Predicting the microstructural evolution in a multi-layered corrosion resistant coating on a Ni-base superalloy

Protective metallic MCrAlY or diffusion type (NiAl) coatings enhance the oxidation and corrosion resistance of the underlying high temperature materials employed in aeroengines and industrial gas turbines by ensuring the growth of a slowly growing protective alumina scale. However, a chromia forming coating would provide a better resistance against sulphur induced corrosive attack. A hybrid coating system combining both chromia and alumina forming coating layers would provide optimum protection in oxidising-sulphidising environments. The microstructural stability and applicability of such a coating system (SmartCoat) containing alternate layers rich in chromium and aluminium respectively on the Ni-base superalloy CMSX-4 was evaluated after various exposure times at 800 C. Scanning electron microscopy (SEM) and electron microprobe analyses (EPMA) provided the element concentrations. Phases were identified by electron backscatter diffraction, and correlated with SEM and high-resolution TEM/EDX analyses. A computational approach was employed to describe the mechanisms of the phase transformations occurring in the coating system