Thermal barrier coating toughness: Measurement and identification of a bridging mechanism enabled by segmented microstructure

Failure mechanisms in thermal barrier coatings (TBCs) often involve the propagation of delamination cracks within the top coating. The work presented in this paper makes two principal contributions: first, the development of a straightforward testing geometry and analysis approach enables the accurate determination of the mode I fracture toughness of these coatings and second, the application of the approach to technologically relevant coatings produces new insights into the impact of the dense vertically cracked (DVC) microstructure on the toughness. Mode I toughness of air plasma-sprayed 8 wt% yttria-stabilized zirconia DVC TBCs is measured by sandwiching the freestanding coatings in a modified double cantilever beam configuration. Digital image correlation measurements and finite element analysis provide a pathway to quickly and accurately extract toughness values from displacement data alone. Results show R-curve behavior and unexpectedly high steady-state toughness values of Gss≈300-400J/m2. The observation of this elevated toughness can be rationalized by a crack bridging model that is consistent with the TBC's starting microstructure and features of the fracture surfaces.