Sodium sulfate corrosion of silicon carbide fiber-reinforced calcium aluminosilicate glass-ceramic matrix composites

Abstract

Hot corrosion effects of Sodium Sulfate (NaSO4) coated Calcium Aluminosilicate (CAS)/Silicon Carbide (SiC) reinforced glass-ceramic matrix composite were investigated using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Analysis (EDX) and X-ray Diffraction (XRD). The samples provided by the Naval Air Warfare Center (NAWC) were unidirectional SiC/CAS as follows: (1) as received, (2) uncoated in air, (3) Na2SO4, coated in air and (4) Na2SO4 coated in argon. A heat treatment was conducted at 900 deg C for 100 hours. Experimental observations indicated that the Na2S04 coating in an oxidising environment had severely corroded the silicon fiber resulting in a silica rich, Nepheline (NaAlSi04), Wollastonite (CaSiO3), Rankinite (CaSi2O7), Albite (NaAlSi3O8 and glassy phases. In the argon atmosphere fiber degradation was present although less severe than in the oxygen environment. Similar phases of silica rich, Nepheline, Albite, Rankinite, Mullite (Al6Si2O13), Pseudo-Wollastonite (CaSiO3) and a glassy region were present. Minimal fiber and matrix degradation was observed in the uncoated sample beat treated in air