Thermal barrier coatings (TBCs) are key components of multilayered thermal protection systems for gas turbines. Failure mechanisms often involve the propagation of delamination cracks within this zirconia-based ceramic layer. Increasing the toughness of this layer is an essential element of any failure mitigation strategy. Progress hinges on (i) developing an ability to assess the toughness of TBCs and (ii) cultivating knowledge on how to optimally exploit toughening mechanisms for these materials.

Mode I toughness of air plasma-sprayed TBCs can be measured by sandwiching free-standing coatings in a modified double cantilever beam configuration. Quantification of R-curve behavior is straight-forward provided that: (i) the crack tip position can be correctly determined as a function of the beam deflection, (ii) the energy release rate can be calculated accurately while accounting for complicating factors such as the finite compliance of the ceramic layer and transverse shear effects, and (iii) the fracture process zone is small compared to other relevant dimensions. These interrelated challenges are addressed by a combination of digital image correlation (DIC) measurements and finite element (FE) analyses.

A key insight in the present approach, which facilitates the extraction of the crack tip location from DIC data, is that the centerlines of the beams experience negative displacements at a characteristic distance, which is insensitive to reasonable variations in specimen dimensions and TBC properties, ahead of the crack tip. Thus, a simple method is developed to determine the crack length from DIC displacement measurements using a fitting procedure with FE results.

Applying this methodology to 8wt% yttria-stabilized zirconia (8YSZ) dense vertically cracked (DVC) material reveals R-curve behavior with steady-state fracture resistance that is appreciably higher than the initiation toughness. Features observed on the fracture surface strongly suggest that a crack bridging and pull-out mechanism, enabled by the DVC microstructure, is responsible for the substantial improvement of the TBC's macroscopic resistance to fracture. There is also evidence of ferroelastic switching, a mechanism reported to provide 8YSZ with superior intrinsic toughness. Contrasting the performance of the DVC coatings to those with a standard "porous" microstructure reveals the impact of the bridging mechanism. The efficacy of this mechanism in a material with low intrinsic toughness is investigated through DVC coatings of 20wt% yttria-stabilized zirconia (20YSZ), a composition that does not exhibit ferroelastic switching. Analysis of novel TBC compositions further contributed to understanding of the effect of each mechanism and their interplay.

To probe the evolution of toughness during service, coatings that had been exposed to high temperature were examined. While these experiments were not conclusive, this area may be attractive for future work.

This research contributes to the understanding of current state-of-the-art TBCs, enabling improved durability predictions. The tools, analysis techniques, and comparisons now available can be implemented as guides in assessing the mechanical properties when devising new coatings.