Corrosion behavior of Gd <sub>2</sub>Zr <sub>2</sub>O <sub>7</sub> thermal barrier coatings under Fe-containing environmental sediment attack
Lei Guo, Yanyan Li, Kai Yan
Abstract
Environmental sediments mainly consisting of CaO-MgO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> (CMAS) corrosion is a series threat to thermal barrier coatings (TBCs), in which Fe element is usually ignored. Gd<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> TBCs are famous for their excellent CMAS resistance. In this study, the characteristics of Fe-containing environmental sediments (CMAS-Fe) and their corrosiveness to Gd<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> coatings were investigated. Four types of CMAS-Fe glass with different Fe contents were fabricated. Their melting points were measured to be 1322-1344 ℃, and the high-temperature viscosity showed a decreasing trend with increasing Fe contents. The corrosion behavior of four types of CMAS-Fe to Gd<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> coatings at 1350 ℃ was investigated. At the initial corrosion stage (0.1 h), anorthite was precipitated in CMAS-Fe with a high Ca:Si ratio, while Fe-garnet was formed in the melt with the highest Fe content. Prolonging the corrosion time resulted in the formation of a reaction layer, which exhibited an interpenetrating network composed of Gd-oxyapatite, ZrO<sub>2</sub>, and residual CMAS-Fe. Some spinel was precipitated within the reaction layer. After 1 h or even longer time, the reaction layers tended to be stable and compact, which had comparable hardness and fracture toughness to those of Gd<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> coatings. Under the cyclic CMAS-Fe attack, the residual CMAS-Fe in the interpenetrating network provided a pathway for the redeposited CMAS-Fe infiltration, resulting in the continuous growth of the reaction layer. As a result, Gd<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> coatings had a large consumption in thickness, degrading the coating performance. Therefore, Gd<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> coatings exhibit unsatisfactory corrosion resistance to CMAS-Fe attack.