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 Ma2–x MxMb2O7, where x lies between 0 and 0.5 and M denotes a rare earth or other suitable element.
In experiments, powders of various compositions were synthesized by a modified sol-gel method and calcined at appropriate temperatures to convert them into compounds of pyrochlore structure as confirmed by x-ray diffraction. These powders were hot pressed into dense disks of 1-in. (2.54-cm) diameter. The thermal conductivities of the disks were measured at various temperatures up to 1,550 °C by use of a steady state laser heat-flux technique. The figure presents results of such measurements performed on several materials of general composition La2–x(Gd and/or Yb)xZr2O7, where x = 0 or 0.3. The thermal conductivities of all doped samples (x = 0.3) were less than those of the undoped (x = 0) sample [La2Zr2O7]. The lowest conductivity — ranging from 40 to 50 percent below that of undoped sample — was exhibited by the sample co-doped with both Gd and Yb.
This work was done by Narottam P. Bansal of Glenn Research Center and Dongming Zhu of the U. S. Army Research Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category.
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