Analysis of Service-Retrieved TBC-Coated Industrial Gas Turbine Components

Abstract

Thermal barrier coatings (TBCs) have been widely used in gas turbine applications such as aerospace and power generation. TBC systems serve numerous purposes – the bond coat acts as a sacrificial layer for oxidation and the ceramic top coat works together with internal cooling systems to protect the superalloys from extreme temperature environments. With the rising demand for better fuel efficiency, the hot gas temperature within modern gas turbine engines has exceeded the working temperature of most advanced superalloys. The state-of-the-art TBCs have raised the high temperature capabilities of modern superalloys to a new level and have become an absolute necessity in modern gas turbine applications. While most research focus on the improvements of TBCs, the present study examines the environmental attacks which could lead to failures of TBCs. To be more exact, sand and dust particles often enter the mainstream hot gas flow path of gas turbines due to the powerful suction of its compressors, and internally generated particles, such as wear debris of the components, also could enter the mainstream hot gas flow path of the gas turbine. Once these particles (external and internal) pass through the combustion stage of the gas turbine engine, some of these particles become molten and adhere onto the surface of TBC-coated turbine components. These sand particles and debris gradually build-up in thickness and cause discoloration on the TBC surface. In some cases, the accumulated deposits could reduce the lifetime of TBCs. In other cases where particles carried by the mainstream hot gas flow path remain in solid state after passing through the hot combustion stage, such solid particles are very likely to impact the TBC coated turbine components, mainly the nozzle guide vanes and turbine blades right after the combustion chamber, damaging the TBCs. Since the gas stream inside the gas turbine engine travels at a high velocity, even micron size particles could build-up high kinetic energies. Upon striking the TBCs, these particles wear down the thickness of the TBCs, reducing its thermal insulation capability. Ex-service roll one (R1) turbine nozzle guide vanes and second stage turbine blades were retrieved from a land-based gas turbine for power generation and an aero-engine for transportation respectively, and the three nozzle guide vanes received were contaminated with surface deposits of various colours, while the turbine blades suffered from erosion and foreign object damage. These turbine components were analyzed using laboratory techniques, primarily by scanning electron microscopy with energy dispersive X-ray analysis and X-ray diffraction. In addition, heat treatment tests are also conducted to study the effect of these environmental attacks on the performance and lifetime of TBCs to determine the response of the deposit to prolonged thermal exposure.