A research team involving the U.S. Department of Energy’s Argonne National Laboratory, the German Aerospace Center and the universities of Central Florida and Cleveland State has developed an in-situ facility for use at Argonne's Advanced Photon Source (APS) that accurately simulates extreme turbine engine conditions.
In particular, the Florida team developed an improved furnace system and the German team developed a coolant system to add to the mechanical testing system at Sector 1 of the APS, where high-energy X-rays were able to penetrate all layers of a coated test blade.
Researchers say this goes beyond other in-situ capabilities to allow the influence of temperature, stress and thermal gradients to be studied together. This trifecta enables scientists and engineers to view the microstructure and internal strain in both the substrate and thermal barrier coating system during real operating conditions and in real time.
The ability to operate turbine blades at higher temperatures improves efficiency and reduces energy costs. For example, researchers say that energy companies estimate that raising the operating temperature by 1% at a single electric generation facility can save up to $20 million a year. In order to achieve temperatures of 1,832 degrees Fahrenheit in engines, metallic turbine blades are coated with ceramic thermal-barrier coatings and actively air cooled, which together allows operating temperatures exceeding the metal’s melting point. Adding to these extreme conditions, during high-temperature operation, blade rotation induces thermo-mechanical stresses throughout the blade components.
Because of the difficulty of monitoring engines in operation, most manufacturers test blades either after flight (in the case of aircraft turbines) or rely on simulated tests to give them the data on how the various coatings on the blades are performing.
The research team captured high-resolution images of evolving strains and plans in future experiments to pinpoint when and where defects start. This would allow for an accurate lifespan estimate on material and improve the process for applying ceramic thermobarrier coatings. Industry could benefit through improved plasma spray applications and reduced cost of electron beam physical vapor deposition applications.
“This integrated approach allows us to simulate the engine conditions so manufactures are getting interested,” says Jon Almer, a co-author of a paper published in the July issue of the journal Nature Communications.