Environmental degradation in CMCs

基于碳化硅纤维和具有氮化硼（BN）中间层的碳化硅基质的陶瓷基质复合材料（CMC）正在被开发用于下一代航空航天发动机的一系列应用。这些复合材料的部署将使发动机更有效地运行，并促进新的发动机架构减少对环境的影响。然而，由于缺乏对降解过程的机械理解，服务中的使用受到阻碍。特别地，存在两个感兴趣的领域：1）BN与使用中的蒸汽之间的反应。这通过改变界面的机械性能来控制机械行为。2)BN与基于稀土硅酸盐的环境阻隔涂层（EBC）的高温反应，其中该反应可影响阻隔涂层的完整性，该阻隔涂层用于保护下面的复合材料。在这两种情况下，了解反应将导致预测使用寿命的重要信息。该项目将使用最先进的能量电子损失谱（EELS）与高分辨率透射电子显微镜（HR-TEM），以了解降解样品的局部结构和化学环境。这将包括来自工业合作者的模拟服务样品和使用新安装的蒸汽降解装置在牛津降解的样品。该设施允许在高度受控的温度下对样品进行蒸汽暴露，通过高精度天平监测质量损失，并使用质谱法研究气态反应产物。EELS光谱包含元素特定的键合信息，可以在整个样品中映射，提供有关元素分布和键合类型如何在材料中变化的信息。为了解释EELS谱，将使用现有软件对一系列潜在结构进行密度泛函理论（DFT）建模。这将使剩余的固体反应产物的化学和结构的全面了解。目前关于BN与蒸汽在航空航天条件下的反应的信息有限，并且基本上没有BN与稀土硅酸盐（有或没有蒸汽）的反应的信息。该项目将提供有关反应产物和途径的重要信息，但也允许将其与机械性能数据相关联。该项目将建立在一位博士后研究人员目前正在进行的工作的基础上，该研究人员从未降解的CMC样品中获得了EELS数据，并表明光谱内的精细结构包含了丰富的信息。它将使用建模来解释实验光谱，并使用它来了解降解材料中反应产物的类型和空间分布。该项目正在与罗尔斯·罗伊斯公司合作进行。该项目属于EPSRC未来制造（材料工程-复合材料）和能源（子主题：能源效率）主题的福尔斯。

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).