STUDY AND DEVELOPMENT OF ENVIRONMENTAL BARRIER COATINGS FOR LIFETIME EXTENSION

Abstract

For the application of gas turbine engines, Silicon based ceramic matrix composites (CMCs) are considered as a promising candidate to replace their supper alloy counterparts and thermal barrier coating systems due to higher temperature capabilities and lower densities. However, the presence of high-pressure steam in the combustion environment of a turbine leads to rapid surface recession. In order to eliminate hot corrosion and recession of CMCs, environmental barrier coatings (EBCs) are needed. Nevertheless, neither the materials nor the processing method for EBCs has been established yet. Ytterbium based silicates are considered as one of the most promising materials group to be used in EBC application owing to high temperature phase stability and high melting point (>1800 °C). However low steam corrosion resistance of ytterbium disilicates Yb2Si2O7 and high coefficient of thermal expansion (CTE) of the ytterbium monosilicate Yb2SiO5 limit the employment of these materials for EBCs. In addition, effect of the long-term thermal exposure on structural characteristics have not been well studied before. In the present work, firstly sintering behaviour of Yb2Si2O7, Yb2SiO5 and Yb2Si2O7 / Yb2SiO5 composites were investigated on the compacted specimen. Initial sintering temperature were determined using dilatometry. Activation energy for the grain growth were calculated via subsequent examination of the microstructure in the course of thermal ageing at 1350 °C. Moreover, the evolution of the mechanical properties were examined using indentation technique. Among the two compounds of ytterbium silicate, Yb2SiO5 showed higher resistance to sintering than that of Yb2Si2O7. Hence, the microstructure and the mechanical properties did not evolve even after 1000 hours of thermal ageing. Yb2Si2O7 on the other hand, showed further densification after initial sintering (that applied prior the 22 thermal ageing experiment) and grain growth. Consequently Young`s modulus increased approximately 10% at the end of thermal ageing in comparison to initial state. For the composite specimens addition of Yb2SiO5 into Yb2Si2O7 retarded the grain growth rates of Yb2Si2O7 and resulted in crack healing specifically at 22/78 Yb2SiO5 to Yb2Si2O7 compositional ratio. This phenomenon were later associated with the CTE mismatch induced thermal stress generation within the Yb2SiO5 and Yb2Si2O7 grains. In order to further improve the sintering efficiency of Yb2Si2O7 at temperature below 1400 °C, alumina (Al2O3) and Yb2Si2O7 powders were mixed and sintered at various ratios. With the addition of 3 wt.% Al2O3, sintering efficiency including densification rates and grain growth of Yb2Si2O7 significantly enhanced due to chemical reaction between Yb2Si2O7 and Al2O3 that transformed the sintering mechanism from solid state sintering to viscous/liquid phase sintering. Later, effect of the Al2O3 addition on steam corrosion behaviour has been investigated by exposing the Yb2Si2O7 and Yb2Si2O7 /Al2O3 sintered specimens to steam at 1350 °C. Correlative microscopy and spectroscopy analysis after steam exposure test indicated that surface of Yb2Si2O7 transformed to porous Yb2SiO5, while Al2O3 addition stopped this reaction by resulting in formation of protective ytterbium alumina garnet phase (Yb3Al5O12) on the outer most surface. Residual Al2O3 grains on the surface of composite specimen was found to originate the Yb3Al5O12 formation via surface diffusion of aluminium into silica depleted Yb2Si2O7 phase. In terms of processing technique, although thermal spraying were well studied in the literature, complex resulting microstructure and challenges to maintain the phase composition, directed the researchers to prospect alternative processing techniques. In the present work slurry based process were developed to deposit ytterbium-based silicates onto SiC. In order to obtain well packed, defect free green coatings, first composition of the slurries were optimised based on th

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Ping Xiao (Supervisor) & David Hall (Supervisor)