Effect of the Air Plasma Spray Process on Rare Earth Silicate Coating Life
Parker, Cory, Materials Science - School of Engineering and Applied Science, University of Virginia
Opila, Elizabeth, EN-Mat Sci/Engr Dept, University of Virginia
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, PH2O 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.
PHD (Doctor of Philosophy)
Rare earth silicates, Water vapor, Air plasma spray, Thermal expansion, Computation