In order to increase combustion temperatures, the com bustion chamber is protected against excessive temperatures by the applicat ion of a multi-layered coating consisting of a thermally insulating ceramic layer and a corrosion resistant layer. By considering the mechanical behav iour separately from the corrosion behaviour, it becomes clear that this ty pe of coating detrimentally affects the creep properties of the substrate m etal. The purpose of the investigation was to determine the microstructural cause of this behaviour. The deterioration in the creep properties of the alloy NiCr22Co12Mo9 after coating was found to be caused by the degradation of th e M23C6-carbide precipitate in the substrate material and by a weakening in the mechanisms of Solid solution hardening and precipitation hardening tak ing place at the interface, on which the creep strength of the material rel ies.