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Comparative studies of the oxidation of MoSi₂ based materials Samadzadeh, Seyed Mostafa

Abstract

Molybdenum disilicide (MoSi₂) is a promising intermetallic material for high temperature applications (above 1000°C). However, it rapidly oxidizes at temperatures ranging from 400 to 600°C, which given enough time, can lead to its disintegration. Above 1000°C, MoSi₂ exhibits better oxidation resistance due to the formation of a continuous SiO₂ layer (or alumina layer for the materials doped with aluminum). The experiments in this study were divided into two main categories: low temperature oxidation (300 to 900°C; high oxidation rate expected) and high temperature oxidation (1000 to 1600°C; lower oxidation rate expected due to rapid formation of the protective oxide films). The isothermal exposure time in the low temperature oxidation experiments was from 4 to 240 hrs while it was from 2 to 144 hrs for the high temperature oxidation experiments. Five different, commercially available, MoSi₂ based heating elements, i.e. Kanthal Super (labelled by the manufacturer as KS-1700, KS-1800, KS-1900, KS-ER and KS-HT) were used in the experiments. It was found that the oxidation behavior of different materials under investigation depended strongly on their chemical and phase composition, exposure time and temperature. KS-ER and KS-1800 showed excellent resistance against the low temperature (300 to 900°C) degradation for up to 240 hrs, while KS-HT and KS-1900 underwent significant degradation after 240 hrs of air exposure within the same temperature range. In high temperature oxidation experiments, a dense barrier alumina film (1.5 µm thick at 1000°C to 50 µm thick at 1500°C for up to 144 hrs) formed on KS-ER samples. A dense glassy SiO₂ film (3 µm thick at 1000°C to 50 µm thick at 1600°C for up to 144 hrs) formed on the other types of samples. The glass scale on the surface of KS-1700, KS-1800 and KS-HT was significantly thicker (~3 times) than that on KS-HT over the temperature range of 1200°C to 1600°C after 144 hrs. The rate of alumina formation of KS-ER was relatively higher than the glass film formation of the other types of composite MoSi₂ materials. The differences in the oxidation behavior of various MoSi₂-based materials were linked to their chemistry and phase compositions.

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