Response of molten silicate infiltrated Gd₂Zr₂O₇ thermal barrier coatings to temperature gradients

The delamination tendency for bilayer gadolinium zirconate/yttria-stabilized zirconia (GZO/YSZ) thermal barrier coatings (TBCs) was investigated using a continuous laser-based thermal gradient test and a computational model for the analysis of multilayer structures. TBCs with different architectures were exposed to molten silicate compositions, representative of aero-engine deposits, and subjected to thermal cycles prescribed to impart specified levels of strain energy in the coating. The exposed surface of the intercolumnar gaps in the outer part of the GZO layer was found to rapidly dissolve into the intruding molten silicate and precipitate reaction products that seal the flow paths, limiting the penetration depth and the ensuing stiffening of the TBC. Nevertheless, the stiffened layer magnifies the thermal stresses in the coating upon thermal cycling. The influence of the thermal history and multilayer structure on the driving force for delamination was modeled and compared with the experimental results. The effects that the substrate coefficient of thermal expansion, the temperature gradient, and the TBC thickness have on the driving force for delamination were analyzed and the critical amount of stored elastic strain energy for failure under different scenarios was assessed.