Hot corrosion behavior of coated covalent ceramics

Ceramic thin films were deposited on hot-pressed silicon nitride and silicon carbide substrates to provide corrosion protection to the underlying material. High-purity silicon nitride, silicon carbide, and silicon oxynitride (SiO{sub 2}/Si{sub 3}N{sub 4} composite) coatings were deposited by plasma-enhanced chemical vapor deposition and rf sputtering to form an effective diffusion barrier for cation migration into the silica corrosion product. Preliminary results indicated that the extent of corrosion in Na{sub 2}SO{sub 4} melts at T = 900{degree}, 950{degree}, and 1,000{degree}C was dependent on the outward diffusion of cations in the grain-boundary phase and the crystallinity of the grain-boundary phase in the uncoated ceramics. Accelerated corrosion occurred in these ceramics because of breakdown of the protective silica film formed on the surface. Corrosion rates in Na{sub 2}SO{sub 4} melts were reduced by separating the oxide grain-boundary phases from the corrosion products with high-purity covalent ceramic coatings.