Oxidation behaviour of a (Mo, W)Si2-based composite in dry and wet oxygen atmospheres in the temperature range 350–950 °C

The oxidation of a (Mo, W)Si2-based composite was investigated in the temperature range (350–950 °C). The influence of temperature and water vapour on the oxidation was examined. The kinetics was studied using a thermobalance whereas the morphology and composition of the oxides were examined using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive X-ray (EDX). Focused ion beam (FIB) milling was performed on some of the oxide scales which allowed us to look at a non-mechanically disturbed scale/oxide in cross-section. Rapid oxidation was found to occur in the 550–750 °C temperature range. The mass gains were significantly larger in O2 than in O2 + 10%H2O. The different mass changes in the two exposure atmospheres were attributed to the higher vapour pressure of the volatile MoO2(OH)2 and WO2(OH)2 species in O2 + 10%H2O than that of (MoO3)3 and (WO3)3 in dry O2. The peak mass gain was found to occur at a temperature of about 750 °C in O2 and 650 °C in O2 + 10%H2O. At temperatures above 850 °C, especially when water vapour is present, the removal of Mo and W from the oxide scales is rapid enough to allow partial healing of the silica, causing the oxidation rate to drop. At 950 °C in O2 + 10%H2O, a protective SiO2 scale could be re-established quickly and maintained, causing the oxidation to essentially cease.