Meeting the demands for ever increasing operating temperatures in gas turbines requires concurrent development in cooling technologies, new generations of superalloys, and thermal barrier coatings (TBCs) with increased insulation capability. In the case of the latter, considerable research continues to focus on new coating material compositions, the alloying/doping of existing yttria stabilized zirconia ceramics, and the development of improved coating microstructures. The advent of the electron beam physical vapor deposition coating process has made it possible to consider the creation of multiple layered coating structures to meet specific performance requirements. In this paper, the advantages of layered structures are first reviewed in terms of their functions in impeding thermal conduction (via phonons) and thermal radiation (via photons). Subsequently, the design and performance of new multiple layered coating structures based on multiple layered stacks will be detailed. Designed with the primary objective to reduce thermal radiation transport through TBC systems, the multiple layered structures consist of several highly reflective multiple layered stacks, with each stack used to reflect a targeted radiation wavelength range. Two ceramic materials with alternating high and low refractive indices are used in the stacks to provide multiple-beam interference. A broadband reflection of the required wavelength range is obtained using a sufficient number of stacks. In order to achieve an 80% reflectance to thermal radiation in the wavelength range 0.3–5.3μm, 12 stacks, each containing 12 layers, are needed, resulting in a total thickness of 44.9μm. Using a one dimensional heat transfer model, the steady state heat transfer through the multiple layered TBC system is computed. Various coating configurations combining multiple layered stacks along with a single layer are evaluated in terms of the temperature profile in the TBC system. When compared with a base line single layered coating structure of the same thickness, it is estimated that the temperature on the metal surface can be reduced by as much as 90°C due to the use of multiple layered coating configurations. This reduction in metal surface temperature, however, diminishes with increasing the scattering coefficient of the coating and the total coating thickness. It is also apparent that using a multiple layered structure throughout the coating thickness may not offer the best thermal insulation; rather, placing multiple layered stacks on top of a single layer can provide a more efficient approach to reducing the heat transport of the TBC system.
Microstructure Evolution and Thermal Durability with Coating Thickness in APS Thermal Barrier Coatings
