Comparative study on microstructure evolution and failure mechanisms of ordinary and refurbished EB-PVD TBC under cyclic oxidation

Refurbishment of thermal barrier coating (TBC) has become a valuable technique to prolong the service life of high-temperature components. This study investigates the effect of the refurbishment process (coating removal and recoating) on the microstructure evolution and physical properties of TBC, including oxidation characteristics, element diffusion behavior, and crack failure mechanisms. The results showed that a certain amount of interdiffusion zone (IDZ) with Cr-rich would be retained in DD6 superalloy substrate after coating removal. The microstructure of the refurbished specimens showed equiaxed β-NiAl phases, while the ordinary specimens have elongated grain shapes with a high aspect ratio. Moreover, mixed oxides in the refurbished TBC specimens were earlier observed during cyclic oxidation, with a greater thickness compared to ordinary TBC, due to the influence of BC layer phase sizes. The growth mechanism of thermally grown oxide (TGO-Al₂O₃ layer) in the refurbished TBC specimens was also different, resulting from the different mechanisms of mixed oxides growth. Furthermore, under cyclic oxidation with water quenching at 1100 ℃, the cracks in the refurbished specimen tend to occur in the mixed oxides layer, while the cracks in the ordinary specimen occur in the top coat (TC) layer, attributing to the earlier and thicker mixed oxides layer formed in refurbished specimens.