Although thermal barrier coatings (TBCs), most commonly composed of yttria-stabilized zirconia (YSZ), provide beneficial thermal protection for turbine engine components, the risk of premature failure compromises the reliability of TBCs. Nondestructive diagnostic tools that could reliably assess the damage state of TBCs would alleviate the risk of premature TBC failure by indicating when the TBC needed to be replaced before the level of TBC damage threatened engine performance or safety. To meet this need, researchers at the NASA Glenn Research Center in collaboration with Penn State University successfully implemented a new TBC coating design that is self-indicating for delamination.
Concept for a TBC that is self-indicating for delamination. This TBC is translucent to 532-nm excitation, which excites europium ion (Eu3+) emission at 606 nm from a buried Eu-doped layer. Emissions of 606-nm Eu3+ through the overlying undoped YSZ are enhanced by delamination-crack-induced total internal reflection. PtAl, platinum aluminide.
The preceding figure illustrates the concept of using luminescence sensing to detect TBC delamination. The YSZ TBC incorporates an initial sublayer that is doped with europium (Eu) below the overlying undoped YSZ. Strong luminescence at a wavelength of 606 nm (red) from the buried Eu-doped layer can be excited by a 532-nm (green) wavelength excitation that travels through the overlying undoped YSZ. Luminescence enhancement occurs when delamination cracks occur within or below the Eu-doped layer (see the preceding figure). The enhanced luminescence arises because of the total internal reflection of a large fraction of both the excitation and emission radiation at the interface between the Eu-doped layer and the delamination crack.
TBCs with a 7-μm-thick Eu-doped YSZ sublayer beneath a 125-μm undoped YSZ overlayer were deposited by multiple-ingot electron-beam physical-vapor deposition without disrupting TBC growth. To demonstrate delamination indication, researchers induced a localized TBC delamination by scratching the coating with a stylus. The delaminated region can be faintly discriminated in a standard white light image (see the following image on the left), but it stands out strongly in a luminescence image because of the greatly enhanced red emission originating from that area (see the image on the right). Luminescence imaging was very simple to implement and can be achieved by using only a light-emitting diode (LED) illumination source and a camera with a bandpass filter. High-resolution luminescence images were obtained within a few seconds that immediately identified regions of TBC delamination that would otherwise be difficult to detect, thereby showing great promise for the routine inspection of TBCs. Funding for this work was provided by the Propulsion 21 Thermal Management and Advanced Cooling Task.
Delamination-indicating thermal barrier coating. Enhanced Eu3+ 606-nm luminescence detected from scratch-induced delaminated region. The TBC consists of a 7-μm-thick Eu-doped YSZ sublayer beneath a 125-μm undoped YSZ overlayer. Left: White light image. Right: Eu3+ luminescence image.
Find out more about the research of Glenn’s Durability and Protective Coatings Branch: http://www.grc.nasa.gov/WWW/EDB/
Glenn contacts:
Dr. Jeffrey I. Eldridge, 216-433-6074, Jeffrey.I.Eldridge@nasa.gov; and Timothy J. Bencic, 216-433-5690, Timothy.J.Bencic@nasa.gov
Authors:
Dr. Jeffrey I. Eldridge and Timothy J. Bencic
Headquarters program office:
Aeronautics Research
Programs/Projects:
Propulsion 21, UEET
Last updated: October 16, 2006
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