In Situ Monitoring of Cracking Mechanisms in Multi-Layered Suspension Plasma-Sprayed Thermal Barrier Coatings

Abstract

In this study, the in situ technique was used to observe crack formation and growth in multilayer suspension plasma spray (SPS) thermal barrier coatings (TBCs). Utilizing synchronized three-point bending (3 PB) and scanning electron microscopy, coupled with digital image correlation, we gained real-time insights into strain field dynamics around cracking zones. This approach allowed us to induce bending-driven failure in both single and multi-layered SPS coatings to explore crack behavior in these cauliflower-like multilayer TBCs. Our observations revealed that columnar gaps facilitate crack initiation and propagation from the coatings’ free surfaces. The triple-layer SPS coating showed a reduced susceptibility to vertical cracking compared to other SPS structures, due to a dense gadolinium zirconate layer on the top. Additionally, the splat structure of the bond coat (BC) layer contributes to crack relative path deflection, which could enhance the fracture toughness of the SPS coatings by dissipating the energy needed for crack propagation. Moreover, it was revealed that grit particles at the BC/substrate interface appear to promote crack branching near the interface, localized coating delamination, and serve as nucleation sites for crack development. Therefore, optimizing the grit-blasting process of the substrate prior to BC layer deposition is essential for minimizing the likelihood of crack formation under operational conditions, thereby enhancing durability and extending the lifespan of the coatings. This study highlights the critical role of in situ observation in unraveling the complex failure mechanisms of multi-layered coatings, paving the way for the design of advanced coatings with improved performance in extreme environments.