Synthesis and Processing of Ytterbium Based Silicates for Application of Environmental Barrier Coatings

Abstract

SiC/SiC ceramic matrix composites (CMC) have been assigned to replace the superalloys used in the hot sections of gas turbine engines due to superior high temperature capability, lightweight nature, and excellent corrosion resistance in dry environments. However, SiC suffers from degradation in the presence of high temperature water vapour. In order to prevent this degradation, environmental barrier coating (EBC) have been developed and Yb2Si2O7 is the most promising candidate as an EBC material. In this study, firstly, Yb2Si2O7 powder was fabricated using the solution combustion synthesis (SCS) to investigate the importance of material properties in EBCs. Then, Electrophoretic deposition (EPD) is proposed to develop Yb2Si2O7 coatings on SiC substrate, and following a performance analysis at high temperature isothermal exposure. The solution combustion synthesis method is a self-sustained high-temperature redox reaction between metal nitrates and fuel sources. A systematic study was conducted to understand the role of fuel sources and calcination temperatures on the physical and structural properties of the fabricated powder. Results showed that the powder's particle size and morphology highly depend on the fuel and calcination temperature. The more gaseous by-products (depending on the fuel source) allowed the fabrication of smaller particles as releasing the gaseous compounds left a porous structure. Using high calcination temperature, on the other hand, caused particle agglomeration; however, it reduced the amount of second phase, Yb2SiO5. After the combustion process, the colloidal behaviour of powder samples was examined using zeta potential measurement and sedimentation test. Electrophoretic deposition was also employed to judge the effect of powder properties on green coating microstructure. Results showed that SCS can be an alternative technique for fabricating nano-scale rare earth silicate with desired phases. EPD is a wet chemical technique consisting of two stages; electrophoresis and deposition. In the first stage of EPD, suspension parameters (solvent type, dispersant concentration, and pH) were varied to understand their roles in the green coating microstructure. The suitable solvent for Yb2Si2O7 was determined using a sedimentation test and microstructure analysis, resulting in ethanol-based suspension, providing long-term stability with a crack-free microstructure. Moreover, the relation between packing density and solvent type was built using micro indentation, stating lower print area for higher packing density. The optimum dispersant concentration was judged via zeta potential measurement and microstructure analysis, showing that high dispersant concentration increased the suspension conductivity and ionic strength. Therefore, uneven and cracked green coatings were obtained when dispersant concentration was used above 0.5 g/L for 20 g/L of Yb2Si2O7 in ethanol-based suspension. The deposition stage of EPD was also investigated to tailor green coatings' porosity level and thickness. Then, sintering was applied at 1380 °C for 10 hours, evaluating the role of green coating properties on sintering. Results indicated that high packing density in a uniform accumulation increased the sintered coating density. Moreover, a thickness threshold, providing a uniform pore distribution and similar packing through the thickness after sintering, was determined for green coatings as 130 μm. Additionally, based on experimental findings, a systematic illustration was overviewed, providing a general understanding of EPD and proposing a recipe to obtain desired properties in the final coating. Finally, the constrained sintering and the effect of isothermal ageing on the densification of green coatings containing either Yb2Si2O7 or alumina (sintering aid) added Yb2Si2O7. Results showed that the sintering of Yb2Si2O7 required high temperature (>1400 °C), low heating rate, and long sintering duration. However, using alumina redu

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