Environmental degradation in CMCs

Ceramic matrix composites (CMCs) based on silicon carbide fibres and silicon carbide matrix with a boron nitride (BN) interlayer are being developed for a range of applications in next generation aerospace engines. Deployment of these composites will allow more efficient operation of engines, and facilitate new engine architectures reducing environmental impact. However use in service is hampered by a lack of mechanistic understanding of the degradation processes. In particular there are two areas of interest 1) the reaction between BN and steam in service. This controls mechanical behaviour through modification of the mechanical properties of the interface. 2) The high temperature reaction of BN with environmental barrier coatings (EBCs) based on rare earth silicates, where the reaction can affect the integrity of the barrier coating, which acts to protect the underlying composite. In both cases understanding the reactions will lead to important information for predicting service lifetimes.
This project will use state of the art energy electron loss spectroscopy (EELS) with high-resolution transmission electron microscopy (HR-TEM) to understand the local structure and chemical environment in degraded samples. This will include simulated service samples from industrial collaborators and samples degraded in Oxford using a newly installed steam degradation rig. This facility allows for steam exposure to samples under highly controlled temperatures, with mass loss monitored through high precision balances and gaseous reaction products to be studied using mass spectrometry.

EELS spectra contain element specific bonding information which can be mapped across the sample providing information on how the elemental distribution and type of bonding vary across the material. To interpret the EELS spectrum, density functional theory (DFT) modelling will be carried on a range of potential structures using existing software. This will allow full chemical and structural understanding of remaining solid reaction products. There is currently limited information on the reactions of BN with steam under aerospace conditions and essentially no information of the reactions of BN and rare earth silicates (with or without steam). This project will provide vital information on both the reaction products and pathways, but also allow correlation of this with mechanical performance data.
This project will build on work currently being carried out by a postdoctoral reseracher who has obtained EELS data from undegraded CMC samples and shown that the fine structure within the spectra contains a wealth of information. It will use modelling to interpret the experimental spectra and use it to understand the type and spatial distribution of the reaction products in the degraded materials. This project is being carried out in collaboration with Rolls Royce. This project falls within the EPSRC themes of Manufacturing the future (Materials Engineering - composites) and Energy (sub-theme: energy efficiency).