Stability Study of EBC/TBC Hybrid System on Si-based Ceramics in Gas Turbines
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Stability Study of EBC/TBC Hybrid System on Si-based Ceramics in Gas Turbines JiaPeng Xu1, Vinod Sarin1,2, Soumendra Basu1,2 1 2
Division of Materials Science and Engineering, Boston University, Brookline, MA 02446, USA Department of Mechanical Engineering, Boston University, Boston, MA 02215, USA
ABSTRACT Currently, ceramics are being used under increasingly demanding environments. These materials have to exhibit phase stability and resist chemical attack during service. This research involves the study of the high-temperature stability of ceramic materials in gas turbines. SiC/SiC ceramic matrix composites (CMCs) are being increasingly used in the hot-sections of gas turbines, especially for aerospace applications. These CMCs are prone to recession of their surface if exposed to a flow of high-velocity water vapor, and to hot-corrosion when exposed to molten alkali salts. The objective of this investigation was the development of a hybrid system containing an environmental barrier coating (EBC) for protection of the CMC from chemical attack and a thermal barrier coating (TBC) that allows a steep temperature gradient across it to lower the temperature of the CMC for increased lifetimes. The EBC used was a functionally graded mullite (3Al2O3·2SiO2) coating deposited by chemical vapor deposition (CVD), while the TBC layer was yttria-stabilized zirconia (YSZ) deposited by air plasma spray (APS). The stability of this system was investigated, via adhesion between the two coating layers and the substrate, the physical and chemical stability of each layer at high temperature, and the performance under severe thermal shock and exposure to hot corrosion. INTRODUCTION Gas turbines operate under harsh environments and often require protective coatings on the hot-zone components. The most common protective coatings are environmental barrier coatings (EBCs) and thermal barrier coatings (TBCs). TBCs are designed to maintain a steep temperature gradient across them. This keeps the temperature of the substrate materials significantly cooler than the temperature of the hot gases in the hot-section. Recently, SiC/SiC ceramic matrix composite (CMC) consisting of SiC fibers in a SiC matrix have gained a lot of interest. However, for SiC based ceramic materials, there are two common degradation mechanisms, i.e. recession and hot-corrosion [1–4]. There is thus a need for EBCs to protect the ceramic materials from the chemical attack. Since EBCs & TBCs both improve the performance and lifetimes of the gas turbines in different aspects, it is logical to combine them into a hybrid coating system. A novel EBC/TBC hybrid coating system is proposed, in which the EBC layer and TBC layer are deposited by two different techniques, namely the functionally graded mullite EBC is deposited via CVD, and YSZ TBC is deposited via APS. At high temperature, protective mullite coatings have shown great potential for silicon based ceramics, and are currently successfully being used in industry as EBC’s [5]. This is mainly due to mullite’s close CTE matc
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