Kinetics and oxidation mechanism of MoSi2–MoB ceramics in the 600–1200 °C temperature range

Abstract The aim of this work was to study the kinetics and oxidation mechanism of MoSi 2 -MoB SHS-ceramics in the temperature range of 600–1200 °C. Due to the low rate of change in the mass of samples during the initial stage of oxidation, the kinetic curves at 600 °C suggest that the oxidation process occurs according to a parabolic rate law. At this temperature, MoO 2 and MoO 3 are formed in samples with the greatest MoB content due to its oxidation, and their intensive evaporation only begins after 700 °C. A thin film consisting of a mixture of Mo, Si and B oxides forms on the sample surface as a result of oxidation with a minimum MoB content. The presence of B 2 O 3 in the oxide film leads to the formation of a borosilicate barrier layer, which prevents oxygen diffusion to the sample surface and MoO 3 evaporation. At temperatures of 900 and 1200 °C, the oxidation process proceeds according to a logarithmic law. At these temperatures, an oxide film based on SiO 2 is formed on the surface layer, which in the case of the maximum MoB content sample, is a mixture of B 2 O 3 -SiO 2 oxides in an amorphous state. At lower MoB concentrations, the formed SiO 2 film has a crystalline structure and contains boron. In addition, during oxidation of the materials at the maximal temperature, a Mo 5 Si 3 silicide layer is formed at the interface between the oxide layer of B 2 O 3 -SiO 2 or B-SiO 2 and the sample material due to the oxidation of MoSi 2 , whose thickness and continuity increase with increases of the temperature and duration of oxidation.