Hot-corrosion behavior of graded thermal barrier coatings formed by plasma-spraying process

The hot-corrosion behavior of thermal barrier coatings (TBCs) has been studied by comparing double-layer coatings and graded coatings. Two types of oxide ceramics, 2CaO·SiO2-15mass%CaO·ZrO2 (C2S-15CZ) and 8 mass% Y2O3·ZrO2 (8YSZ), with a bond coating of NiCrAlY, were applied to metallic substrates in this study. After hot-corrosion testing with V2O5-Na2SO4 corrosive ash for 3 h at 1273 K, the TBCs were investigated by visual inspection, a scanning electron microscope, x-ray diffraction, and electron probe microanalysis. The findings for the resulting coating of C2S-15CZ reacted with V2O5 only where it was in direct contact with the corrosive ash. The affected area from the reaction was limited to the coating surface where V2O5 was present. The coating showed adequate hot-corrosion resistance against V2O5-Na2SO4 corrosive ash for 3 h at 1273 K. The findings for the 8YSZ coating were that Y2O3, the stabilizing component, particularly reacted with V2O5 and lost its function, which led to partial spalling of the coating. It was observed that the hot-corrosion resistance of the double-layer TBC was largely influenced by the performance of a corrosion-resistant NiCrAlY bond coat, which provided protection against corrosive components penetrating through the ceramic topcoat. Last, the graded coating degraded due to the oxidation of NiCrAlY particles that existed near the topcoat surface and affected the durability of the TBC.