Hafnium silicate formation during the reaction of β‐cristobalite SiO 2 and monoclinic HfO 2 particles

Park

Int J Applied Ceramic Tech

et al. 2022

Bond coats in environmental barrier coatings (EBCs) prevent oxidants from penetrating the substrate, mediate the mismatch of the coefficient of thermal expansion (CTE), and improve the adhesion strength between adjacent layers. However, the development of bond coats is rarely studied systematically. In this paper, the research status of the bond coats in EBCs is introduced in detail, including the materials and deposition methods. Thus far, Si, modified‐Si, mullite, etc., have been employed as bond coats. Nevertheless, visible drawbacks of each bond coat limit their application at high‐temperatures in extreme environments. Si bond coat is easily oxidized and forms thermally grown oxides that form cracks, resulting in delamination, spallation, and failure of EBCs. In the Si–HfO2 bond coat, the optimal ratios of Si/HfO2, deposition methods, distribution of Si and HfO2, and oxidation of Si remain completely unsolved. For mullite bond coat, SiO2 suffers selective evaporation in the water vapor environment, and the ratios of the Al2O3 and SiO2 in mullite coatings restrict its service lifetime. HfSiO4 is a potential candidate acting as a next‐generation bond coat in EBCs is proposed. Furthermore, choosing reasonable deposition methods is beneficial to improve the performances of the bond coats in EBCs.

“…Scanning electron microscopy (SEM) micrograph and Energy‐dispersive spectroscopy (EDS) profile of HfO 2 and SiO 2 at 1400°C for 5 h 82 …”

Section: Bond Coatmentioning

confidence: 99%

Section: Prospects For the Bond Coatsmentioning

confidence: 99%

Research status of bond coats in environmental barrier coatings

Park

Int J Applied Ceramic Tech

et al. 2022

“…Therefore, the respective oxidation kinetics of bond coatless EBCs are also of interest. One method to increase EBC temperatures is to modify the Si bond coating with large amounts of HfO 2 [35][36][37][38] to promote HfSiO 4 (hafnon) formation, effectively raising the temperature limit of the parent bond coating. While there are some studies showing reduced oxidation rates and resistance to silica cracking, 36,38 at temperatures above 1400℃ these coatings exhibit rapid rates of steam oxidation, allowing oxidant ingress and direct oxidation of the underlying substrate.…”

Section: Introductionmentioning

confidence: 99%

“…29,30 The physics of resin under pressure is similar to the seepage pr lar approaches have been used also to mod into a mold containing fiber mats. 23,[31][32][33][34][35][36][37][38] T in these cases to determine the resin flow ing times corresponding to different infilt Preceramic polycarbosilane polymers as precursors to make ceramic fibers 10,3 trix. [7][8][9]40,41 The chemical and volume ch pany ceramization of the polymer during documented in several studies.…”

mentioning

confidence: 99%

Evaluating steam oxidation kinetics of environmental barrier coatings

et al. 2021

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℃.

“…20 The current generation of EBCs [21][22][23][24][25][26] is comprised of an Si bond coat and a rare earth silicate-based top coat but this multilayer architecture is temperature limited by the melting point of Si, ∼1414 • C. Achieving higher structural temperatures for SiC-based components therefore necessitates Si-free EBCs. There are several avenues already being explored, like hafnia-modified Si bond coats, [27][28][29][30][31] oxide/oxide multilayer architectures, 32 high entropy silicate EBCs, 33 or single layer ytterbium silicate EBCs applied directly to SiC. [34][35][36] For a multilayer Si/ytterbium disilicate (YbDS) architecture, a Si bond coat is the underlying silica former, while for a bond coat-less single layer YbDS architecture, the SiC/SiC substrate is the underlying silica former.…”

Section: Introductionmentioning

confidence: 99%

Accelerated oxidation during 1350°C cycling of ytterbium silicate environmental barrier coatings

et al. 2021