Influence of Yttria content in YSZ: An evaluation of water-based SPS coatings and spark plasma sintered pellets

The inherent phase instability of the state-of-the-art 7–8 mol% partially stabilised Yttria-Stabilised Zirconia (YO_1.5, 8 YSZ, tetragonal) under a molten CMAS attack lacks the technological readiness needed to increase the gas inlet temperatures for more thermally efficient gas turbine engines. In this study, the concentration of Yttria in YSZ was systemically increased to impart CMAS resistance and their applicability via emerging Suspension Plasma Sprayed (SPS) coatings and Spark Plasma Sintered (SpPS) pellets under simulated conditions was evaluated. The fully stabilised higher Yttria YSZ compositions (21.4 mol% and 50.2 mol%, cubic) severely restricted the CMAS infiltration and interaction areas, with the later composition forming an apatite layer and the trans-granular cracking and chipping of YSZ into layers reduced with an increase in Yttria content. However, their application into water-based SPS coatings resulted in a complete coating failure following the high-temperature CMAS tests. During Furnace Cycling Test (FCT) tests, the 8 YSZ coating survived 34 thermal cycles compared to just one on the 50.2 mol% YSZ coatings. The premature delamination of the higher Yttria coating seems to have been caused by the spallation associated with horizontal cracks within the coating. The YSZ composition could be modified into CMAS-resistant chemistry; however, their applicability as a functioning TBC with inherent microstructural features, such as the Dense Vertically Cracked (DVC) coating strategy examined in this study, requires new coating design strategies that impart sustainable thermal cycling performance.