Researchers from Tianjin University have developed new strategies to protect thermal barrier coatings against environmental corrosion.
Previous research has focused on the role of SiO2, CaO, MgO and Al2O3 in CMAS glass skeleton formation, while the role of Fe2O3 has largely been overlooked. Additionally, the corrosion behavior of Gd2Zr2O7 coatings under Fe-containing environmental deposits remains unclear. In a study published in the Journal of Advanced Ceramics, the effect of Fe on the CMAS-Fe silicate glass skeleton is investigated, examining the microstructural evolution and corrosion degradation behavior of Gd2Zr2O7 coatings under Fe-rich environmental deposits.
The presence of Fe can alter the structure of the CMAS silicate glass skeleton, affecting the melting point and viscosity of the melt: Lei Guo, senior author of the paper and Associate Professor at the School of Materials Science and Engineering at Tianjin University, stated, "CMAS-Fe retains [SiO4] tetrahedra as the basic glass skeleton structure. Fe acts as a network intermediate element in CMAS-Fe glass and can form [FeO4] tetrahedra participating in glass skeleton formation, which increases the melting point of CMAS-Fe."
Dr. Guo, a senior expert in the field of thermal barrier coatings, continued his explanation: "However, at high temperatures, the transformation of [FeO4] tetrahedra to octahedral [FeO6] structures disrupts the glass polymerization. Therefore, the higher the Fe content, the lower the high-temperature viscosity of CMAS-Fe."
Gd2Zr2O7 coating possesses exceptional potential as a new TBC material with extremely low thermal conductivity, an appropriate thermal expansion coefficient, and thermally stable high-temperature phase structure, capable of withstanding temperatures exceeding 1,400°C. The study yielded interesting findings when the coating was exposed to various components of CMAS-Fe. "The precipitation of crystallization products, depending on Fe content and corrosion duration, affects the corrosion resistance of Gd2Zr2O7 coating against CMAS-Fe. During initial corrosion, a high Ca:Si ratio facilitates anorthite precipitation and thereby increases melt viscosity," said Lei Guo, continuing, "High Fe content, on the other hand, promotes Fe-garnet precipitation. Long-term corrosion results in an interlocking network structure composed of Gd-oxyapatite, ZrO2 and residual CMAS-Fe. Attack by CMAS-Fe with higher Fe content leads to more severe degradation of crystallization products."
Under CMAS-Fe cyclic corrosion conditions, the performance of Gd2Zr2O7 coatings is unsatisfactory. Lei Guo stated, "The remaining CMAS-Fe in the interlocking network provides a pathway for renewed CMAS-Fe penetration. The precipitation of crystallization products affects the melt composition, significantly impacting melt viscosity and penetration rate."
Gd2Zr2O7 coatings mitigate the harmful effects of the melt through self-consumption, which leads to continued growth of the reaction layer and degradation in coating performance. In the context of developing new TBC materials against environmental deposit corrosion, it is crucial to comprehensively address the effects of various components. Therefore, more refined research is needed to discover the mechanical effects of specific environmental deposit components on the corrosion resistance of TBCs.
This contributes to the development of TBC modification strategies adapted to specific local conditions. On this basis, Guo also outlined some promising development directions including composition modification by adding rare earth elements to coating modification, surface laser polishing treatment, and composite coating structure design.
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