Modeling and simulation of CMAS infiltration in EB-PVD TBCs

Kurzfassung

To protect the metallic surfaces of the turbine blades, which are exposed to very high temperatures, a thin ceramic coating known as Thermal Barrier coating (TBC) is applied. The TBCs are processed using a prominent deposition method Electron Beam Physical Vapor Deposition (EB-PVD). The process enables custom-tailoring of well grown columns and sub-columns over the coating by altering processing parameters. The gaps among the columns and sub-columns form a porous structure, offering very good strain compliance under thermal cycles in gas turbine engine. This porous structure loses its strength and fails drastically when the external mineral particles (volcanic ash, sand, debris etc.) in the form of molten Ca–Mg–Al–Si oxides (CMAS) infiltrate and solidify within the pores. The TBC structure loses strain compliance and stiffens, which leads to delamination of coatings. The damage in TBC finally reduces the service life of component as well. Therefore, advanced research is required to develop innovative coating systems defiant to erosive wear against CMAS infiltration. Previous researches on infiltration mitigation are mainly based on the arresting of CMAS by chemical reactions that solidify CMAS during the infiltration. In recent experimental and analytical studies, it was shown that the morphology of TBCs also influences the kinetics of CMAS infiltration. However, the infiltration kinetics with respective to the morphologies of coatings has not yet been fully explored. Additional knowledge in this regard can be gained by means of numerical analysis considering multi-physics interactions between CMAS and TBCs. In the current state of the art research, no such model as well as analysis procedure is proposed. In the present work, an advanced modelling approach was proposed that considers a digital prototype of the TBC microstructures and a liquid domain of molten CMAS. The digital microstructure with complex morphologies is constructed taking model parameters from the Scanned Electron Microscope (SEM) images. In order to study the respective microstructural influences on the CMAS infiltration, a Coupled Eulerian Lagrangian (CEL) method was used which allows fluid-solid interaction of TBCs and CMAS. A Python script was developed to automate the preprocessing of geometrical models for a parametric study of infiltration kinetics through different TBC microstructure. Based on the analysis and results, adoptable microstructures for deprecation of infiltration through porous TBCs are proposed. The proposed approach allows one to study the CMAS flow behavior within complex microstructure, to visualize the flow characteristics, and to optimize the microstructure for reducing the infiltration rate.