Non-Destructive Evaluation and Maintenance of Thermal Barrier Coatings

Abstract Thermal Barrier Coatings (TBC) composed of Yttria-stabilized Zirconia (YSZ) ceramic are widely employed in gas turbine engines to insulate nickel-based components from the extreme temperatures of the gas stream. Over the course of extensive operation, these coatings delaminate and eventually succumb to spallation failure. Photoluminescence Piezospectroscopy (PLPS) is employed as a non-destructive evaluation (NDE) technique for the predictive determination of the operational lifetime of these coating systems. A mobile analysis unit has been developed to provide remaining life analysis for EB-PVD applied TBCs of engine-run turbine blades. This technology includes a Laser-Induced Breakdown Spectroscopy (LIBS) capability utilized for precision removal of oxide formations on the coating surface. Expansions to this technology were undertaken to explore its fuller potential, including a performance optimization of the LIBS and PLPS subroutines, and evaluated during field demonstrations. Mobile analysis capability removes the necessity of an off-site laboratory, but the savings in labor and time would compound quickly if in-situ measurements on a mounted engine were enabled. Ground work was laid for a borescope adaptation of the NDE technique, involving the integration of optical fiber components into the laser ablation and lifetime analysis processes. When coating systems reach the end of service life, removal of the topcoat becomes desirable, so that it can be replaced. A prospective laser-ablation TBC removal method was tested to acquire evidence concerning its utility for coated systems