Integratives Design von neuartigen Mo-Si-Legierungen und Schutzschichten für Hochtemperaturanwendungen

Mo Si based alloys with high Ti concentrations represent a new class of high temperature materials offering promising properties for structural applications such as gas turbines, exceeding the thermal capabilities of Ni based superalloys. The combination of these three elements provides high melting temperatures, substantial creep resistance, and exceptional phase stability. Recent alloys in this field have successfully addressed the primary challenge of inadequate oxidation resistance known as "pesting" in air environment. Furthermore, it is recognized that water vapor, constituting approximately 10 vol.% of relevant turbine environments, causes otherwise protective oxides such as SiO2 to react, forming volatile Si(OH)4. However, the exact mechanisms remain inadequately understood.
This study evaluates the high-temperature oxidation behavior of both multi phase (MoSS, T1, T2, A15) (Mo9Si8B and Mo12,5Si8,5B27,5Ti2Fe (at.%)) and two phase (MoSS, T1/D88) (Mo20Si52,8Ti and Mo21Si34Ti0,5B) Mo Si (B) (Ti) (Fe) alloys under dry and water vapor containing (wet) atmosphere. To ensure reliable oxidation protection at lower temperatures in wet atmosphere, Si and Yb silicate Environmental Barrier Coatings (EBCs) are utilized. The goal is to acquire comprehensive knowledge about the oxidation behavior of Mo Si (B) (Ti) (Fe) alloys with and without coating systems in complex atmospheres.
Oxidation tests are conducted at 1200 °C in both dry and wet atmospheres for up to 100 h. Techniques such as thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X ray diffraction (XRD), and transmission electron microscopy (TEM) including electron energy loss spectroscopy (EELS) are employed to assess the oxidation kinetics and to characterize the microstructural development of the oxide layers.
The results from these investigations highlight that an increase in Ti content in general enhances the oxidation resistance in the Mo Si (B) (Ti) (Fe) system, manifesting itself in reduced oxide layer thicknesses and lower specific mass changes. This enhancement is closely linked to a finer and more homogeneous microstructure, which promotes the oxidation stability via shorter diffusion paths. However, a higher Ti content in coated alloys leads to thicker oxide layers and higher specific mass changes, particularly in the Mo20Si52,8Ti alloy as compared to the Mo12,5Si8,5B27,5Ti2Fe alloy.
The use of a wet atmosphere results in increased specific mass changes and thicker oxide layers across all alloys, primarily due to the formation of volatile oxides such as MoO3, which accelerate the oxidation rate. Coated alloys demonstrate exceptional oxidation resistance with minimal specific mass changes and oxide layer thicknesses, especially evident for the Mo12,5Si8,5B27,5Ti2Fe alloy. Additionally, the investigation of the oxide layer on the Mo20Si52,8Ti alloy confirms the significance of a nearly pore-free SiO2 layer for effective oxidation resistance. The supplementary application of Yb silicate layers effectively minimizes the impact of water vapor and shows only a slightly worsened oxidation behavior under wet conditions, underscoring the protective nature of the coatings. These results emphasize the necessity of an effective coating system to ensure satisfactory oxidation resistance in wet atmospheres.