The University of Connecticut, in cooperation with Pratt & Whitney and Siemens Energy, will use a novel solution precursor plasma spray (SPPS) process to deposit low thermal conductivity, high durability yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) and to utilize surface protective layers (SPLs) to provide high temperature contaminant resistance and increased surface temperature capability, while minimizing the use of rare earth elements. The SPPS process provides a unique TBC microstructure that consists of through-thickness vertical cracks for strain tolerance, ultra-fine splats for spallation crack resistance, and the ability to produce very low thermal conductivities. The low thermal conductivities, or low-K (with K representing heat conductivity), will result from planar arrays of fine porosity called inter-pass boundaries (IPBs) and the ability to vary porosity content over wide ranges. The IPBs are generic conductivity lowering features that can be implemented for any TBC. Pratt & Whitney and Siemens Energy will contribute valuable specimens and will test the down-selected low-K TBCs.
The low-K YSZ TBC structure will be created using the SPPS process. Taguchi methods for design of experiments will be used to systematically determine the deposition parameters to minimize the thermal conductivity. Cyclic durability tests of the low-K TBC will be conducted to verify that the excellent durability of the SPPS TBC is preserved. A protective surface layer of gadolinium-zirconium will be added to the low-K YSZ TBC to increase the maximum allowable surface temperature by at least 100 degrees Celsius (°C) and improve its calcium-magnesium-alumina-silicate (CMAS) resistance, while minimizing the heavy use of rare earth elements. This improved contaminant resistance will be verified by testing in CMAS and moisture. Contaminant resistance will be further enhanced by employing two novel approaches, one using aluminum-titanium additions to the YSZ, and the other by deliberately adding calcium sulfate, shown to block CMAS when deposited by natural processes in service engines. The spallation failure mechanisms of the low-K TBC will be defined to provide guidance for subsequent improvements and a sound basis for life prediction methodologies.
This project will use a novel solution precursor plasma spray (SPPS) process to deposit thermal barrier coatings (TBCs). Turbine materials research seeks to improve coating materials that will allow for higher temperature operation and increased durability leading to increased turbine efficiency and reduced maintenance. Specifically, this project will determine the depostion parameters to use with SPPS, use them to apply a TBC with unique microstructure that will have high thermal conductivity with reduced rare earth metals, and analyze the resulting coatings for durability.
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