Degradation of plasma-sprayed yttria-stabilized zirconia coatings via ingress of vanadium oxide

Abstract

V 2O 5 reaction and melt infiltration in plasma-sprayed 7 wt% Y 2O 3–ZrO 2 (YSZ) coatings were investigated at temperatures ranging from 750 °C to 1200 °C using SEM and TEM combined with EDS. The interlamellar pores and intralamellar cracks, common in plasma-sprayed materials, provide pathway for the molten species. The microstructure of the contaminated coatings is therefore the result of the interplay between the dissolution/reaction rates of the V 2O 5 with YSZ coating and the infiltration rates of the molten species. Near the coating surface, the reaction front proceeds in a planar fashion, via dissolution of the lamella and precipitation of fine-grained reaction products composed of ZrV 2O 7 (for reactions at 750 °C and below), m-ZrO 2 and YVO 4. The thickness of this planar reaction zone or PRZ was found to increase as reaction time and temperature increased. The melted V 2O 5 was observed to infiltrate along the characteristic microstructure of plasma-sprayed coatings, i.e. the interconnected pores and cracks, and react with the YSZ. The thickness of this melt infiltrated reaction zone or MIRZ ranged from 5 μm for reactions at 750 °C for 30 min to 130 μm for reactions at 1000 °C for 90 min. At 1200 °C, only a PRZ was observed (i.e. the thickness of the MIRZ was nominally zero), suggesting that the dissolution reaction within the pores/cracks and subsequent formation of reaction products may limit infiltration. Fifty-hour heat-treatments at 1000 °C and 1200 °C prior to reaction with the V 2O 5 at 800 °C for 90 min were used to change the microstructural features of the coating, such as crack connectivity and pore size. The heat-treatment at 1000 °C was found most deleterious to the coating due to large cracks created via a desintering process that afforded deep penetration of the molten V 2O 5.