Multilayer design and evaluation of a high temperature environmental barrier coating for Si-based ceramics

Two types of environmental barrier coatings for silicon nitride are investigated. In Type A coatings, a bilayer of polymer-derived SiCN and zirconia was deposited on silicon nitride. This coating nearly completely suppressed oxidation of Si3N4 at 1350 degrees C in a 900 h-long test. But the coating volatilized in a streaming water vapor environment. In Type B coatings, a topcoat of hafnia was built on to the SiCN coating with an intermediate compliant layer to accommodate the thermal expansion mismatch between hafnia and Si3N4. The three-layer design was successful in preventing both oxidation as well as weight loss in the silicon nitride at temperatures up to 1300 degrees C. The compliant interlayer was made from a porous microstructure, which approximately followed the guidelines subscribed by a model based on the columnar design. The results lead to both expected and unexpected findings. The three-layer design used to accommodate thermal expansion followed the prediction from the model. But the suppression of oxidation by the SiCN and zirconia overlayer, in Type A coatings, was unexpected. Inhibition of oxygen diffusion by zircon, which appears to have formed by a reaction between SiCN and zirconia, is one possible explanation. Another explanation is that ionic diffusion of oxygen in zircon overlayer and the molecular diffusion of oxygen through the silica interlayer just below creates an electrical field, which opposes ionic diffusion of oxygen through zircon.