The effect of superalloy substrate on the behaviour of high-purity low-density air plasma sprayed thermal barrier coatings

Several superalloy-bond coat couples were prepared without ceramic topcoat layers to better understand the effects of superalloy substrate on the oxidation behaviour of NiCoCrAlY bond coats. The same composition NiCoCrAlY bond coats were deposited on three superalloy substrates (Inconel 718, Haynes 188 and Rene N5) via argon-shrouded plasma spraying. The specimens were exposed to cyclic oxidation in laboratory air at 1100 degrees C in a bottom loading furnace. Scaling behaviour and rate of aluminum depletion were compared between the various specimens. The bond coats on all three superalloys experienced some form of chemical failure after an extended number of cycles. The number of cycles until chemical failure was shortest for the IN718 specimen followed by the HA188 specimen, both of which experienced complete bond coat chemical failure, and then the Rene N5 specimen, which experienced localized chemical failure. The cycles to chemical failure coincide with the cycles to thermal barrier coating (TBC) spallation from previous work, indicating chemical failure of the bond coat is a critical event in the lifetime of TBCs. The effect of bond coat surface finish and porosity on the scaling behaviour has been investigated using specimens with the same superalloy substrate but with different bond coat surface finishes and porosity levels which were produced by utilizing two separate sized starting bond coat metallic powders. Bond coats with minimal porosity and smooth surface finishes did not experience chemical failure, at least in the time frame they were tested; however, oxide scale spallation was more apparent in the smooth bond coats as compared to the specimens with the rough surface finishes and high levels of porosity.