Effect of the Air Plasma Spray Process on Rare Earth Silicate Coating Life

SiC/SiC ceramic matrix composites (CMCs) entered commercial jet turbine service in 2016. SiC forms a protective SiO₂ layer at operating temperatures in oxidizing environments, however, H₂O(g) produced in the combustion environment reacts with the SiO₂ layer to form Si(OH)₄(g). Environmental barrier coatings (EBCs) are required to protect the underlying CMC from H₂O(g) in the combustion stream. Rare earth (RE) disilicate (RE₂Si₂O₇) coatings, where RE is either Y or Yb, are state-of-the-art EBCs. These coatings are typically deposited using an air plasma spray (APS) process resulting in a heterogeneous microstructure with multiple phases present such as RE₂O₃, RE₂SiO₅, and RE₂Si₂O₇. Cracks and pores are also likely to result from this process. This complex microstructure results in thermal expansion and thermochemical response to H₂O(g) that differ from homogeneous RE₂Si₂O₇ materials processed in conventional ways. The aim of this research is to quantify property differences that arise due to the APS process and incorporate them into a lifing model to predict EBC behavior at temperatures, pressures, gas velocities, and times relevant for turbine engine applications.

Thermal expansion of Y₂Si₂O₇ material deposited using APS was measured by dilatometry at temperatures between 25 and 1400°C. APS Y₂Si₂O₇ had a coefficient of thermal expansion (CTE) higher than phase pure Y₂Si2O₇ tested in the same manner, likely due to the presence of constituent phases such as Y2O₃ and Y₂SiO₅ that have CTE values higher than Y₂Si₂O₇. In addition, anisotropy in CTE was determined for δ-Y₂Si₂O₇, X2-Y₂SiO₅, β-Yb₂Si₂O₇, and X2-Yb₂SiO₅ using
high-temperature XRD performed at the Advanced Photon Source at Argonne National Laboratory. Low expansion planes were found along with large differences in expansion along different axes, especially in the monosilicates.

The thermochemical stability of RE₂Si₂O₇ is an important factor in the lifetime prediction of EBCs for SiC/SiC CMCs. Samples of Y₂Si₂O₇ processed with SPS have been exposed to H₂O(g) with a gas velocity of 125-250 m/s at 1200 and 1300°C for 60h and 1400°C for times of 60 – 250 h in a steam-jet furnace. Samples were then observed in plan view and cross section using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Phase pure SPS Y₂Si₂O₇ reacted with H₂O(g) forming Y₂SiO₅ and porosity, releasing Si(OH)₄(g). Analysis of the change in Y₂SiO₅ layer thickness with time has shown parabolic reaction kinetics, suggesting a diffusion limited mechanism. For the first time the reaction of Y₂SiO₅ with H₂O(g) to form Y₂O₃ was also observed.

APS Yb₂Si₂O₇ provided by Rolls-Royce was tested in the steam-jet furnace as well, showing reaction between H₂O(g) and Yb₂Si₂O₇ similar to that seen in the Y₂O₃-SiO₂ system. Local microstructural features in the APS coatings, such as splats, cracks, and pores, were found to heavily influence SiO₂ depletion.

A hybrid Potts/diffusion model of the phase and microstructural evolution was developed. Currently the model simulates the formation of Y₂SiO₅ and porosity from phase pure Y₂Si₂O₇ with parabolic kinetics. Once calibrated, the model will be used to predict Y₂Si₂O₇ reactions with H₂O(g) and the corresponding microstructural evolution as a function of time, temperature, P_H2O and gas velocity. The model also allows for the introduction of new phases like the formation of Y₂O₃ from Y₂SiO₅ reacting with H₂O(g) and pores and cracks in the microstructure.

This work together will give a better understanding of how the APS process used in EBC deposition alters material properties compared to phase pure materials and how these alterations must be considered for lifetime prediction of the coatings.