Research status of bond coats in environmental barrier coatings

Abstract

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.

Water vapor and CMAS corrosion tests of Y2SiO5/Si thermal and environmental barrier coating

“…After measuring, the sample is put back into the tube furnace for the next testing rotation. The taking-out and putting-back process during the test is accompanied by a great temperature change, which introduced great stress into the Y 2 SiO 5 /Si TEBC due to the different CTEs between the Y 2 SiO 5 top layer (6.86 × 10 −6 /K −1 ) [ 22 ] and bond layer (4.5–5.5 × 10 −6 /K −1 ) [ 23 ]. With the influence of stress, a small number of microcracks in the Y 2 SiO 5 top layer become wider and longer [ 24 ].…”

“…The water vapor reacts with TGO to form volatile Si(OH) 4, as shown in Figure 5 (b). Besides, the TGO layer (10.3 × 10 −6 /K −1 ) [ 23 ] has a large mismatch of CTE with the Si layer and the Y2SiO5 layer, which can also lead to the generation of cracks. The weight gain caused by the oxidation of the bond layer and the weight loss caused by the volatilization of Si(OH) 4 cancel each other out.…”

Exploration of the oxidation behavior and doping ratio of the Si–HfO2 bond layer used in environmental barrier coatings

“…[1][2][3][4] To enhance the service performance of withstanding high temperatures, chemical corrosion, and high gas flow, current EBC systems generally adopt a threelayer structure, including the top, intermediate, and bond layers. 5,6 Silicon (Si) is a widely used bond layer due to its beneficial coefficient of thermal expansion (CTE) (2.6-3.5 × 10 −6• C − 16 ) matching with that of SiC (4.5 × 10 −6• C −16 ), low Young's modulus, and advantageous adhesion property [7][8][9] . Si is oxidized into SiO 2 with a low oxygen diffusion rate at temperatures lower than its softening point, which can slow the further oxidation of the bond layers.…”

“…Silicon (Si) is a widely used bond layer due to its beneficial coefficient of thermal expansion (CTE) (2.6–3.5 × 10 −6 °C −1 6 ) matching with that of SiC (4.5 × 10 −6 °C −1 6 ), low Young's modulus, and advantageous adhesion property 7–9 . Si is oxidized into SiO 2 with a low oxygen diffusion rate at temperatures lower than its softening point, which can slow the further oxidation of the bond layers 10 .…”

Evaluation of environmental barrier coatings: A review

“…10,11 The intermediate layer is used to adjust the mismatch in thermal expansion coefficient (CTE) and chemical incompatibility between the bond and top layers, as well as to protect the bond layer. 12 The top layer is directly exposed to external environments as a first shield. 13 According to the service condition of CMCs with EBCs, physical ablation, high temperature, and chemical corrosion are the dominant environmental threats, 14,15 which are summarized in Figure 1A.…”