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 12001C. 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 7471C. However, at temperatures above 7471C, 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 1V 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 (6101C), which was found to degrade YSZ by the formation of YVO 4 at a relatively lower temperature of 7001C. 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 2001C and as high as 12001C, 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 0 -YSZ) that promoted the formation of the fluorite-cubic ZrO 2 phase.

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Degradation of free-standing yttria-stabilized zirconia (YSZ) and CoNiCrAlY coatings (300 lm) due to V 2 O 5 and a laboratory-synthesized CMAS was investigated at temperatures up to 1400°C. Reactions, phase transformations, and microstructural development in coatings were examined by using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The molten deposits destabilized the YSZ and reacted with the thermally grown oxide with various phase transformations and reaction product formation. A dense, continuous environmental barrier overlay, based on oxides, applied by electrophoretic deposition was effective in mitigating the molten deposit attack. Enriching CMAS composition with Al promoted the crystallization of anorthite platelets and MgAl 2 O 4 spinel, and mitigated CMAS ingression. EPD MgO overlay was also effective in protection against V 2 O 5 melt by formation of magnesium vanadates. EPD alumina overlay deposited on thermal barrier coatings with APS 8YSZ and bond-coated IN939 superalloy retained its adhesion and structural integrity after prolonged furnace thermal cycle test at 1100°C.