Project Final Report “ Spectroscopic In - Situ Non - Destructive Evaluation to Monitor the Health of Thermal Barrier Coatings ”

Thermal barrier coatings (TBC) on turbine hot section components require regular maintenance by visual inspection for which the engine is taken out of service and partially disassembled. Early detection of the TBC degradation and failure is critical to prevent catastrophic engine failure. Under this project it was proposed to incorporate Li2O as a maker material in the TBC away from the surface (preferably near the bond coat) and to explore the feasibility of using the lithium emission spectral response as an in situ NDE tool to monitor the health of TBCs (cracking, spallation, etc.) on gas turbine engine hot section component. TBC coating formulation and architecture were identified that wwere resistant to thermal cycling damage typically encountered in gas turbine engines. The oxyacetylene welding flame and flat-flame burner tests on coated superalloy disk coupons indicated that lithium emission signal intensity was proportional to the amount of lithium present in the Li2O-YSZ coat, the intensity increased with increasing temperature, and crack and spall simulation yielded increased intensity. However, tests in the high gas-flow and high-temperature environment of the burner rig were not satisfactory. Even though lithium emission signals were readily detectable for the lowest marker level (1-wt% Li2O and 99-wt% YSZ) examined in this study even in the very bright environment of the burner-rig exhaust stream, the plasma-sprayed YSZ top coat were found to be pervious to lithium vapor. As a result lithium vapor continued to leak through the TBC causing a decrease in the emission intensity as a function of time. An impervious top coat TBC layer is required for the emission detection technique proposed under this project to work.