Oxide-Assisted Degradation of Ni-Base Single Crystals During Cyclic Loading: the Role of Coatings

The role of oxidation‐induced layers in the failure process of aluminide‐coated nickel base single crystals subject to high‐temperature fatigue cycling has been investigated experimentally and via finite element analysis. Isothermal strain‐controlled compressive fatigue experiments (R=−∞) with 120 s holds in compression were conducted at 982° and 1093°C. Surface‐initiated cracks containing a layer of alumina progressively grew through the coating layers into the superalloy substrate, ultimately causing failure. Growth stresses in the oxide provided a driving force for extension of the oxide into the softer coating and substrate layers. Finite element modeling shows the rate of growth of the oxide‐filled cracks is sensitive to the strength of the constituent layers and the magnitude of the oxide growth strains. Implications for design of failure‐resistant coating–substrate systems are discussed.