Experimental and Finite Element Study of an Air Plasma Sprayed Thermal Barrier Coating under Fixed Cycle Duration at Various Temperatures

Abstract

Cyclic furnace tests were conducted on Air Plasma Sprayed (APS) Thermal Barrier Coating (TBC) samples which were coated with NiCoCrAlY on a nickel‐based superalloy at three temperatures. From these tests the following were determined: oxidation kinetics and activation energy, bond coat rumpling rate, and cyclic failure lives. All failures occurred in the top coat consistent with engine experience. The measured rumpling rate and oxidation kinetics were used as input for a finite element model that utilized a highly realistic top coat constitutive model that included creep and relaxation behavior as well as tension/compression flow stress asymmetry consistent with available experimental data. The modeling results based on these measured inputs show that the primary sources of stress and strain in descending order are rumpling, oxide growth and finally thermal expansion mismatch between the top coat and underlying metal. Three important implications of these results are; predicted stresses are too low to predict failure based on single crack fracture mechanics, cycling increases accumulated inelastic strain in a fixed time interval providing an additional reason why cycling is damaging and finally to get realistic stresses and strains it is necessary to accurately characterize the geometry changes associated with rumpling.

A semi-analytical model for predicting stress evolution in multilayer coating systems during thermal cycling

“…Fig. 6 shows the experimentally measured temperature-dependent kinetics (taken from [37]) and the corresponding fitting curves using the parameters in Table 2. The comparisons indicate that the selected k op works quite well.…”

“…Note: The k op and η g are selected in this work, and other parameters are taken from literature[37].…”

Cyclic furnace testing and life predictions of thermal barrier coating spallation subject to a step change in temperature or in cycle duration

“…In the data collected the standard deviation shows a modest increasing trend in life fraction as shown in [7] …”

“…For example, in one recent study, life prediction required finite element modeling using an advanced constitutive model as well as measurements of rumpling and oxidation kinetics collected from over 1000 micrographs [7]. Mechanism based approaches such as in [7] are beyond the scope and intent of this experimental program and often infeasible in an industrial context. Second, with respect to using measured TGO stress to predict remaining life there are several challenges.…”

Zirconia and Pyrochlore Oxides for Thermal Barrier Coatings in Gas Turbine Engines

“…Undulations in the TGO layer generate locally high stress levels, which can lead to failure at the interface between the TGO and the thermal barrier coating. [102][103][104][105][106][107][108] Grit blasting of the alloy prior to coating deposition has been shown to lead to a more planar TGO and thus improved lifetime. [109] However, the failure in this location highlights the importance of compatibility of the thermal barrier coating material with the substrate alloy.…”