Slurry Based Coatings on Silicon Based Ceramics

The use of advanced structural ceramics, such as Si3N4 (SN282), SiC, and SiC/SiC composites, is a promising technology to accomplish the high performance goals for advanced turbine applications. These materials exhibit excellent corrosion resistance by forming a dense silica scale. However, severe degradation of the silica scale has been observed due to the presence of certain environmental species, like alkali salts and water vapors, limiting the long-term reliability of structural ceramics. Presence of water vapor in the turbine environment leads to the formation of a gaseous silicon hydroxide species (Si(OH)4) via a reaction with silica layer, SiO2(s) + 2H2O(g) = Si(OH)4(g). The Si(OH)4 is swept away in turbine environments resulting in continued recession of the substrate. Purpose of this research is to develop environmental barrier coatings (EBCs) for silicon nitride substrates (Si3N4 and SN282) that will protect them from water/salt vapor damage and enhance their usage. Slurry dip coating is chosen as the coating method in this research because it is easy, cost- efficient, and suitable to coat on complex shaped ceramic components because of its non-line-of-site nature. Mullite (3A12O3.2SiO2) is chosen as coating material because it has good mechanical properties at high temperatures, and a coefficient of thermal expansion (CTE) that is very close to Si3N4 [SN282] (CTE of Mullite is 5.3 x 10-6 *C-1 and CTE of SiC is 4.7 x 10-6 *C-1 and Si3N4 [SN282] is 3-4 x 10-6 *C-1). Mullite slurry is modified with sintering enhancing additives of rare earth silicates, such as gadolinium, lutetium, hafnium, and erbium. Mullite/ rare earth silicate slurry, sintered at high temperatures (1375*C - 1450⁰C) and coated on Si3N4 and SN282 substrates, did not show good bonding with the substrates. Yttria-alumina containing Si3N4 substrates coated with Mullite slurry and sintered at high temperatures (1375*C - 1430*C) also showed lack of good bonding between the coat and substrate. 100 mullite coating sintered at 1475*C an