Characterization of Si3N4 Coated with Chemically‐Vapor‐Deposited Mullite after Na2SO4‐lnduced Corrosion

Si{sub 3}N{sub 4} substrates coated with chemically-vapor-deposited, crystalline mullite (3Al{sub 2}O{sub 3}{center_dot}2SiO{sub 2}) were subjected to a corrosive environment containing Na{sub 2}SO{sub 4} and O{sub 2} at 1,000 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 Si{sub 3}N{sub 3} substrates. However, a small degree of Na penetration through mullite grain boundaries was observed to a coating depth of {approximately}1 {micro}m.