Corrosion resistant thermal barrier coating materials for industrial gas turbine applications

Thermal Barrier Coatings are ceramic materials that are deposited on metal turbine blades in aircraft engines or industrial gas turbines which allow these engines to operate at higher temperatures. These coatings protect the underlying metal superalloy from creep, oxidation and/or localized melting by serving as an insulating barrier to protect the metal from the hot gases in the engine core. While for aircraft engines, pure refined fuels are used, it is desirable for industrial gas turbine applications that expensive refining operations be minimized. However, acidic impurities such as sulfur and vanadium are common in these "dirty" fuels and will attack the thermal barrier coating causing reduced coating lifetimes and in the worse case catastrophic failure due to spallation of the coating. The industry standard coating material is stabilized zirconia with seven weight percent yttria stabilized zirconia being the most common. When used in industrial gas turbines, the vanadium oxide impurities react with the tetragonal zirconia phase causing undesirable phase transformations. Among these transformations is that from tetragonal to monoclinic zirconia. This transformation is accompanied by a volume expansion which serves to tear apart the coating reducing the coating lifetime. Indium oxide is an alternative stabilizing agent which does not react readily with vanadium oxide. Unfortunately, indium oxide is very volatile and does not readily stabilize zirconia, making it difficult to incorporate the indium into the coating. However, by pre-reacting the indium oxide with samarium oxide or gadolinium oxide to form a stable perovskite (GdInO₃ or SmInO ₃) the indium oxide volatilization is prevented allowing the indium oxide incorporation into the coating. Comparison of EDX data from evaporated coatings containing solely indium oxide and those containing GdInO₃ are presented and show that the indium is present in greater quantities in those coatings containing the additional stabilizer. Corrosion tests by reaction with vanadium pentoxide were performed to determine the reaction sequence and to optimize the chemical composition of the coating material. Lastly, select x-ray diffraction phase analysis will be presented.