Hot Corrosion Behavior of Graded Thermal Barrier Coatings Formed by Plasma Spraying Process

Hot corrosion behavior of Thermal Barrier Coatings (TBCs) has been studied by comparison between double layer coatings and graded coatings. Two types of oxide ceramics, 8 mass % Y₂O₃-ZrO₂(8YZ) and 2CaOSiO₂-15mass% CaOZrO₂ (C₂S-15 CZ), with a bond coating of NiCrAlY were applied to metallic substrates in this study. After a hot corrosion test by V₂O₅-Na₂SO₄ corrosive ashes, hot corrosion behavior of TBC has been investigated by visual inspection, metallography, X-ray diffraction and EPMA. The C₂S-15%CZ coating reacted with V₂O₅ only where it was in direct contact with the material. The affected area from the reaction was limited to the coating surface where V₂O₅ existed. The coating showed adequate hot corrosion-resistance. It was found on the 8YZ coating that Y₂O₃, the stabilizing component, particularly reacts with V₂O₅ and loses its function; this led to partial spalling of the coating. It was observed that the durability of the double layer TBC was largely influenced by the performance of a corrosion resistant NiCrAlY undercoat which provided protection against corrosive components penetrating through the ceramic topcoat. It was observed that the graded coating degraded due to oxidation of NiCrAlY particles which independently existed near the coating surface and affected the durability of TBC.