On the Oxidation Behaviour of MoSi2 and (Mo,W)Si2 - The Influence of Oxygen and Water Vapour

Doctoral thesis, 2003

This thesis deals with the oxidation of a clay-bonded MoSi2 composite (KanthalSuper 1800) and a clay-bonded (Mo,W)Si2-based composite (KanthalSuper 1900) which are used as heating elements at temperatures of up to approximately 1800°C and 1850°C, respectively, in industrial furnaces. These ceramics are ideal for use at these temperatures due to their excellent oxidation behaviour at high temperatures, which is ascribed to the formation of a protective silica scale. However, at lower temperatures, these materials suffer from severe corrosion, due to the formation of a non-protective oxide scale. In this thesis, the oxidation behaviours of the clay-bonded MoSi2 composite and the clay-bonded (Mo,W)Si2-based composite were investigated at 400-700°C and 350-950°C, respectively, in well-controlled laboratory environments, i.e., in O2/H2O mixtures and Ar/H2O mixtures, as well as in ambient laboratory air. After exposure, the samples were analyzed by a number of analytical techniques, which included XRD, ESEM, SEM, TEM, EDX, and AAS. It is known that the oxidation rate of MoSi2 is significantly influenced by temperature. In this thesis, this phenomenon is elucidated further, and it is also shown that the oxidation of MoSi2 is strongly influenced by the water vapour concentration in the gas phase. The results show that water vapour-induced Mo evaporation is a key process that causes the breakdown of the poorly protective oxide scale at low temperatures (400-550°C), while promoting rapid establishment of a protective scale at high temperatures. In the presence of O2+H2O, Mo is vaporised in the form of MoO2(OH)2. Mo evaporation results in a depletion of Mo in the oxide scale. At low temperatures, Mo depletion creates pores in the oxide scale, which leads to quick diffusion paths for oxygen into the bulk material, which results in faster oxidation of the material. At high temperatures (>550°C), diffusion within the scale is rapid enough to heal the pores that are created by Mo depletion. Thus, the removal of Mo from the oxide scale makes it possible for a pure protective SiO2 scale to be established, which results in a lower oxidation rate. The protective scale may also be established in a dry O2 atmosphere, although more time is needed to remove all of the Mo and to form the protective, pure SiO2 scale. The rate of Mo evaporation depends on the water vapour concentration and the temperature. An oxidation mechanisms is proposed that describes the breakdown of the poorly protective oxide scale at low temperatures and the accelerated formation of the protective scale at high temperatures in O2/H2O and dry O2 atmospheres. A clay-bonded, (Mo,W)Si2-based composite was studied. Although the addition of W makes the oxidation process more complex, the results are similar to those obtained for the MoSi2-based composite, in that (Mo,W)Si2 also undergoes accelerated oxidation within aspecific temperature range. This range is broader (˜550°C-850°C) than that observed for MoSi2. A possible reason for this is the formation of WO3, which undergoes volatilisation depending on the temperature and water vapour content of the exposure atmosphere. The W-containing species WO2(OH)2 has a higher vapour pressure in O2+10%H2O than (WO3)3 has in dry O2. Within the temperature range of accelerated oxidation, evaporation results in an open scale that offers little protection against further oxidation. At high temperatures, the evaporation of W-containing species is beneficial in promoting the formation of a protective SiO2 layer.