Coated turbine blades
The Argonne National Lab website has a nice article discussing the work of a research team from Northwestern University, Rolls-Royce and ANL to improve the performance of materials used in power-generating turbines, improvements that can decrease weight, increase temperature tolerance, extend component life and, thus, increase the overall efficiency of these units that are expected to play an increasing role in future power grid planning. In particular, the group is addressing problems related to environmental barrier coatings that are applied to silicon-based ceramic substrates, such as silicon carbide, to limit oxidation and other types of degradation. The coatings generally work, but there have been concerns related to “mismatches” between the EBCs and the substrates as they are exposed to huge temperature and pressure changes
However, as temperatures are raised and lowered during the combustion cycle, internal mismatch stresses arise as the coatings and substrate expand and shrink at different rates. This is especially problematic in turbine engines, since temperatures can change over 1200º C in a single cycle. Stresses are exacerbated over tens of thousands of hours of operation, so even small mismatches in expansion can cause fractures in coatings and subsequent component failure. Minimization of these stresses is critical to protecting the materials.
ANL houses the Department of Energy’s Advanced Photon Source, so the researchers used the high-energy synchrotron radiation at the APS to measure these stresses using combinations of materials to learn which ones reduce the mismatch.
Results from the studies demonstrated that coating mismatch stresses could be minimized in a three-layer coating system heat-treated to provide a low-expansion equilibrium topcoat. The heat treatment also served to heal many of the cracks formed during coating deposition. It was determined that the low stress levels made the heat-treated coating an ideal system to withstand the harsh environments of gas-turbine engines.
Kang N. Lee, Senior Specialist of Materials, Processes and Repair Technology at Rolls-Royce told ANL, “These studies shed light on the development of next generation EBCs, enabling the implementation of silicon-based ceramic components in turbine engines.” Lee is a co-author (along with B. J. Harder, J. D. Almer, C. M. Weyant and K. T. Faber) of a paper about this research that appeared in the February 2009 edition of the Journal of the American Ceramic Society.