Evaluating steam oxidation kinetics of environmental barrier coatings

Abstract

Environmental barrier coatings (EBCs) are a commercially proven means of protecting SiC-based materials in gas turbine environments. However, there are little specific data in the literature on the impact of coatings like Yb 2 Si 2 O 7 on preventing accelerated SiO 2 growth in the presence of H 2 O. Quantification of reduced rates are necessary for evaluating and comparing EBC effectiveness and incorporation of silica growth rates into future EBC lifetime models. In this study, baseline kinetics of silica formation on bare Si and chemically vapor deposited (CVD) SiC in the 1250-1425℃ range were obtained via 100 h isothermal exposures in dry air and steam environments utilizing a SiC reaction tube to mitigate specimen volatility. An Arrhenius plot of the resulting rates was constructed, representing baseline minimum and maximum rates for Si and SiC oxidation at ambient pressure. Various EBC systems on CVD SiC substrates including air plasma sprayed (APS) EBCs with and without a Si bond coating and with surface roughening to enhance Yb 2 Si 2 O 7 adhesion were subjected to 1-h furnace cycle testing in air with 90vol%H 2 O at 1250-1350℃ for up to 500 cycles. After exposure, silica formation rates were measured and compared to the baseline rates to assess EBC effectiveness, where EBC effectiveness is gauged as the propensity to reduce underlying rates of silica formation. With a Si bond coating, a ~180 µm Yb 2 Si 2 O 7 (YbDS) top coating reduced rates over the entire 1250°-1350℃ range. Without a Si bond coating, ~60 µm (YbDS) coatings deposited directly onto CVD SiC exhibited poor adhesion, and had to be deposited onto substrates with enhanced roughness at 1350℃. While exhibiting good adhesion at 1350℃, overall the single layer YbDS coating exhibited a decreasing effectiveness from 1250° to 1350℃.

Steam oxidation and microstructural evolution of rare earth silicate environmental barrier coatings

“…Furnace cyclic testing (FCT) was performed in a vertical automated cyclic rig with a thoroughly aged alumina reaction tube 16 , 21 at 1350°C, with each cycle consisting of a 1‐h exposure at temperature, followed by air cooling for 10 min. A 90 vol.% H 2 O (g)/10 vol.% air environment was achieved through preheating liquid H 2 O at 400°C.…”

“…This work compares the microstructure evolution and the steam oxidation kinetics of the Si-bond coating beneath two unique EBCs, (Y 1.2 Yb 0.8 ) 2 Si 2 O 7 , termed (Y/Yb)DS throughout this work, and Yb 2 Si 2 O 7 , termed YbDS/YbMS, to determine the impact of EBC composition on steam oxidation kinetics. Furnace cyclic testing (FCT) was performed in a vertical automated cyclic rig with a thoroughly aged alumina reaction tube 16,21 at 1350 • C, with each cycle consisting of a 1-h exposure at temperature, followed by air cooling for 10 min. A 90 vol.% H 2 O (g)/10 vol.% air environment was achieved through preheating liquid H 2 O at 400 • C. An airflow rate of 130 cm 3 /min and liquid H 2 O pump rate of 54 ml/h resulted in a calculated flow rate of 1.5 cm/s.…”

Raman spectroscopic characterization of SiO2 phase transformation and Si substrate stress relevant to EBC performance

“…Yet, the bond coating also oxidizes via gas transport through the EBC. Formation of a TGO at the EBC interface has been shown to greatly reduce EBC adhesion and encourage coating delamination after extended exposure times at elevated temperatures 2–4 . As such, bond coating oxidation represents one of the major EBC failure modes that limits the lifetime of EBC/CMC systems.…”

“…Formation of a TGO at the EBC interface has been shown to greatly reduce EBC adhe-sion and encourage coating delamination after extended exposure times at elevated temperatures. [2][3][4] As such, bond coating oxidation represents one of the major EBC failure modes that limits the lifetime of EBC/CMC systems. Therefore, it is important to understand the microstructural and mechanical details of the SiO 2 TGO similar to the insights that were gained from characterizing the Al 2 O 3 TGO for thermal barrier coatings.…”

Steam Degradation of Ytterbium Disilicate Environmental Barrier Coatings: Effect of Composition, Microstructure and Temperature

“…Secondly, CMCs exposed to high temperatures under clean, dry oxygen form a protective SiO2 scale that provides protection against corrosion and recession [9]. Under the presence of steam, a naturally occurring combustion product [10,11], the CMCs components show increased oxidation [12][13][14] and accelerated corrosion due to the volatilization of the SiO2 scale to form gaseous Si-O-H species, such as Si(OH4) [15], as shown below:…”