Stability Study of EBC/TBC Hybrid System on Si-based Ceramics in Gas Turbines

Abstract

ABSTRACTCurrently, 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.