Mechanisms controlling the performance and durability of thermal barrier coatings

Thermal protection systems based on thermal barrier coatings are widely used in turbine engines for propulsion and power generation. The benefit of these coatings resides in their ability to inhibit degradation of the underlying structural superalloy component by thermo-mechanical fatigue and oxidation. Existing commercial coatings are well-engineered with established durability and cost benefits. However, they lose adhesion and spall from the underlying metal with cyclic thermal exposure. Because of varied manufacturing approaches and operating scenarios, several specific mechanisms are involved. Ultimately, failure is connected to the large residual compression in the thermally grown oxide and its interaction with imperfections at the coating/substrate interface. This interaction induces an energy release rate at cracks emanating from the imperfections that eventually leads to buckling and spalling of the TBC. Recent work to understand these phenomena has highlighted several nuances and challenges. This paper provides an overview of the current understanding of factors affecting coating durability and presents relationships between the durability, the governing material properties and the salient morphological features.