Characterization of Si3N4 coated with chemically-vapor-deposited mullite after Na2SO4-induced corrosion

Si3N4 substrates coated with chemically-vapor-deposited, crystalline mullite (3Al(2)O(3) . 2SiO(2)) were subjected to a corrosive environment containing Na2SO4 and O-2 at 1000 degrees C for 100 h. The composition and microstructure of the as-deposited and corroded specimens were examined and compared. The coating appeared to be effective in preserving and therefore protecting the surface microstructure of the underlying Si3N3 substrates. However, a small degree of Na penetration through mullite grain boundaries was observed to a coating depth of similar to 1 mu m.