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

The hot-corrosion behavior of uncoated SiC, bulk mullite and CVD grown mullite coatings in contact with Na2SO4 were investigated at 1200A degree C. The range of thermodynamic activity of Na2O (10a degree 4 to 10a degree 6) simulated in this study makes the oxide very basic leading to high reaction rates with SiO2. Uncoated SiC corroded severely, forming various Na2OA.SiO2 compounds with a significant weight gain. Even though the thermodynamic activity of silica is reduced in mullite, several compounds in Na2OA.SiO2A.Al2O3 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 SiO2 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 SiO2 activity in Al-rich mullite. a-[ordm Uncoated SiC corroded severely in Na2SO4 environments. a-[ordm Stoichiometric mullite corroded less than SiC. a-[ordm CVD coated SiC did not undergo corrosion.