Thermoresistant Oxide Nanocomposite: Synthesis and Characterization
Thermal barrier coatings (TBC) used for protection of the gas turbines hottest parts must enable modern engines to operate at enhanced temperatures that increases efficiency and improve performance of advanced gas turbines . So, TBC consisting of two layers: an oxidation-resistant metallic bond coat and a porous and thermally insulating ceramic topcoat must meet severe requirements. The operation under extreme conditions leads to phase transition and accelerated sintering of yttria-stabilized zirconia (YSZ) coatings usually used as topcoat. Hence, novel complex oxides with sufficiently low thermal conductivity, high sintering resistance and high phase stability were suggested as high performance TBC topcoats .
In this work, nanocomposites comprised of La2Zr2O7 and LaAlO3 or LaCuAl11O19, LaMnAl11O19 were prepared and studied both as bulk materials and thin layers supported on top-coat YSZ layer loaded on Ni superalloy substrate with NiCrAlY bond coat. Particles of NiCrAlY alloy and YSZ have been deposited by detonation spraying from a hot gas stream as dry powders. Complex oxides were prepared via several methods including Pechini route, soft mechanochemistry and microwave heating of salts mixture with carbon. Nanocomposites were prepared by ultrasonic treatment of the oxides mixture or their precursors in isopropanol with addition of surfactants. Thin layers of oxide nanocomposites were supported on YSZ/ NiCrAlY/NiCr-superalloy substrate by slip casting of these suspensions. Genesis of bulk composites and coatings texture, their composition and real/defect structure after annealing under air and argon up to 1200 oC were studied by combination of diffraction methods (high resolution SEM and TEM with EDX, XRD) and spectroscopic methods (FTIRS of lattice modes, UV-Vis, Raman, XPS, SIMS, laser-excited luminescence spectra). The thermal diffusivities of the bulk materials and supported layers were characterized by the laser ﬂash system (Netzsch LFA 427, Germany) from room temperature up to 1273 K in an air atmosphere. For separate oxides, well-crystalline samples are obtained after sintering at 1100 oC, though surface layers are disordered due to their enrichment by smaller Zr and Al cations. In nanocomposites substantial disordering of coordination spheres of cations and residual mesoporosity remain even after sintering at 1300 oC, while redistribution of cations between phases is small. Thin nanocomposite layers on YSZ/NiCr substrates sintered up to 1300 oC demonstrate a good matching with YSZ along with smaller particle sizes of pyrochlore and perovskite phases and their higher disordering as compared with bulk nanocomposite. Good thermal shock resistance (heating up to 1100 oC) was demonstrated for this multilayer design of thermal barrier coating.
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Support by FP7 THEBARCODE project is gratefully acknowledged.