The Microstructural and Chemical Composition Changes in TGO and BC of HALT Pt–Al–APS YSZ Coating After Cyclic Oxidation

In this research, the microstructural and chemical composition changes of thermally grown oxide (TGO) and bond coat (BC) layer were investigated after cyclic oxidation of Pt–Al–YSZ coating at 950°C. First, the Pt–Al layer was coated by the high-activity-low-temperature (HALT) method on the Rene-80 substrate, and then the YSZ top coat (TC) layer was coated by the atmospheric plasma spraying (APS) method on the HALT Pt–Al layer. Cyclic oxidation was performed in ambient air for 50 cycles. After cyclic oxidation, the Pt–Al layer was divided into upper and lower regions. Upward diffusion of Al from the Pt-Al layer to TGO led to the transformation of β to γ′ and γ in the lower region, rumpling in the upper region, and internal oxidation at the interface of the upper and lower region of the Pt–Al layer. The TGO layer included mixed oxides of NiO, Cr₂O₃, and NiCr₂O₄, along with discontinuous Al₂O₃ veins. Depletion of Al from Pt–Al led to the ratcheting of the TGO, the formation of NiAl₂O₄ at the Pt–Al/TGO interface, and, subsequently, the nucleation of lateral microcracks in the upper region of the Pt–Al. A mechanism for changing the chemical composition of TGO after cyclic oxidation was proposed.