| In an aero-engine, thermal barrier coatings (TBCs) play a key role in protectingits metallic components in a severe environment and enhancing its efficiency. Withthe improvement of the turbine thrust-weight ratio, the gas temperature of thecombustion chamber is on the increase continuously, high-temperature corrosion ofTBCs with volcanic ash has been an outstanding question limiting TBCs life. Theapplication of various microscopic characterization methods to obtain detailedmicrostructure information has become a vital means to analysis TBCs failuremechanism. High-temperature interaction of natural volcanic ash (VA) and artificialvolcanic ash (AVA) with a standard7wt%Y2O3-stabilized ZrO2(7YSZ)electron-beam physical vapor deposition (EB-PVD) thermal barrier coating isinvestigated to evaluate the failure behaviour of TBCs related to the high pressureburbine blades after volcanic ash deposition in this thesis. Then the microstructureevolution of ceramic layer and the ceramic/alumina interface are characterized byusing X-ray diffraction (XRD), Raman spectroscopy (Raman), scanning electronmicroscope (SEM) and transmission electron microscope (TEM). Finally, thecorrosion characteristics of two volcanic ashes on TBCs are compared; moreover, theeffect of trace elements (such as Fe) is revealed in the process of TBCs corrosion. Themain results are stated as following:Thermal-chemical properties tests show that Sakurajima volcanic ash iscomprised of a series of rock-forming minerals with different ratios. Silicon accountsfor60%of the total composition by converting the silicate and the quartz into thesilica. It also contains a large amount of Fe in Fe2O3and silicate cationic (Fe2+/Fe3+)forms. The average particle diameter of the natural volcaninc ash powder used forcorrosion experiments is about10.3μm. There exists a wide softening/meltingtemperature range for Sakurajima volcanic ash. However, there is a quasi-meltingpoint for the artificial volcanic ash (C10M7A13S70) with melting endothermic peak at1250°C.Sakurajima volcanic ash deposited on TBCs surface becomes semi-soften at1150°C for1h. There appears ZrSiO4at top of the YSZ columns after4h, identified asthe chemical reaction product of YSZ and the volcanic ash, but without monoclinicZrO2. Some corrosion spots on the surface of YSZ columns are found by TEM after24h. The edge shape of VA-corroded holes is controlled by the lattice plane (001) with a high atomic density or a low corrosion rate. The variety of materials filled inthe YSZ column gaps is related to the chemical environment. The filling material isamorphous ash at the top of the ceramic layer. However, the filling material isanorthite crystal at the bottom. It is beneficail to precipitate anorthite crystal due to anAl-rich local chemical environment adjacent to the YSZ/Al2O3interface. Meanwhile,some cracks are found in these areas, where the formation of anorthite leads to thegrowth stain and the thermal strain, thus the cracks are in favor of initiating andpropagating.At1250°C, when TBCs are corroded by natural volcanic ash and artificial onesrespectively, the morphology of YSZ coating certainly both change from originalfeather-arm columns to random network structures filled with some grains. Thereappears ZrSiO4at the top of the ceramic layer and CaAl2Si2O8at the interface. On theother hand, YSZ is still cubic or tetragonal structure. TEM analysis shows that theceramic layer is seriously damaged after TBCs corroded by two kinds of volcanicashes, and a large number of spherical particles exist at YSZ/Al2O3interface.Monoclinic ZrO2is only observed in the column gaps for the articial ash corrosion.These indicate that the Fe or other trace elements do not directly affect the corrosionmorphology and the phase distrubution of TBCs. When temperature reaches1400°C, anew reaction product-Ca2Si4ZrO12crystal is found near the AVA/YSZ interface, aswell as monoclinic ZrO2in the ceramic coating. |
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