Oxidation behaviour of a Mo(Si,Al)2 based composite at 1500 °C

The oxidation of a Mo(Si,Al)2 composite is investigated at 1500 °C in dry air using exposure times from 1 to 1000 h. Cross sections are examined with Scanning Electron Microscopy (SEM) and the phase composition is analyzed by X-ray diffraction (XRD). The material forms a continuous and protective alumina layer, the growth of the alumina layer following parabolic kinetics. Immediately below the scale Mo(Si,Al)2 is replaced by a Mo5(Si,Al)3 layer due to the flux of aluminum to the scale. The Al concentration in the Mo(Si,Al)2 phase in the underlying substrate decreases from 27% before exposure to 16–17% after 1000 h. The continuous alumina layer becomes covered by a top layer consisting of alumina grains embedded in a viscous melt with approximate composition 7 Na2O–15 Al2O3–78 SiO2. With time, sodium is volatilized from the melt and the top scale layer transforms to a mixture of alumina, mullite and silica melt.