The Fabrication, High Heat Flux Testing, and Failure Analysis of Thermal Barrier Coatings for Power Generation Gas Turbines

Thermal barrier coatings (TBC) are used to protect the hot components of gas turbines engines to enhance thermal efficiency and component service life. The coating, based on yttria stabilized zirconia, is used in this study. In this paper high heat flux testing with a temperature gradient across the coating thickness of TBC coated coupons is presented. These buttons are subject to precisely-controlled laser heating on the top side and compressed air cooling on the bottom side. Analysis of the thermal conductivity change with respect to heating time and peak temperature, failure assessment, and metallurgical examination are also presented. Some important results of using this method of testing are: definition of the service time vs temperature relationship for TBC lifetime; improved durability of TBCs under severe environmental conditions; determination of effective steady-state sintering conductivity; identification of onset of coating cracking and delamination; adjustable peak temperature, automated and accelerated thermal cycling, etc. This leads to faster testing turn-around for TBC development. Two different types of heating modes can be employed: soak test and cycle test. In soak tests, coated coupons are subjected to steady laser heat flux for up to 12 hr. In cycle tests, the laser heat flux is on for one hour and then off to cool the coated coupons for three minutes. Coupon top surface temperatures from 1200 to 1528°C are maintained in various test cases. At the highest temperature test cases, delamination of TBC (cycle test) and surface crack (soak test) are observed. All key measurements (temperatures, laser power to coupon, compressed air flow rate, etc.) are recorded per second. The normalized thermal conductivity can be computed in real time or processed after the test. It is found that the normalized thermal conductivity increases in the first few hours or cycles of heating and it either reaches a near steady state value or decreases due to surface cracking or delamination in the later state of testing.