Effect of Pre-Oxidation and Thermal Exposure on the Microstructure of Superalloys and Thermal Barrier Coating Systems

Thermal barrier coatings (TBCs) enable nickel-base superalloy gas turbine blades to run at their highest possible temperatures. Currently, a bond coat (BC) is utilized to adhere the yttria-stabilized zirconia (YSZ) top coat to the nickel-base superalloy substrate. However, this increases cost. In this study, YSZ top coats were applied directly to the nickel-base superalloy René N5. The René N5 samples were pre-oxidized at 1150°C in atmosphere in order to form an α-Al2O3 layer to enhance bonding to the YSZ top coat. YSZ dense vertically cracked (DVC) and columnar morphologies were then applied. These samples were then cyclically and isothermally tested in atmosphere at 1150°C. Both SPS DVC and SPS Columnar coatings were found to have poor adhesion. The lack of adequate α-Al2O3 layer thickness, the rapid formation of spinel and the grit blasting process preceding coating deposition were determined to be the primary causes of poor performance.