Water vapor corrosion of mullite: Single crystals versus polycrystalline ceramics

Water vapor corrosion of mullite single crystals and polycrystalline mullite ceramics was investigated in rapidly streaming water vapor at 1200 °C. Both materials behave quite differently: mullite ceramics display non-isotropic corrosive attack resulting in superficial α-alumina formation with high nucleation density. Preferential corrosion occurs on crystal surfaces oriented more or less perpendicular to the crystallographic c-axis (i.e. on or near to the (0 0 1) plane), while parallel to c no corrosion effects are observed after short-term treatments. In single crystals, on the other hand, the α-alumina formation rate does not depend on crystal orientation; moreover, the nucleation density of alumina is relatively low. In case of stoichiometric mullite ceramics, having approximately stoichiometric 3/2-mullite composition (i.e. ≈72 wt.% Al2O3) the early stage of water vapor-induced mullite to α-alumina conversion is interpreted in terms of true corrosion. This process is stronger parallel to the c-axis than perpendicular to it, reflecting the non-isotropic bonding character of mullite. In the case of mullite single crystals, having approximately 2/1-composition (i.e. ≈78 wt.% Al2O3) the early stage of mullite/alumina conversion is considered as a hydrothermally activated isotropic α-alumina precipitation process of mullite, supersaturated in Al2O3.