Correlation of thermal characteristics and microstructure of multilayer electron beam physical vapor deposition thermal barrier coatings

摘要

Referring to the expected increasing amount of aviation until the year 2030, energy and fuel efficiency as well as harmful emissions are of prime importance to the technological and ecological advancement of aircraft engines. Minimizing fuel consumption and improving energy efficiency of jet engines can be reached by increased turbine entry temperatures. However, the permitted combustion temperatures are restricted by material-dependent maximum service temperatures. Nowadays, yttria stabilized zirconia (YSZ) is commonly used as thermal barrier coating (TBC), only withstanding a permanent surface temperature of T similar to 1200 degrees C. In recent years, research projects have shown that multilayer systems might be a solution to withstand higher combustion temperatures. In order to assess this potential of multilayer systems, quadruple TBC consisting of 7 wt.% YSZ and La2Zr2O7 were deposited on INCONEL (R) 600 by electron beam physical vapor deposition. The quadruple TBC were fundamentally characterized with regard to their microstructure as well as their thermal conductivity lambda. Considering the requirements with regard to the application, the long term behavior under isothermal and thermal cycling load is of great importance. To classify this behavior, atmospheric annealing tests were performed at temperatures of T = 1200 degrees C and T = 1300 degrees C for t = 50 h as well as thermal cycling tests were conducted at a temperature of T = 1150 degrees C for N = 1000 cycles. The results of the quadruple layer TBC were compared to the prior investigated single and double layered TBC coatings consisting of YSZ or 7YSZ/La2Zr2O7. The investigations showed no major influence of multilayer architecture regarding the thermal conductivity lambda compared to conventional 7YSZ single layer architecture. Moreover, multilayer architecture leads to the increase of porosity inside the TBC. However, no strong correlation between porosity and thermal conductivity can be revealed. The analyses of the thermal cycled TBC show that TBC delamination is mainly caused by accelerated growth of thermally grown oxide and sintering effects. Furthermore, no improvement of the thermal cycling behavior due to quadruple multilayer architecture compared to 7YSZ single layer can be determined. The comparison of thermal cycling behavior regarding single, double and quadruple layer architecture emphasizes increased TBC lifetime of double layer TBC.