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

This paper shows how the fracture properties of thermal barrier coatings (T BCs) can be determined for different crack orientations, and demonstrates t he complex interaction between these properties during coating failure. Atm ospheric plasma-sprayed ZrO2 coatings removed from the substrate were broke n in three-point bending using micro-bending test equipment. Linear elastic fracture mechanics was used to calculate the toughness of a macroscopic th rough-thickness crack as a function of crack length. A strong R-curve was i dentified. The problem of using linear elastic fracture mechanics is addres sed. Additionally the work of fracture of delamination cracks which propaga te parallel to the interface was measured. Comparison of the work of fractu re of delamination and through-thickness cracks showed a strong anisotropy. Coatings were annealed at different temperatures to investigate aging effe cts on the properties. The critical energy release rate of through-thicknes s 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 turbi ne application. Using time-dependent safety maps the interaction between th rough-thickness cracking and coating delamination is shown. If the ratio be tween the critical energy release rates for the two species of cracks is fa vorable, segmentation of coating takes place prior to delamination, which t hus can be prevented by the reduction of strain energy in the coating. The influence of aging effects and creep deformation on coating failure is disc ussed. (C) 2000 Elsevier Science S.A. All rights reserved.