Research status of bond coats in environmental barrier coatings

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.

Automated summary

This paper introduces the research status of bond coats in environmental barrier coatings (EBCs), including the materials and deposition methods. The existing bond coats have limitations in high-temperature extreme environments, such as oxidation of Si bond coat and selective evaporation of SiO2 in mullite bond coat. HfSiO4 is proposed as a potential candidate for the next-generation bond coat in EBCs, and it is emphasized that choosing reasonable deposition methods can improve the performance of bond coats in EBCs.