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    Effect of high temperature preoxidation treatment on the oxidation behavior of MoSi2 - and WSi2 -Al2O3 composites
    (Elsevier Science Sa, 2020) Yakaboylu, Gunes A.; Yumak, Tugrul; Sabolsky, Katarzyna; Sabolsky, Edward M.
    Composites of 30 vol% MoSi2 -70 vol% Al2O3 and 30 vol% WSi2-70 vol% Al2O3 were prepared via sintering at 1600 degrees C in argon. The high-temperature preoxidation treatments were applied at 1000 degrees-1200 degrees C for 10 - 120 min in air. The non-isothermal oxidation tests were conducted in air at temperatures ranging between 50 degrees and 870 degrees C to study and understand the influence of the alumina phase and preoxidation process on the low-temperature oxidation behavior. The optical and scanning electron microscopy, X-ray diffraction, X-ray photoelectron and Raman spectroscopy were used to characterize the surface layers formed. The oxidation-induced mass gains were substantially reduced by 82.1-99.8%, implying their enhanced oxidation resistance. This was due to the formation of highly dense, protective surface layers with a sufficient thickness (3.1-14.2 mu m). The structural and surface analyses revealed their complex chemistries, since the ternary (Mo-Si-Al, W-Si-Al) intermetallic, mullite, alumina and silica phases with high oxidation resistances were identified within these surface features. In addition, the high-temperature electrical properties of the composites were highly preserved after the preoxidation treatment, and their electrical conductivities were measured as 45.1-78.6 S/cm at 800 degrees C and 40.1 - 69.1 S/cm at 900 degrees C. (C) 2019 Elsevier B.V. All rights reserved.