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 (V2O5), phosphorus pentoxide (P2O5), and sodium sulfate (Na2SO4) were investigated at temperatures up to 1200 degrees C. Phase transformations and microstructural development were examined using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Molten V2O5 reacted with solid YSZ to form zirconium pyrovanadate (ZrV2O7) at temperatures below 747 degrees C. However, at temperatures above 747 degrees C, molten V2O5 reacted with YSZ to form yttrium vanadate (YVO4). The formation of YVO4 led to the depletion of the Y2O3 stabilizer and deleterious transformation to the monoclinic ZrO2 phase. In addition, studies on YSZ degradation by Na2SO4 and a Na2SO4+V2O5 mixture (50-50 mol%) showed that Na2SO4 itself had no effect on the degradation of YSZ. However, in the presence of V2O5 at high temperatures, Na2SO4 forms vanadate compounds having a lower melting point such as sodium metavanadate (610 degrees C), which was found to degrade YSZ by the formation of YVO4 at a relatively lower temperature of 700 degrees C. P2O5 was found to react with APS YSZ by the formation of zirconium pyrophosphate (ZrP2O7) at all the temperatures studied. At temperatures as low as 200 degrees C and as high as 1200 degrees C, molten P2O5 was observed to react with solid YSZ to yield ZrP2O7, which led to the depletion of ZrO2 in YSZ (i.e., enrichment of Y2O3 in t\u27-YSZ) that promoted the formation of the fluorite-cubic ZrO2 phase