Nonuniform creep-induced alumina scale spallation on FeCrAl coatings

Alumina scales play a pivotal role in the failure of thermal barrier coatings, and their cooling rate-dependent spallation remains a major limitation in high-temperature applications. This study presents an analytical model to quantify dynamic creep relaxation in alumina scales ($\alpha -{\text{Al}}_2{\text{O}}_3$) and its role in blistering and eventual spallation. Particularly, cooling rate-dependent residual stress and stress relaxation kinetics are integrated to characterize the nonuniformity of creep relaxation, elucidating the mechanism of scale detachment at room temperature (Tolpygo and Clarke, 2000). The established model reveals that localized pockets of tensile stress at the scale-metal interface govern crack nucleation, while through-thickness bending from in-plane radial stress gradients leads to blister formation. Additionally, the model introduces spatial stress heterogeneity and energy threshold as the universal criteria for predicting spallation. The pocket of energy concentration model shows strong agreement with experimental observations. This study provides a comprehensive understanding of the interactions among mechanical stress, interface fracture toughness, and scale stability, enhancing predictive capabilities for failures in extreme thermal environments.