XPS investigation of impurity phase segregation in 25.5 wt.% CeO 2 −2.5 Y 2 O 3 −72 ZrO 2 plasma‐sprayed thermal barrier coatings

X‐ray photoelectron spectroscopy (XPS), angular‐dependent XPS and x‐ray‐inducd Auger electron spectroscopy (XAES) have been used to investigate impurity (Si, Na and Al) segregation in 25.5 wt.% CeO 2 −2.5 Y 2 O 3 −72 ZrO 2 plasma‐sprayed thermal barrier coatings (TBCs) as a function of high‐temperature air thermal treatment (up to 1460°C). The segregated phase, present in a similar composition both on the as‐thermally treated and fracture surfaces, forms a thin layer of ∼20 nm thick. This phase contains both silicon and sodium at a temperature ranging between 900 and 1350°C and it is also enriched with aluminium at higher temperatures. The XPS and XAES results are used to identify the chemical composition of this surface phase.