Steam degradation of ytterbium disilicate environmental barrier coatings

Abstract

Ytterbium silicates, ytterbium monosilicate (Yb2SiO5, YbMSi) and ytterbium disilicate (Yb2Si2O7, YbDSi), have been identified as promising environmental barrier coatings (EBC) candidate materials due to their high melting point (> 1850 C) and superior thermal-physical properties compared with other RE silicates. YbDSi is of particular interest to researchers due to a more compatible CTE with SiC/SiC CMC substrates than YbMSi and so has less chance of a thermal expansion mismatch, whilst still displaying moderate resistance to silica volatilization in high temperature water vapour. The influence of high temperature water vapour on YbDSi EBC lifetimes can be significant and complex, with much research required to understand the complex interactions in such environments. Therefore, sintered compacts are initially used to model the effect of different microstructural features in steam, before applying the knowledge gained to the degradation of APS deposited free standing EBC coatings in steam. Firstly, in this work the effect of microstructure, particularly porosity, on steam corrosion of YbDSi was studied for sintered compacts of different porosities at 1350 C. A porous layer of YbMSi formed on the surface after exposure to low velocity steam, with the level of porosity determining corrosion rate independent of grain size. The effective gaseous diffusion via pores was found to be significantly reduced by mixed molecular/Knudsen diffusion. Furthermore, a thin deposit of dense ytterbium aluminium garnet (YbAG, Yb3Al5O12) was formed on the compact surface which offered protection from corrosion to the underlying ceramic. Therefore, it is proposed that microstructural control of pore formation and the addition of alumina (Al2O3) in YbDSi coatings can significantly improve the steam corrosion resistance of EBCs. The development of more steam resistant YbDSi based EBCs will be assisted by understanding the role of YbMSi as a secondary phase in the steam corrosion. Therefore, the steam corrosion of two sintered compact compositions, YbDSi, and YbDSi containing 20% YbMSi, was evaluated at 1350 C. It is shown that pre-existing YbMSi grains serve as nucleation sites for the development of new porous YbMSi grains, contrary to previous assumptions of grain boundary formation in literature. Through a crack healing process, most prominent at 20% YbMSi content, it has been demonstrated that an amount of YbMSi content in YbDSi coatings as a secondary phase can be beneficial for steam corrosion protection. Finally, to determine the effect of low velocity steam exposure at 1350 C on EBCs representative of those deployed in industry, the steam corrosion of 3 ytterbium silicate free standing coatings with different microstructural characteristics was evaluated. Utilising the knowledge gained from sintered compacts described above, the corrosion of these coatings and the mechanisms underlying this corrosion have been explored. The loss in mechanical properties caused by steam exposure is shown by nano-indentation maps of cross-sections of annealed EBC coatings before and after steam exposure. It was discovered that the advancement of surface fractures, which are influenced by the amount of YbMSi present, accelerates coating degradation. Investigations to optimise corrosion conditions in low velocity steam are detailed, and it has been shown that the surface area of the samples is an important but often-overlooked factor in experimental design.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Withers (Supervisor) & Ping Xiao (Supervisor)