Protective Al-rich mullite coatings on Si- based ceramics against hot corrosion at 1200 °C

Abstract

The hot-corrosion behavior of uncoated SiC, bulk mullite and CVD grown mullite coatings in contact with Na 2SO 4 were investigated at 1200 °C. The range of thermodynamic activity of Na 2O (10 − 4 to 10 − 6 ) simulated in this study makes the oxide very basic leading to high reaction rates with SiO 2. Uncoated SiC corroded severely, forming various Na 2O·SiO 2 compounds with a significant weight gain. Even though the thermodynamic activity of silica is reduced in mullite, several compounds in Na 2O·SiO 2·Al 2O 3 system were formed in case of bulk mullite. CVD based mullite coatings with high alumina content at the top surface of the coating, and therefore reduced SiO 2 activity, offered protection to the underlying SiC in corrosive environments. As predicted correctly by thermodynamic calculations, the trend in weight gain with the coated SiC samples followed the theoretically calculated SiO 2 activity in Al-rich mullite.