CMAS corrosion of dense-vertically cracked thermal barrier coatings: influence of the coating materials

Thermal barrier coatings (TBCs) are currently most used in applications such as industrial gas turbines (IGT), where high thermal stability and resistance are required. Degradation by molten CaO-MgO-Al₂O₃-SiO₂ (CMAS) glassy deposits remains a crucial issue in terms of coating durability. The present study is focused on the development of new advanced TBC designs that can be a valid alternative to conventional atmospheric plasma spraying (APS) 7YSZ coatings for improved resistance against CMAS attack. Rare earths doped-ZrO₂, Gd₂ZrO₇, ZrO₂-55 wt.%Y₂O₃ and standard 7YSZ as a reference were the selected coating materials. All systems were sprayed by Atmospheric Plasma Spraying (APS) in order to obtain a dense-vertically cracked (DVC) microstructure, because the latter is known to exhibit better corrosion resistance against CMAS in comparison to porous TBCs. The coatings were sprayed onto Hastelloy-X substrates with a HVOF-sprayed NiCoCrAlY bond coat. Six ceramic bilayers, where 7YSZ (with a porous or Dense Vertically Cracked (DVC) microstructure) was employed as an intermediate layer and the top layers were composed of rare earths doped-ZrO₂, Gd₂ZrO₇, ZrO₂-55 wt.%Y₂O₃, respectively, were sprayed on the same bond coats. Corrosion tests (1 h at 1250 °C) were performed on both the four different ceramic monolayers and on the six bilayer coatings. Thermal Cycling Fatigue (TCF) tests were also carried out to assess the thermomechanical resistance of the investigated coating materials and architectures.

CMAS tested coatings were characterized by FEG-SEM + EDX, XRD and Raman spectroscopy techniques to evaluate the chemical interaction between the glassy deposit and the different coating chemistries and their resistance against degradation and chemical dissolution.