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Impermeable Thin Al2O3 Overlay for TBC Protection From Sulfate and Vanadate Attack in Gas Turbines Quarterly Report

Description: In order to improve the hot corrosion resistance of yttria-stabilized zirconia (YSZ), an Al{sub 2}O{sub 3} overlay has been deposited on the surface of YSZ by electron-beam physical vapor deposition. Currently, hot corrosion tests were performed on the YSZ coatings with and without Al{sub 2}O{sub 3} overlay in molten salt mixture (Na{sub 2}SO{sub 4} + 0 {approx} 15wt%V{sub 2}O{sub 5}) at 950 C in order to investigate the effect of amount of vanadate on the hot corrosion behaviors. The results showed that the presence of in V{sub 2}O{sub 5} the molten salt exacerbates the degradation of both the monolithic YSZ coating and the composite YSZ/Al{sub 2}O{sub 3} system. The formation of low-melting Na{sub 2}O-V{sub 2}O{sub 5}-Al{sub 2}O{sub 3} liquid phase is responsible for degradation of the Al{sub 2}O{sub 3} overlay. The Al{sub 2}O{sub 3} overlay acts as a barrier against the infiltration of the molten salt into the YSZ coating during exposure to the molten salt mixture with <5wt% vanadate. In the next reporting period, we will use XPS and SIMS to study the interactions between alumina overlay and molten salt containing vanadate.
Date: June 10, 2003
Creator: Mao, Scott X.
Partner: UNT Libraries Government Documents Department

Impermeable Thin Al2O3 Overlay for TBC Protection From Sulfate and Vanadate Attack in Gas Turbines Quarterly Report

Description: In order to further improve the hot corrosion resistance of yttria-stabilized zirconia (YSZ), an Al{sub 2}O{sub 3} overlay of 58 {micro}m thick was deposited on the surface of YSZ by electron-beam physical vapor deposition. Hot corrosion tests were performed on the YSZ coatings with {gamma}-Al{sub 2}O{sub 3} overlay and {alpha}-Al{sub 2}O{sub 3} overlay in molten salt mixture (Na2SO4 + 5wt%V2O5) at 950 C. The {alpha}-Al{sub 2}O{sub 3} overlay was obtained by the post-annealing of g-Al{sub 2}O{sub 3} overlay at 1200 C for 1h. The results showed that compared with the hot corrosion resistance of YSZ coating with 25 {micro}m thick {gamma}-Al{sub 2}O{sub 3} overlay, either thickening {gamma}-Al{sub 2}O{sub 3} overlay or employing {alpha}-Al{sub 2}O{sub 3} overlay could impair the hot corrosion resistance of YSZ coating, because the tensile stresses developed in the alumina overlay in both cases due to the mismatch in thermal expansion coefficient (TEC) between alumina and zirconia resulted in cracking of Al{sub 2}O{sub 3} overlay. The formation of cracks increased contact area between molten salt and Al{sub 2}O{sub 3} overlay, and also the penetration rate of molten salt into Al{sub 2}O{sub 3} overlay and YSZ coating, leading a faster and greater degradation of YSZ coating upon exposure. In the next reporting period, we will study the effect of Al{sub 2}O{sub 3} overlay thickness on hot corrosion and spalling of YSZ coatings.
Date: June 30, 2004
Creator: Mao, Scott X.
Partner: UNT Libraries Government Documents Department