Effects of Boria on Environmental Barrier Coating Systems

SiC/BN/SiC ceramic matrix composites (CMCs) with rare earth silicate (Yb2Si2O7) environmental barrier coating (EBC) systems entered use in civilian turbine engines in 2016. The BN fiber/coating interphase has been found to oxidize to form boria on the surface of the CMC. Exposures of CMC/ EBC systems in combustion environments have shown possible spinodal decompositions and dendritic structure formation attributed to boria formation and reaction with the Yb2Si2O7. However, no thermodynamic data and few experimental studies exist on the B2O3-SiO2-Yb2O3 system to confirm these interactions. Due to the lack of data on this system, this dissertation seeks to determine the effect of boria on the EBC system through the following objectives:

1. Understand the reactions and equilibrium phases that occur when B2O3 reacts with the phases found in the rare earth silicate EBC; Yb2Si2O7, Yb2SiO5, and Yb2O3.

2. Develop a phase diagram for the Yb2O3 – SiO2 – B2O3 ternary system.

3. Determine the properties of any reaction products and equilibrium phases that are found to form in the Yb2O3 – SiO2 – B2O3 ternary system.

4. Determine the impact of the formation of these reaction products on the oxidation of SiC/BN/SiC CMC -Si bond coat, and thermochemical stability of theYb2Si2O7 EBC systems