By using high energy X-rays produced by the synchrotron facility at the advanced photon source (APS) in Argonne near to Chicago, an international research team with members from DLR has succeeded in direct strain measurement in the different layers of a protective coating for jet engine turbine blades at 1000°C under close to service conditions. The results are important for modelling the lifetime of the coated components and could ultimately help manufacturers to improve the coating systems.
In nearly all modern jet engines protective coatings are employed and are particularly important. On the turbine blades directly behind the combustion chamber, which are most affected by thermal and mechanical loads, are multiple thin layers applied: a ca. 100 micron thin metallic coating provides protection against oxidation while a ca. 150 – 200 micron thin ceramic layer maintains a temperature difference of about 100°C between the hot combustion gases and the metallic substrate.
In the Institute of Materials Research protective coating systems are developed and tested in the laboratory under close to service conditions. However, it was not yet possible to view into the coating system during the thermal and mechanical testing and to measure the strain in the individual layers in real time.
In a collaborative work with Kevin Knipe and Albert Manero from the team of Prof. Seetha Raghavan at the University of Central Florida the DLR researcher Janine Wischek, Carla Meid, and Prof. Marion Bartsch developed a new testing facility similar to that employed at DLR. Important differences are that the new apparatus fits exactly into the testing machine operating in a laboratory of the APS in Argonne and provides a duct for the high energy X-ray beam to the coated test specimen. The testing machine with the clamped coated specimen can be positioned with an accuracy of one micron with respect to the X-ray beam. The X-rays are diffracted at the crystalline materials of the coating system and then collected by a detector. Analysing the pattern of the diffracted X-rays on the detector the strain of the single layers is calculated.
For conducting the experiments the teams from Orlando in Florida and Cologne met in Argonne. Within four days of round-the-clock testing under the supervision of Dr. Jonathan Almer und Dr. John Okasinski the data have been collected, which now have been published in Nature Communications. Also involved in the current project is a team led by Prof. Anette Karlsson, Cleveland State University, who uses the experimental data base for developing numerical models of the coating system.