Environmental degradation of ceramic matrix composites

Thesis

SiC/SiC Ceramic Matrix Composites (CMCs) are a promising material for hot section components of future generations of jet engines. Understanding the degradation processes in these materials is essential before CMCs can be safely adopted, and is a problem that requires further research. In particular the composition of the interphase in these materials, the atomic structure it adopts, and the way the structure changes during environmental degradation is not well understood. Due to the constituent parts these materials are made from, as well as the length scales present in the microstructure of these materials, understanding these materials has been a challenge for the scientific community.

Here, Scanning Transmission Electron Microscopy combined with Electron Energy Loss Spectroscopy, is used to determine the nature of interphases in as received CMC samples. This reveals information about what elements are present in the materials and how consistent the composition is within a single sample and across multiple samples. This characterisation process was also performed for material that had undergone testing at high temperature, revealing information about how the interphase changes as a result of exposure to a degrading environment.

This thesis also aims to use first principles modelling and structure prediction methods to determine the atomic structures and local bonding environments of materials in the interphases. This is done using Density Functional Theory, Ab-Initio Random Structure Searching, and Ephemeral Data Derived Potentials to predict the structures of the atoms present in these materials. This thesis will demonstrate a workflow for combining these computational techniques, along with the experimental data from electron microscopy, to make predictions about the atomic structures in these materials, and to further scientific understanding of the complex spectra of these materials.

Authors: Ginnis, T

• DOI: 10.5287/ora-goqavdpo1
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: electron energy loss spectroscopy; density functional theory; structure prediction; ceramic matrix composites