Degradation characteristics of thermal barrier coatings for hot corrosion and CaO-MgO-Al2O3-SiO2

Hot corrosion and CaO-MgO-Al2O3-SiO2 (CMAS) resistance of thermal barrier coatings (TBCs) are required in an industrial gas turbine employed with impure fuels and high turbine inlet temperatures. Therefore, various TBC systems with different structural combinations were investigated in thermochemical aspects of hot corrosion and CMAS causing the degradation of TBCs. Four types of 8 wt% yttria-stabilized zirconia (8YSZ), namely dense microstructure (8YSZ), porous microstructure (C-8YSZ), no monoclinic phase (NoM-8YSZ), and low thermal conductivity (low-k), were employed to prepare topcoats using an atmospheric plasma spray on the two bondcoats coated by a vacuum plasma spray with feedstocks of NiCoCrAlY and NiCoCrAlY+Hf+Si. Hot corrosion components penetrated through the whole topcoat layer at 1000 degrees C however did not infiltrate to the bondcoat layers because of the thermally grown oxide layer formed between the topcoat and the bondcoat. In addition, the elements Hf and Si contained in the bondcoat composition suppressed diffusion of beta phase aluminum. In the visual inspection after the hot corrosion at 1000 degrees C and CMAS test at 1250 degrees C, the TBC system with the NoM-YSZ topcoat layer showed a sound condition without failure unlike other TBCs, and no monoclinic phase after hot corrosion and the CMAS test, indicating that the phase stability plays a critical role in TBCs' lifetime in the hot corrosion and CMAS environments.

Automated summary

The study investigated various TBC systems with different structural combinations and found that the TBC system with the NoM-YSZ topcoat layer showed excellent performance in both hot corrosion and CMAS tests. The phase stability plays a critical role in the lifetime of TBCs in harsh environments.