Thermal cycling behavior of EBPVD TBC systems deposited on doped Pt-rich γ–γ′ bond coatings made by Spark Plasma Sintering (SPS)

Abstract

In the last decade, an increasing interest was given to Pt-rich γ–γ′ alloys and coatings as they have shown good oxidation and corrosion properties. In our previous work, Spark Plasma Sintering (SPS) has been proved to be a fast and efficient tool to fabricate coatings on superalloys including entire thermal barrier coating systems (TBC). In the present study, this technique was used to fabricate doped Pt-rich γ–γ′ bond coatings on AM1® superalloy substrate. The doping elements were reactive elements such as Hf, Y or Zr, Si and metallic additions of Ag. These samples were then coated by electron beam physical vapour deposition (EBPVD) with an yttria partially stabilized zirconia (YPSZ) thermal barrier coating. Such TBC systems with SPS Pt rich γ–γ′ bond coatings were compared to conventional TBC system composed of a β-(Ni,Pt)Al bond coating. Thermal cycling tests were performed during 1000-1h cycles at 1100°C under laboratory air. Spalling areas were monitored during this oxidation test. Most of the Pt rich γ–γ′ samples exhibited a better adherence of the ceramic layer than the β-samples. After the whole cyclic oxidation test, cross sections were prepared to characterize the thickness and the composition of the oxide scales by using scanning-electron microscopy. In particular, the influence of the doping elements on the oxide scale formation, the metal/oxide roughness, the TBC adherence and the remaining Al and Pt under the oxide scale were monitored. It was shown that RE-doping did not improve the oxidation kinetics of the studied Pt rich γ–γ′ bond coatings, nevertheless most of the compositions were superior to “classic” β-(Ni,Pt)Al bond coatings in terms of ceramic top coat adherence, due to lower rumpling kinetics and better oxide scale adherence of the γ–γ′-based systems.