Effect of the HfO2/SiO2 pre-mixing ratio and heating temperature on the formation of HfSiO4 as a bond coat of environmental barrier coatings

As the operating temperature of advanced gas turbines typically exceeds 1400 °C, it has been required to replace conventional Si bond coat in environmental barrier coatings (EBCs) with materials possessing higher thermal stability. Since HfSiO₄ has excellent thermal properties such as a high melting point, phase stability over 1400 °C, and CTE matches with that of the SiC-based ceramic matrix composites, it has attracted much attention as a next-generation bond coat material. In this study, HfSiO₄ bond coat was successfully formed by atmospheric plasma spray with pre-mixed HfO₂-SiO₂ powders (molar ratios: 7:3 and 5:5) followed by heat treatment. Effect of molar ratios of the HfO₂-SiO₂ and post-heat treatment temperature (1375 and 1475 °C) on the formation of HfSiO₄ were studied. An oxidation test of the HfSiO₄ coating was carried out at 1475 °C with the conventional Si bond coat to verify whether the new bond coat was suitable for use in a thermal environment of 1400 °C or higher. From the results, the HfO₂/SiO₂ ratio of 5:5 was suitable for the formation of HfSiO₄ than that of 7:3. After heat treatment at 1475 °C, the ratio of HfSiO₄ phase was 84.35%. The higher content of HfSiO₄ formed under 1475 °C, meaning the higher heat treatment temperature accelerated the HfSiO₄ formation. In the oxidation test at 1475 °C, the new HfSiO₄ bond coat showed no cracks and maintained its integrity, but the Si bond coat was oxidized and cracked severely. Therefore, it can be concluded that the new HfSiO₄ bond coat formed from 5HfO₂–5SiO₂ coating is a potential candidate as a next-generation bond coat material in EBCs.