Microstructural evolution and cyclic oxidation of Pt-Al-YSZ coating under thermal shock

Abstract

In this research, the effect of thermal shock has been investigated on the microstructure and cyclic oxidation of Pt-Al-7%YSZ coating applied on Rene-80 superalloy. The microstructural evaluation and phase identification were performed by field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD). The thermal shock test consisted of repeated cycles of rapid cooling between 900 °C and 400 °C. Prior to the thermal shock test, the specimen coated by Pt-electroplating, high-activity low-temperature (HALT) aluminizing and the thermal spray of YSZ ceramic layer. From surface towards the substrate; the coating consisted of 5 layers including: 7% YSZ, PtAl2 + β-(Ni,Pt)Al, β-(Ni,Pt)Al with fine precipitates rich in α-(Cr) and α-(W), precipitate-free β-NiAl, and inter-diffusion zone next to the specimen surface. After the thermal shock test, spallation and delamination have been observed in the YSZ layer. The Pt-Al layer was found in upper and lower parts of the coating with different Pt concentration. Also, the chemical composition of the Pt-Al layer demonstrated the removal of PtAl2 particles and β transformation to both γ and γ′ phases. Degradation of Pt-Al layer occurred by ratcheting the thermally grown oxide (TGO), rumpling, and cavitation. Comparison of weight changes showed that the thermal shock resistance is improved by Pt-Al-7% YSZ coating.