Mechanistic Understanding of the Damage and Fracture in Ceramic-Matrix Composites under Extreme Conditions

陶瓷基复合材料（CMCs）具有强、韧、轻、高温稳定等特点；在许多应用中，它们被认为是取代高温合金的重要候选材料，例如在燃气涡轮发动机的核心，目的是提高工作温度，减少对空气冷却的需求，从而实现卓越的燃油效率，减少有害排放。在过去的20年里，cmc已经被用于大型军用发动机的增强部分。随着众多公司和研发机构（主要在美国、欧盟和日本）的大量投资，新的碳化物技术已经被开发出来，以帮助cmc向商业燃气涡轮发动机过渡，更不用说未来在高超音速领域的应用了。尽管CMC市场增长迅速，但英国尚未建立CMC材料供应链。在国际上，碳纤维的设计和加工由于其复杂的微观结构（纤维、基体和孔隙率）仍然是一个挑战。因此，英国有一个重要的机会参与并最终领导，或至少分享，CMC发展的全球努力。这些绝对是不久的将来的主要结构材料。作为一种结构材料，cmc在高温下的力学性能一直是材料验证和采用的关键考虑因素。为了实现特定应用的优化设计，正确理解cmc中损伤和失效机制的演变，以及它们在极端条件下与内在加工-微观结构-性能关系的关系，无疑是关键。这就确定了拟议的工作方案的必要性和范围。在该项目中，将使用一种独特的、逐步变化的实时3D成像方法来捕捉cmc在代表潜在使用条件的超高温（~1000C至1800C）下的变形和断裂。通过结合衍射、微尺度力学和多尺度建模方法等技术，可以理解在前所未有的温度下控制这些材料损伤演变的潜在力学，并与改进材料设计的处理相关。本项目研究的材料将在英国加工或设计，目的是提高英国在cmc方面的工业专业知识，同时也能获得可用的国际材料。我们感兴趣的主要材料是两种CMC类型，它们在航空航天、汽车和能源应用中需求量很大：连续纤维增强SiC-SiC和氧化铝-氧化铝CMC，前者最重要的是作为先进、轻量化、超高效推进装置的游戏规则改变者。然而，与传统的高温合金相比，这些材料是新的；从科学的角度来看，缺乏的是它们在两个关键方面的特征：(i)单个成分的局部特性，纤维/基体界面强度和纤维/基体中的残余应力作为工艺参数的函数，以及（ii）在实际使用条件下，即超高温下，与其3D微观结构相关的损伤积累导致裂纹萌生的实时成像，以模拟这些cmc的工作环境。该项目将在材料加工合作伙伴（如伯明翰大学）和最终用户（如罗尔斯·罗伊斯plc、Cross-Manufacturing和西屋电气）的支持下，针对两种材料类型。最后但并非最不重要的是，该项目将与建模专家（例如，牛津大学，代尔夫特理工大学和建筑科学研究所Eduardo Torroja）密切合作，提供多个长度尺度的实验结果，以开发基于微结构的机制模型框架，用于评估cmc的损伤容限。

Ceramic-matrix composites (CMCs) have the qualities of being strong, tough, lightweight and stable at high temperatures; they are considered as a serious material candidate to replace superalloys for many applications, such as in the core of gas-turbine engines with the aim of increasing operating temperatures, reduce the need for air cooling, and thus enable superior fuel efficiency to reduce harmful emissions. Over the last ~20 years, CMCs have been used in the augmentor sections of large military engines. Followed major investment from numerous companies and R&D organisations, mainly in the US, EU and Japan, new carbide technologies have been developed to aid the transition of CMCs to commercial gas-turbine engines, not to mention future applications in hypersonics. Despite the fast-growing CMC market, there is not yet an established CMC materials supply chain in the UK. Internationally, the design and processing of the CMCs also are still a challenge due to their complex microstructure (fibre, matrix and porosity). Therefore, there is an important opportunity for the UK to participate and ultimately lead, or at least share in, the global effort in CMC development. These are definitely the prime structural materials of the immediate future.As a structural material, the mechanical performance of CMCs at elevated temperatures has been a critical factor for consideration in materials validation and adoption. To achieve an optimised design for a particular application, a sound understanding of the evolution of damage and failure mechanisms in CMCs, and how they relate to the intrinsic processing-microstructure-property relationships under extreme conditions, is undoubtedly the key. This sets the imperative and the horizon of the proposed work programme. In this project, a unique and step-changing, real-time, 3D imaging method will be used to capture the deformation and fracture of CMCs at ultrahigh temperatures (~1000C to 1800C) representative of potential service conditions. By combining with techniques such as diffraction, micro-scale mechanical and multi-scale modelling methodologies, the underlying mechanics controlling the damage evolution in these materials at unprecedented temperatures can be understood and related to processing for improved material design.The materials studied in this project will be processed or designed in the UK with the aim of enhancing UK-based industrial expertise in CMCs, but also access to international materials that are available. The primary materials of interest are two CMC types that are of high demand in aerospace, automotive and energy applications: continuous fibre reinforced SiC-SiC and alumina-alumina CMCs with the former being most important as a game-changer for advanced, lightweight, super-efficient propulsion units. However, compared to conventional superalloys, these materials are new; what has been lacking from a scientific perspective has been their characterisation in terms of two key aspects: (i) the local properties of the individual constituents, fibre/matrix interfacial strength and residual stresses in the fibre/matrix as a function of process parameters, and (ii) the real time imaging of their damage accumulation leading to crack initiation, in relation to their 3D microstructures, at realistic service conditions, i.e., ultrahigh temperatures, to simulate the working environment of these CMCs. This project will target at both material types with the support from materials processing partners (e.g., Birmingham Univ.) and end-users (e.g., Rolls-Royce plc, Cross-Manufacturing and Westinghouse).Last but not the least, this project will work closely with modelling experts (e.g., Oxford Univ., Delft Univ. of Technology, and Institute Eduardo Torroja of Construction Sciences) by providing experimental results over multiple length-scales to develop a framework of a microstructure-based mechanistic model for the evaluation of the damage tolerance of CMCs.