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 = -infinity) with 120 s holds in compression were conducted at 982 degrees and 1093 degrees 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.