Enhancing the Hot Corrosion Resistance of 8-Yttria Stabilized Zirconia Based Thermal Barrier Coatings

Gas-turbine hot section components such as vanes, blades, and combustion liners are key to gas-turbine durability. Gas-turbine thermal efficiency improves at higher temperature of the gas flow exiting the combustor and entering the turbine. For Ni-based superalloys, thermal-barrier coatings (TBCs) applied on hot-section blades of gas-turbine engines provide thermal insulation which allows the blades to operate at a mean temperatures of 900-1050C, while the hot gas temperatures may be 1100-1275C. 8 wt.% yttria-stabilized zirconia (8 YSZ) is considered as the working horse for TBCs applied for land-based gas-turbines for power generation in Saudi Arabia as well as worldwide. ZrO2-topcoat has been used for decades owing to its low thermal conductivity and high thermal expansion coefficient. The coating microstructure is essentially comprised of lamellar splat structures with high volume fraction of globular porosity and interlamellar porosity and micro-cracks. Yttria (Y2O3) stabilizer is incorporated in the ceramic solid solution to prevent phase transformation of ZrO2 during thermal cyclic. The stable crystal structure of ZrO2 at the usual gas turbine operating temperatures is tetragonal (t-ZrO2) while the stable phase at low temperature is monoclinic (m-ZrO2). The phase transformation during thermal cycling is accompanied with 3-5% volume change which is detrimental to in-service life of TBCs. Gas turbines are best operated using clean gaseous fuels. However, the power generation industry is increasingly using distillates and even light crudes. These lower grade fuels contain high concentrations of impurities such as vanadium, sodium, and sulphur. Molten salts of vanadates and sulfates form during combustion and deposit along the hot path components causing various hot corrosion degradation mechanisms of the multi-layer TBC system. Molten vanadates have been associated with degradation of the ceramic topcoat and the phenomena is described as vanadium-induced hot corrosion. It has long been known that this form of hot corrosion involves depletion of the yttria stabilizer to form yttria vanadates (YVO4) and consequently causing transformation of the tetragonal zirconia to monoclinic zirconia phase. Different methods have been proposed to mitigate vanadium-induced hot corrosion of the topcoat of TBCs: using pyrochlore, sacrificial topcoat, alternative stabilizer, fuel additive, and surface sealing. The aim of this research work is to investigate a cost-effective modification of 8YSZ with improved hot corrosion resistance. Two modifications of 8YSZ will be examined. First, mixing 8YSZ based TBC powder with highly stabilized TBC systems including ceria stabilized zirconia (CSZ) and zirconia yttria titania (YTiSZ). The second follows a more cost-effective approach through mixing 8YSZ powder with potential ceramic oxides that have great affinity to react with vanadates and thus enhances the stability of 8YSZ. Among these potential oxides are CeO2 and TiO2. The performance of 8YSZ and their modifications will be evaluated in terms of hot-corrosion resistance, thermal conductivity, thermal expansion, inhibition response to fuel additive, and their characteristics when applied using air plasma spray technique.