Corrosion behavior of Gd₂Zr₂O₇ thermal barrier coatings under Fe-containing environmental sediment attack

Environmental sediments mainly consisting of CaO–MgO–Al₂O₃–SiO₂ (CMAS) corrosion are a serious threat to thermal barrier coatings (TBCs), in which Fe element is usually ignored. Gd₂Zr₂O₇ TBCs are famous for their excellent CMAS resistance. In this study, the characteristics of Fe-containing environmental sediments (CMAS-Fe) and their corrosiveness to Gd₂Zr₂O₇ 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 Gd₂Zr₂O₇ 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, ZrO₂, 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 Gd₂Zr₂O₇ 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, the Gd₂Zr₂O₇ coatings had a large consumption in the thickness, degrading the coating performance. Therefore, the Gd₂Zr₂O₇ coatings exhibit unsatisfactory corrosion resistance to CMAS-Fe attack.