Microstructures evolution, corrosion and oxidation mechanisms of EB-PVD thermal barrier coatings exposed to molten salt corrosion

Hot corrosion caused by molten salt attack has been one of the main factors leading to the thermal barrier coating (TBC) degradation and premature failure. In this work, the molten salt corrosion behaviours of EB-PVD TBCs exposed to V₂O₅ + Na₂SO₄ at 1050 ℃ were and characterized. Hot corrosion mechanisms, microstructures evolution, and failure behaviours of TBCs under hot corrosion were investigated. The molten salt would react with the stabilizer Y₂O₃, which caused the depletion of Y₂O₃ phase in the ceramic coating, promoting the destabilization of metastable zirconia (t′-ZrO₂) into monoclinic zirconia (m-ZrO₂), resulting in obvious volume expansion of ceramic coating. The corrosion and formation of new products would densify the ceramic coating and generate high level stress in the TBCs, leading to the formation of micro cracks and pores in the ceramic layer. In addition, the growth behaviour and elemental composition of thermally grown oxide (TGO) under molten salt corrosion were determined. Hot corrosion significantly accelerated the growth of TGO, as the thickness of TGO layer under corrosion was about five times that after isothermal oxidation. Hot corrosion also destroyed the integrity of TGO layer, and the loose and porous TGO layer formed during corrosion would lead to cracking at the interface between the ceramic layer and bonding layer. The formation of cracks and pores under corrosion finally resulted in the premature failure of TBCs.