Improving the Corrosive Durability of Thermal Barrier Coatings

Conventional yttria stabilized zirconia (7YSZ) thermal barrier coatings (TBCs) that are currently used in gas turbine engines undergo hot corrosion by sulfate/vanadate deposits commonly found in marine environments. These deposits destabilize the zirconia by reacting with and leaching out yttria from the zirconia structure, eventually causing a destructive monoclinic phase transformation that ultimately results in the TBC detaching from the underlying alloy component. In response to the current failures of TBCs, an investigation of the effectiveness of stabilizing the zirconia structure by co-doping with yttrium and tantalum is being investigated. Through reverse co-precipitation with precursors, three baseline samples of yttria stabilized zirconia (YSZ), and two samples of YSZ co-doped with tantalum were made. The samples were then pyrolized to obtain an oxide in powder form, pressed into pellets, coated with a Na2SO4-30mole%NaVO3 salt as a corrodent to simulate the sulfate/vanadate deposits, and then tested at 900oC in air. Analysis of changes in microstructure and composition were done with the use of X-Ray Diffraction, scanning electron microscopy, electron dispersive spectroscopy, and transmission electron microscopy in order to compare the relative stability of the compositions and their resistance to hot corrosion.