Advanced multicomponent thermal and environmental barrier coatings were developed at the NASA Glenn Research Center for low-emission SiC/SiC ceramic matrix composite (CMC) combustor liner and vane applications by extending the component temperature capability to 1650 °C (3000 °F) in oxidizing and water-vapor-containing combustion environments (refs. 1 to 4). The coating systems demonstrated improved phase stability, lower lattice and radiation thermal conductivity, and improved sintering and thermal stress resistance under simulated engine heat-flux and thermal cycling conditions. These improvements were due largely to the defect-clustering structures, which are purposely designed to promote the creation of thermodynamically stable oxide defect clusters and/or nanophases within the coating systems.
High-resolution transmission-electron-microscopy lattice image with moiré patterns that indicate defect clusters in a plasma-sprayed HfO2-(Y,Nd,Yb)2O3 coating.
The preceding figure is a high-resolution transmission-electron-microscopy lattice image that reveals atomic-level defect clusters through moiré patterns in a plasma-sprayed multicomponent HfO2-(Y,Nd,Yb)2O3 coating. This is the first time that defect clusters have been characterized and demonstrated for advanced HfO2 systems.
As mapped in the left graph, the thermal conductivity of plasma-sprayed HfO2-based coatings was determined by a laser heat-flux technique at 1650 °C. The initial thermal conductivity and 20-hr sintering thermal conductivity of the HfO2-Y2O3 coatings generally decreases as the Y2O3 concentration increases, indicating that thermal stability increases with Y2O3 content. However, as shown in this example, the advanced, multicomponent rare-earth-doped HfO2-(Y,Nd,Yb)2O3 coatings achieved even lower thermal conductivity and greater thermal stability than the HfO2-Y2O3 pseudobinary oxide systems.
Left: Thermal conductivity of plasma-sprayed HfO2-based coatings determined by a laser heat-flux technique. Right: Multicomponent HfO2-18mol%(Y,Gd,Yb)2O3/rare-earth-doped mullite/HfO2-based thermal and environmental barrier coating systems demonstrated 300-hr long-term 1650 °C sintering and cyclic durability on a SiC/SiC CMC substrate under thermal gradient cyclic testing. Surface temperature, 1650 °C (3000 °F); interface temperature, 1316 °C (2400 °F).
The multicomponent HfO2-based coatings demonstrated long-term cyclic durability and water vapor stability. The graph on the right illustrates the durability exhibited by the multicomponent HfO2-18mol%(Y,Gd,Yb)2O3 coating on a modified mullite/mullite + 20wt%BSAS/Si environmental barrier coating (EBC) system during 1650 °C thermal cycling testing. The coating system possessed a relatively low and stable thermal conductivity, without visual damage after the testing, demonstrating sintering stability and cyclic durability on SiC/SiC composites under the thermal-gradient cyclic-testing conditions (300 hr at 1650 °C). In addition, 6-atm high-pressure burner rig results (see the final figure) indicated the excellent water vapor stability of the coating system in burner rig combustion environments. Continued coating system development is underway to further improve the coating temperature capability and durability under simulated engine environments while reducing the overall coating thickness and surface roughness.
Multicomponent HfO2-(Y,Gd,Yb)2O3/rare-earth-doped mullite/Si thermal and environmental barrier coating systems demonstrated excellent water vapor stability and durability in high-pressure burner rig testing environments. Gas temperature, 1454 °C (2650 °F) at 6 atm; gas velocity, 25 m/sec. Left: Weight changes versus time. Right: Specimen after testing.
U.S. Army Research Laboratory at Glenn contact:
Dr. Dongming Zhu, 216-433-5422, Dongming.Zhu@nasa.gov
Authors:
Dr. Dongming Zhu and Dr. Robert A. Miller
Headquarters program office:
Aeronautics Research
Programs/Projects:
UEET, LEAP
Last updated: October 16, 2006
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