Hf Addition by Sputtering in β-NiPtAl Bond Coating for TBC Systems and its Effect on Thermal Cycling Behaviour

During thermal cycling under oxidizing conditions, thermal barrier systems undergo microstructural and morphological changes as well as stress development. These evolutions initiate cracks whose propagation leads to spallation. Works by Streiff et al. and Pint et al. showed that Hf additions improve spallation resistance of nickel aluminide coatings or alloys. Based on these results, Hf is added to a beta-NiPtAl thermal barrier system. The chosen manufacturing process consists in depositing, onto the superalloy, alternative Hf and Pt layers by sputtering. A conventional diffusion treatment is performed before out-of-pack aluminizing and final heat treatment. To study the effect of Hf on TGO adherence and rumpling, the systems are thermally cycled, at 1100 degrees C in air, in same time as conventional Pt-modified nickel aluminide thermal barrier systems. After failure, TGO and bond coating microstructure and composition are analyzed by SEM and compared to the "as-processed" systems.