Additive Effects on Si 3 N 4 Oxidation/Volatilization in Water Vapor

Two commercially available additive‐containing silicon nitride materials were exposed in four environments which ranged in severity from dry oxygen at 1 atm pressure, and low gas velocity, to an actual turbine engine. Oxidation and volatilization kinetics were monitored at temperatures ranging from 1066° to 1400°C. The main purpose of this paper is to examine the surface oxide morphology resulting from the exposures. It was found that the material surface was enriched in rare‐earth silicate phases in combustion environments when compared with the oxides formed on materials exposed in dry oxygen. However, the in situ formation of rare‐earth disilicate phases offered little additional protection from the volatilization of silica observed in combustion environments. It was concluded that externally applied environmental barrier coatings are needed to protect additive‐containing silicon nitride materials from volatilization reactions in combustion environments.