Studies of the bond-coat oxidation and phase structure of TBCs

To increase the power efficiency of stationary gas turbines, the turbine inlet temperature will in future increase to more than 1400 degrees C. At the same time, the cooling air flow has to be controlled carefully, avoiding an increase in the NO, level. In the new generation of gas turbines, thermal barrier coatings (TBC) are an essential tool to save cooling air. Electron beam physical vapour deposition (EB-PVD) deposited TBCs show better results in terms of life expectancy under the stresses which occur in stationary gas turbines. The durability of EB-PVD deposited TBCs is influenced by induced stresses and the growth of the TGO (thermally grown oxide) with temperature and time. To study TGO formation between the bond coat and the TBC, exposure experiments up to 3000 h were performed at 900, 950 and 1000 degrees C with post-oxidation studies using electron microprobe analysis. In situ measurements using high-temperature X-ray diffraction were carried out on a pulverized yttrium partially stabilized (YPS) zirconia ceramic and on the same ceramic as an EB-PVD deposited coating during heating and cooling between 25 and 1200 degrees C. The tetragonal lattice parameters a and c were determined as a function of the temperature, and showed a different expansion behaviour for the pulverized ceramic and for the EB-PVD deposited coating. (C) 1999 Published by Elsevier Science S.A. All rights reserved.