High Temperature Oxidation of Mo-Si-B Base Alloys

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 for further oxidation. Oxidation is then controlled by the diffusion through the borosilicate glass layer. The focus of this research was to study the mechanisms, thermodynamics and kinetics of Mo-Si-B base alloy high temperature oxidation. The base alloy has a composition of Mo-3Si-1B (wt%) and was studied in a variety of gas environments as well as 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 that develops is believed to contain channels at early stages 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 the formation of MoO2 and additional borosilicate occurring at the alloy MoO2 interface