SiC-based ceramic matrix composites offer the promise of superior mechanical properties at higher temperatures than the nickel superalloys currently used for structural components in the hot section of modern gas turbine engines, enabling improved engine efficiency and reduced emissions. The hostile conditions within the engine, however, necessitate the use of environmental barrier coatings (EBC) to prevent the volatilization of the protective SiO2 scale. Current rare earth (RE) silicate EBCs effectively control the volatilization reactions but are susceptible to mechanical damage and are not sufficiently robust to withstand the corrosive attack of molten silicate deposits.

A systematic environmental protection strategy is proposed to address these protection deficiencies. This approach is intended to improve the intrinsic volatilization resistance of the composite matrix while introducing new materials into the EBC architecture to mitigate the degradation caused by siliceous deposits. The incorporation of Y-bearing compounds to the SiC-based matrix enables the oxidative growth of a Y-silicate scale. The lower SiO2 activity of this silicate presumably reduces the scale volatility. The scale morphology can be controlled by including AlO1.5 to promote the generation of a transient liquid phase. The availability of yttrium at the surface is also strongly correlated to relative hermeticity of the matrix material; less hermetic configurations enable internal oxidation that, while not desired because of the potential degradation of fibers or fiber coatings, promotes the outward migration of yttrium.

The initial design of the improved coating system, termed a T/EBC, was based on the requirement to use phase stable, thermochemically compatible materials to meet individual protection goals. Coatings optimized in terms of these requirements were unable to mitigate molten silicate infiltration. This deficiency is related to nuances in the reaction equilibria that render the smaller RE cations (eg. Yb) less effective in rapidly crystallizing the melt than larger RE cations (eg. Gd or La). Regardless of the coating composition, the efficiency of the crystallization reactions is reduced with increasing temperature due to increased partitioning to the reprecipitated Hf/ZrO 2 fluorite phase. Based on this new understanding further improvements to the T/EBC architecture including compositional modulation or the incorporation of additional layers are proposed.