Untersuchungen zu den Versagensmechanismen von Wärmedämmschicht-Systemen im Temperaturbereich von 900 C bis 1050 C bei zyklischer Temperaturbelastung
The present work examines the failure mechanisms of TBC-coated systems during cyclic temperature exposure without additional mechanical load in the temperature range between 900°C and 1050°C. If failure appeared until an testing duration of 5000h in the temperature range between 900°C and 950°C, it was driven by the formation of a brittle chromium-carbide layer in the interdiffusion zone between the substrate and the MCrAlY bond coating. It can be distinguished between two cases. At a low activity of chrome in the MCrAlY, the carbon from the substrate can diffuse into the bond coating and leads there to local precipitations of chromium-carbide, that doesn't exert, however, any effect on the bonding of the MCrAlY. If the activity of chrome in the bond coating is however high, the formation of a closed chromium-carbide layer caused cracks and subsequent spallation during temperature changes. It could be shown that specifically the elements rhenium and aluminum contribute to a significant increase of the chrome activity. If spallation of the TBC occurred in the temperature range between 1000°C to 1050°C, it was correlated with the oxidation of the bond coating. A flawless a-Al2O3 oxide layer doesn't necessarily need to contribute to increased lifetimes. Since cracks have their origin generally at defects, the crack propagation takes place along the border between the bond coating and the oxide layer for components with a pure oxide layer and a smooth MCrAlY-surface. Considering a rough bond coating surface, a crack will be initiated along the border between oxide and TBC. In a defect containing oxide layer, however, cracks that develop within the oxide can be stopped at flaws. Consequently, an optimal bond coating seems to be one that develops an existing optimal surface roughness and trains a defect containing oxide layer with a slow growth rate.
info:eu-repo/classification/ddc/620, Ingenieurwissenschaften, Wärmeisolierstoff, Dünne Schicht, Dynamische Belastung, Hochtemperatur, Rissbildung, Mechanisches Versagen, Keramischer Werkstoff, Haftvermittler
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