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

Abstract

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.