High temperature oxidation behavior of a Mo–3Si–1B(wt%) alloy

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Molybdenum base alloys with the addition of small amounts of silicon (2–4.5 wt.%) and boron (∼1 wt.%) can form a passivating layer protecting the alloy from further rapid oxidation. When such molybdenum base alloys are exposed to oxidizing environments at high temperatures, a borosilicate glass layer can form that will reduce the transport of oxygen to the alloy to limit further oxidation. Oxidation is then controlled by diffusion through the borosilicate glass layer. The focus of this research was to study the mechanisms and kinetics of high temperature oxidation of a Mo–Si–B alloy. The base alloy has a composition of Mo–3Si–1B (wt.%) and was studied in a variety of gas environments over a range of temperatures in order to elucidate the critical factors that allow it to develop a protective borosilicate glass layer. The borosilicate glass layer is protective when no continuous channels exist in the layer extending from the gas interface to the alloy interface. The borosilicate layer is believed to contain channels in the early stages of development and the elimination of the channels is obtained by appropriate control of the temperature and gas flow conditions whereby MoO3 is removed via vaporization while the borosilicate viscosity is not increased due to loss of B2O3. Once the borosilicate layer is continuous and free of channels, subsequent oxidation occurs by inward diffusion of oxygen and the outward diffusion of molybdenum through this layer with vaporization of MoO3 occurring at the gas/borosilicate layer interface, and MoO2 and additional borosilicate forming at the alloy/MoO2 interface.