Failure Of Multi-Layer Suspension Plasma Sprayed Thermal Barrier Coatings In The Presence Of Na2SO4 and NaCl at 900 oC

The present work focuses on understanding the corrosion mechanism in a multi-layer suspension plasma spray (SPS) thermal barrier coatings (TBCs) when subjected to cyclic corrosion test. Two different TBC systems, a two-layer (composite gadolinium zirconate + yttria stabilized zirconia (YSZ)/YSZ), and a three-layer, dense gadolinium zirconate/ composite of gadolinium zirconate + YSZ/YSZ were studied in this work. Cyclic corrosion tests were conducted between 900 ^oC and 25 ^oC with an exposure time of 8h at 900 ^oC. A mixture of 75 wt.% Na₂SO₄ + 25 wt.% NaCl was used as the corrosive salts at a concentration of 6 mg/cm². SEM analysis of the samples’ cross-sections showed that severe bond coat degradation had taken place for both the TBC systems and the extent of bond coat degradation was relatively more severe in the three-layer system. The corrosion mechanism is discussed in detail in this paper.