Toughness anisotropy and damage behavior of plasma sprayed ZrO2 thermal barrier coatings

This paper shows how the fracture properties of thermal barrier coatings (TBCs) can be determined for different crack orientations, and demonstrates the complex interaction between these properties during coating failure. Atmospheric plasma-sprayed ZrO2 coatings removed from the substrate were broken in three-point bending using micro-bending test equipment. Linear elastic fracture mechanics was used to calculate the toughness of a macroscopic through-thickness crack as a function of crack length. A strong R-curve was identified. The problem of using linear elastic fracture mechanics is addressed. Additionally the work of fracture of delamination cracks which propagate parallel to the interface was measured. Comparison of the work of fracture of delamination and through-thickness cracks showed a strong anisotropy. Coatings were annealed at different temperatures to investigate aging effects on the properties. The critical energy release rate of through-thickness cracks increases with annealing temperature while the work of fracture of delamination cracks decreases. A finite element calculation was performed to simulate the state of stress in a coating system for a typical gas turbine application. Using time-dependent safety maps the interaction between through-thickness cracking and coating delamination is shown. If the ratio between the critical energy release rates for the two species of cracks is favorable, segmentation of coating takes place prior to delamination, which thus can be prevented by the reduction of strain energy in the coating. The influence of aging effects and creep deformation on coating failure is discussed.