Cyclic surface undulations in YSZ coatings: Coupled evolution of microstructure, mechanical behavior, and lifespan

The cyclic evolution of surface undulations in EB-PVD YSZ thermal barrier coatings (TBCs) during long-term high-temperature oxidation service is systematically studied. A well-defined three-stage morphological evolution (comprising an initial undulated configuration, a subsequent stage of surface flattening, and a final re-establishment of undulations reminiscent) has been identified and is shown to exhibit a strong correlation with the sequential growth behavior of the thermally grown oxide (TGO). SEM observations combined with nanoindentation measurements reveal that surface morphology evolution is closely linked to mechanical parameters such as hardness (H), elastic modulus (E), H/E, and H³/E². This surface undulations behavior results from the interplay of four mechanisms: non-uniform TGO growth, sintering-induced densification, stress accumulation and relaxation, and phase transformation-induced volume changes. A coupled thermomechanical–microstructural framework is proposed to describe these interactions.