Polymer-Derived Ceramics as Innovative Oxidation Barrier Coatings for Mo-Si-B Alloys
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resently, state-of-the-art Ni-based superalloys used as turbine materials have reached their application limits at applied homologous temperatures of around 0.75. To further increase the gas inlet temperatures in stationary gas- or aircraft turbines in order to lower thermodynamic inefficiency losses, new high temperature alloys are under development and investigation.[1–5] One category of these high temperature materials is based on Mo, which offers great potential due to its high melting point above 2273 K (2000 C), excellent high temperature strength, and excellent creep properties.[6] The oxidation behavior of Mo-based alloys was studied and investigated by various research groups.[1,7–11] Pure Mo suffers from a poor oxidation resistance due to the formation and evaporation of MoO3 at temperatures ranging from 673 K to 1073 K (400 C to 800 C). By alloying with Si and B, the oxidation resistance can be improved. For example, adding 9 at. pct Si and 8 at. pct B leads to a three-phase material which consists of a Mo solid solution (MoSS) and the two intermetallic phases Mo3Si and Mo5SiB2 (T2). In the present work, this three-phase material serves as substrate for the coating experiments. The intermetallic phases are the main reservoirs for Si and B and oxidize at higher temperatures to a SiO2-B2O3 GEORG HASEMANN, Researcher, TORBEN BAUMANN and SEBASTIAN DIECK, Students, STEFAN RANNABAUER, Researcher, and MANJA KRU¨GER, Professor, are with the Institute of Materials and Joining Technology, Otto-von-Guericke University Magdeburg, Universita¨tsplatz 2, 39106 Magdeburg, Germany. Contact e-mail: georg. [email protected] Manuscript submitted July 29, 2014. Article published online February 10, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
glassy layer protecting Mo-alloys from further oxidation.[12] The high amount of SiO2 is the reason for the very good oxidation behavior at temperatures above 1273 K (1000 C). However, uncoated Mo-Si-B alloys show a pronounced initial mass loss during heating for the first time until a continuous glass layer is formed to protect the material’s surface. At lower temperatures between 973 K and 1173 K (700 C and 900 C), the evaporation of MoO3 leads to disintegration of the materials[9] comparable to the so-called pesting phenomenon, which is well known for MoSi2. Different coating systems and techniques, such as pack cementation, magnetron sputtering, and plasma spraying, are discussed in the literature to control the oxidation properties of such alloys.[13–18] According to Perepezko et al.,[15,19] it is important to design a coating that consists of phases which are in thermodynamic equilibrium with the substrate material. Following this principle, a double-layer coating of MoSi2/MoB on a Mo-8.9Si-7.1B substrate (all concentrations are atomic percent if not stated otherwise) was obtained by pack cementation. An additional heat treatment led to the formation of a protective double coating system consisting of the Mo5Si3/Mo5SiB2+MoB phases.[19] Following Perepezko et al.’s approach, Lange
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