Improvement of Calcium–Magnesium–Alumino-Silicate (CMAS) and Hot Corrosion Resistance of Functionally Graded Al₂O₃/CYSZ Thermal Barrier Coatings (TBCs)

CMAS dusts and hot corrosion salts are one of the most serious threats to gas turbine blades. Both of these, at high temperatures, penetrate into the thermal barrier coating (TBC) on the turbine blade surface and/or degrade the chemical structure of the coating. Therefore, it usually causes stress-induced damage. In this study, a laser surface modification process was performed to increase the CMAS and hot corrosion resistance of functionally graded alumina/ceria-yttria stabilized zirconia (Al₂O₃/CYSZ) TBCs. Single layered CYSZ was chosen as the comparison to the developed and tested TBC material because it has high corrosion stability. Thermal cycling and thermal gradient tests were performed to understand the simultaneous effect of CMAS and hot corrosion, on CYSZ TBC and the laser surface modified Al₂O₃/CYSZ TBC. For thermal cycling and thermal gradient tests, TBCs with a mixture of CMAS powder and hot corrosion salt on their surfaces were heated to 1200°C by using a laser beam. The thermal cycle test was performed as heating from the surface of the TBC and then followed by air cooling, while the thermal gradient test was performed as heating from the surface of TBC and simultaneous water cooling from the back surface of the substrate. Damages in both coatings and their mechanisms were characterized using detailed macroscopic, microscopic (SEM) and phase (XRD) analyses.