Dry air cyclic oxidation of mixed Y/Yb disilicate environmental barrier coatings and bare silica formers

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

“…Further, (Y/Yb)MS was found to be concentrated near the bond coating after FCT exposure (Figure 3), suggesting preferential removal of (Y/Yb)MS near the gas–EBC interface. This behavior was not observed for (Y/Yb)DS exposed to dry air under the same conditions, where the (Y/Yb)MS and porosity concentrations remained at 7% ± 2% and 6% ± 3% area fraction, respectively, for all exposures up to 500 h 20 . Rare earth cation transport to the gas–EBC interface has been shown to be prevalent for YbDS and YbMS exposed to steam, while oxidant transport occurs in the opposite direction toward the interface 23–25 .…”

“…Both EBC systems are proven effective barriers to oxidation, as both compositions result in decreased oxidation kinetics compared to the bare Si reference (solid green line). Further, the reference lines for steam FCT (as opposed to steam isothermal oxidation) of Si and SiC may be higher on Figure 8 (i.e., result in much faster oxidation rates) as dry air FCT rates of silica formation were recently found to be higher than isothermal rates 20 . The parabolic rate constant for (Y/Yb)DS exposed to cyclic steam was approximately 30% of the value determined for YbDS/YbMS.…”

“…The YbDS/YbMS EBC was nominally thicker and contained similar initial porosity as (Y/Yb)DS, yet also displayed thicker silica scales upon steam cycling, which suggests that EBC thickness may not be as primary of a tuning parameter as other deposition parameters or EBC chemistry for decreasing steam oxidation of the bond coating. During cyclic air oxidation, (Y/Yb)DS EBC thickness in the 90–240 μm range was found to have no impact on oxidation kinetics of the underling Si‐bond coatings 20 . The air oxidation results suggest that the rate‐limiting step for bond coat oxidation is oxidant transport through the TGO.…”

“…The relation between oxygen diffusivity and TGO growth rate is visible when comparing dry air and steam oxidation rates, as both silicate phases are expected to have higher oxygen diffusivity in the presence of steam 8 , 28 – 30 . Figure 8 is an Arrhenius plot of the parabolic oxidation rate constants for both compositions exposed to air and steam environments 20 . Data for bare Si and SiC exposed to isothermal conditions in air and steam are also presented in Figure 8 as a method to define EBC performance 31 , 32 .…”

“…Stack et al. performed dry air cyclic oxidation experiments on (Y 1.2 Yb 0.8 ) 2 Si 2 O 7 to determine silica scale growth rates at the Si‐bond coating–EBC interface, where the thermally grown oxide (TGO) kinetics were almost an order of magnitude slower than that of a Yb 2 Si 2 O 7 /Yb 2 SiO 5 EBC 20 . The impact of multiple rare earth cations on TGO growth rates has not yet been explored in steam environments, which are more representative of the combustion environment.…”