Lower-Conductivity Ceramic Materials for Thermal-Barrier Coatings

Thermal conductivities of certain pyrochlore oxides can be reduced by doping.

Doped pyrochlore oxides of a type described below are under consideration as alternative materials for high temperature thermal barrier coatings (TBCs). In comparison with partially yttria stabilized zirconia (YSZ), which is the state of the art TBC material now in commercial use, these doped pyrochlore oxides exhibit lower thermal conductivities, which could be exploited to obtain the following advantages:

• For a given difference in temperature between an outer coating surface and the coating/substrate interface, the coating could be thinner. Reductions in coating thicknesses could translate to reductions in weight of hot-section components of turbine engines (e.g., combustor liners, blades, and vanes) to which TBCs are typically applied.
• For a given coating thickness, the difference in temperature between the outer coating surface and the coating/substrate interface could be greater. For turbine engines, this could translate to higher operating temperatures, with consequent increases in efficiency and reductions in polluting emissions.

TBCs are needed because the temperatures in some turbine-engine hot sections exceed the maximum temperatures that the substrate materials (superalloys, Sibased ceramics, and others) can withstand. YSZ TBCs are applied to engine components as thin layers by plasma spraying or electron-beam physical vapor deposition. During operation at higher temperatures, YSZ layers undergo sintering, which increases their thermal conductivities and thereby renders them less effective as TBCs. Moreover, the sintered YSZ TBCs are less tolerant of stress and strain and, hence, are less durable.

The materials that are sought as alternatives to YSZ are required to have and retain lower thermal conductivities and to be better able to withstand temperatures that degrade TBCs made of YSZ. The undoped versions of the doped pyrochlore oxides of the type now under consideration as alternatives to YSZ are of general composition Ma₂Mb₂O₇, where Ma denotes a 3+ cation (for example, La to Lu) and Mb a 4+ cation (for example, Zr, Hf, Ti). Doping has been investigated as a means of reducing thermal conductivities even further below those of YSZ coatings. In the doping approach investigated thus far, another cation is substituted for part of Ma, yielding a general composition of Ma_2–x M_xMb₂O₇, where x lies between 0 and 0.5 and M denotes a rare earth or other suitable element.