CHARACTERIZATION OF SI3N4 COATED WITH CHEMICALLY-VAPOR-DEPOSITED MULLITE AFTER NA2SO4-INDUCED CORROSION

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