Title

Novel low-thermal-conductivity ceramics for thermal barrier coating applications

Date of Completion

January 2004

Keywords

Engineering, Metallurgy|Engineering, Materials Science

Degree

Ph.D.

Abstract

Thermal barrier coatings (TBCs) made from ceramics with low thermal conductivities will provide greater insulation relative to the current 7YSZ (7 wt% yttria-stabilized zirconia) TBCs, and permit higher turbine operating temperatures and/or increased durability. Ceramics in the xGdO1.5-(1 − x)ZrO 2 system have been identified for use as new TBCs. The thermal conductivity of these ceramics (in monolithic, dense form) as a function of composition and temperature has been measured using the laser-flash method. These results have been analyzed using a thermal conductivity model based on phonon-scattering theory established in this study. It has been shown that the thermal conductivities of GdlZr2O7 (with pyrochlore and fluorite structures) and fluorite 0.15GdO1.5-0.85ZrO2 in this system are ∼30% lower than that of the conventional TBC ceramic, i.e., 7YSZ. However, Gd 2Zr2O7 was found to react with α-Al 2O3 at high temperatures, which is undesirable for TBC applications. We have determined that 0.15GdO1.5-0.85ZrO2 is a composition of interest for novel TBC applications due to its low thermal conductivity and chemical compatibility with α-Al2O3 at high temperatures. ^ The selected composition, i.e., 0.15GdO1.5-0.85ZrO 2, has been electron-beam physical-vapor deposited (EB-PVD) on (Pt, NO Al bond-coated superalloy substrates as TBCs. Phase stability and initial microstructure of the new TBCs have been investigated. These TBCs have been thermo-mechanically tested using thermal cycling. The evolution of the microstructures and damage/failure modes in the thermally-cycled TBCs have been investigated. Spallation within the topcoat ceramic near the topcoat/TGO interface was observed. It was found that the unexpected short cyclic life of these new coatings can be attributed to the weakness of initial microstructure and the non-uniform TGO growth. A new failure mechanism for these TBCs has been proposed. ^