In the pursuit of improving safety and operating economics for gas turbines
The performance of advanced gas turbines, predominating as power sources for aerial and marine transportation, as well as for electricity generation, can be considerably increased by applying higher gas temperatures. The usage of thermal barriers coatings (TBCs) on the critical superalloy turbine components makes such an increase possible by protecting them from thermo-mechanical fatigue and oxidation. Thermal barriers coatings comprise insulation of enough thickness and durability to sustain a sufficient temperature difference between the loads bearing superalloy substrate and the ceramic topcoat. Within the scope of the HIPERCOAT project, scientists at the University of California-Santa Barbara focused on morphological instabilities in the thermally grown oxide (TGO) that forms on the bond coat due to initial interface imperfections. These instabilities occur when the initial non-planarity in the TGO grows in amplitude as the system experiences thermal cycling at very high rates of temperature change. By numerical means, the contribution of displacements induced by the bond coat's plastic deformation and strains that arise due to the thermal expansion misfit with the superalloy substrate was explored. The analytical results of in-plane stresses provided invaluable insights on failure mechanisms governing their failure. Since these cause cracks to form and extend laterally, the multi-layered thermal barriers systems' integrity is jeopardised. While the mechanistic understanding remains incomplete, it has nevertheless provided an informed basis for discerning the attributes of current processing procedures and for guiding durability enhancement efforts. This work, which focused on the microstructure and properties of as-processed and thermally treated materials, will be complemented by experimental evaluation in industrial case studies. The findings should prove useful to a much wider range of applications, where structural integrity must be sustained under severe conditions.
