Durability of plasma sprayed thermal barrier coatings with controlled properties part I: For planar disk substrates

Thermal Barrier Coatings (TBCs) have been qualified for their thermo-mechanical performance and durability using Furnace Cycle Testing (FCT) for decades. FCT provides guidance toward materials and process improvements. Although FCT is a relatively straightforward test, the interpretation can often be subject to uncertainty, making it difficult to link test results directly to engine performance. This is because in FCT, the coating and substrate are isothermally treated, while real turbine components experience strong thermal gradients and time transients. Gradient testing can be time-consuming and expensive, thus FCT tests provide a cost-effective first-order confidence screening for development. A review of the available literature on Air Plasma Spray (APS) TBC durability by FCT does not reveal consistent trends among processing, microstructure (i.e., porosity), and durability. Furthermore, the available literature does not comprehensively capture the influence of microstructural changes (i.e., sintering) on FCT durability. In this two-part study, APS TBCs were produced with systematically-controlled microstructures, using multiple powder types on different bond-coated superalloy substrates and subjected to different FCT conditions. In Part I, the focus is on disk specimens, where edge-delamination failure predominates, while Part II will address curved surfaces with different failure mechanisms. The FCT results for the systematically produced coatings suggest marginal trends between durability and porosity of APS TBCs, with denser/tougher microstructures providing more durable outcomes, i.e., segmented coatings. One coating was sprayed multiple times over six years and tested in FCT to benchmark the durability testing and coating process reproducibility at Stony Brook. The results were compiled in a unified framework, utilizing published formulations to calculate available elastic strain energy, which serves as a driving force for delamination. The current results highlight a few regimes of FCT performance based on coating toughness, stiffness, and nonlinear elastic parameters. In addition, the benefits and limitations of FCT are discussed in this study.