Degradation of Yttria‐Stabilized Zirconia Thermal Barrier Coatings by Vanadium Pentoxide, Phosphorous Pentoxide, and Sodium Sulfate

The presence of vanadium, phosphorus, and sodium impurities in petcoke and coal/petcoke blends used in integrated gasification combined cycle (IGCC) plants warrants a clear understanding of high‐temperature material degradation for the development of fuel‐flexible gas turbines. In this study, degradation reactions of free‐standing air plasma‐sprayed (APS) yttria‐stabilized zirconia (YSZ) in contact with vanadium pentoxide (V 2 O 5 ), phosphorus pentoxide (P 2 O 5 ), and sodium sulfate (Na 2 SO 4 ) were investigated at temperatures up to 1200°C. Phase transformations and microstructural development were examined using X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. Molten V 2 O 5 reacted with solid YSZ to form zirconium pyrovanadate (ZrV 2 O 7 ) at temperatures below 747°C. However, at temperatures above 747°C, molten V 2 O 5 reacted with YSZ to form yttrium vanadate (YVO 4 ). The formation of YVO 4 led to the depletion of the Y 2 O 3 stabilizer and deleterious transformation to the monoclinic ZrO 2 phase. In addition, studies on YSZ degradation by Na 2 SO 4 and a Na 2 SO 4 +V 2 O 5 mixture (50–50 mol%) showed that Na 2 SO 4 itself had no effect on the degradation of YSZ. However, in the presence of V 2 O 5 at high temperatures, Na 2 SO 4 forms vanadate compounds having a lower melting point such as sodium metavanadate (610°C), which was found to degrade YSZ by the formation of YVO 4 at a relatively lower temperature of 700°C. P 2 O 5 was found to react with APS YSZ by the formation of zirconium pyrophosphate (ZrP 2 O 7 ) at all the temperatures studied. At temperatures as low as 200°C and as high as 1200°C, molten P 2 O 5 was observed to react with solid YSZ to yield ZrP 2 O 7 , which led to the depletion of ZrO 2 in YSZ (i.e., enrichment of Y 2 O 3 in t′‐YSZ) that promoted the formation of the fluorite‐cubic ZrO 2 phase.